Applications Engineering Manual
Air-to-Air Energy Recovery in HVAC Systems
1
September 2020
SYS-APM003D-EN
Air-to-Air
Energy Recovery in
HVAC Systems
John Murphy, senior applications engineer
Brenda Bradley, information designer

Preface
As a leading HVAC manufacturer, we believe that it is our responsibility to serve
the building industry by regularly disseminating information gathered through
laboratory research, testing programs, and practical experience. Trane
publishes a variety of educational materials for this purpose. Applications
engineering manuals such as this document can serve as comprehensive
reference guides for professionals who design building comfort systems.
This manual focuses on air-to-air energy recovery. It describes concepts and
mechanical implementation, and identifies system-level characteristics for
effective operation and control. We encourage you to familiarize yourself with
its contents and to review the appropriate sections when designing a comfortsystem application that could benefit from airside energy recovery. ■
Trane, in proposing these system design and application concepts, assumes no
responsibility for the performance or desirability of any resulting system design.
Design of the HVAC system is the prerogative and responsibility of the engineering
professional.
© 2020 Trane. All Rights Reserved
SYS-APM003-EN

Contents
Introduction ...................................................................................................... 1
Why Recover Energy? ................................................................................ 2
Economics ................................................................................................................ 2
Compliance with Codes .......................................................................................... 2
Applications for Air-to-Air Energy Recovery ............................... 5
Basic Concepts ........................................................................................................ 5
Sensible versus Total Energy Recovery .......................................................... 5
Effectiveness Ratio ............................................................................................ 7
Balanced versus Unbalanced Airflows ............................................................ 7
Outdoor-Air (OA) Preconditioning ......................................................................... 9
Sensible versus Total Energy Recovery .......................................................... 9
Effect on First Cost .......................................................................................... 13
Effect on Operating Costs ............................................................................... 16
Other Considerations ...................................................................................... 18
Supply-Air (SA) Tempering .................................................................................. 20
Supply-Air Tempering in Mixed-Air Systems ............................................... 21
Supply-Air Tempering in Dedicated Outdoor-Air Systems ......................... 26
Effect on First Cost .......................................................................................... 32
Effect on Operating Costs ............................................................................... 33
Other Considerations ...................................................................................... 37
Energy-Recovery Technologies .......................................................... 38
Coil Loop ................................................................................................................ 38
Typical Performance ....................................................................................... 38
Capacity Control .............................................................................................. 40
Frost Prevention .............................................................................................. 41
Cross-Leakage .................................................................................................. 42
Coil Loop with Three Coils .............................................................................. 42
Fixed-Plate Heat Exchanger .................................................................................. 43
Typical Performance ....................................................................................... 43
Capacity Control .............................................................................................. 44
Frost Prevention .............................................................................................. 45
Cross-Leakage .................................................................................................. 46
Heat Pipe ................................................................................................................ 47
Typical Performance ....................................................................................... 47
Capacity Control .............................................................................................. 49
Frost Prevention .............................................................................................. 50
Cross-Leakage .................................................................................................. 50
Rotary Heat Exchanger ......................................................................................... 51
Typical Performance ....................................................................................... 52
Capacity Control .............................................................................................. 53
Frost Prevention .............................................................................................. 55
Cross-Leakage .................................................................................................. 57
SYS-APM003-EN
iii

Contents
ARI Standard 1060 ................................................................................................ 60
Standard Rating Conditions ........................................................................... 61
Using EATR and OACF for System Design ................................................... 62
System-Level Control of Recovered Energy .............................. 65
OA Preconditioning:
Dedicated OA System for Neutral-Temperature Air .................................... 68
Dedicated OA Systems for Cold Air .............................................................. 71
CV-MA Systems for Modulated-Temperature Supply Air ........................... 74
VAV-MA Systems for Constant-Temperature Supply Air ........................... 77
SA Tempering:
Dedicated OA System for Neutral-Temperature Air .................................... 80
CV-MA Systems for Modulated-Temperature Supply Air ........................... 82
Applications that Combine OA Preconditioning and SA Tempering ............... 84
Dual-Function, Dedicated OA System for Neutral-Temperature Air .......... 84
Alternatives for Tempering with Sensible-Energy Recovery ...................... 88
Tempering (Recooling) in an Active-Desiccant System .............................. 88
Suggestions for Effective Airside Energy Recovery ............. 90
OA Preconditioning .............................................................................................. 90
Sensible- or Total-Energy Recovery? ............................................................ 90
Central or Local OA Preconditioning? ........................................................... 93
Minimizing Life-Cycle Cost ............................................................................. 95
SA Tempering ....................................................................................................... 98
Sensible- or Total-Energy Recovery? ............................................................ 98
Series or Parallel Arrangement? .................................................................... 98
Minimizing Life-Cycle Cost ........................................................................... 100
Glossary .......................................................................................................... 102
References ..................................................................................................... 106
Index .................................................................................................................. 108
iv
SYS-APM003-EN

Introduction
Energy consumption costs money, uses natural resources, increases air
pollution, and contributes to global warming. All of us share responsibility for
prudent energy use.
The mechanical heating, ventilating, and air-conditioning (HVAC) system
typically accounts for much of the energy costs in a building. Heat gains and
losses from various sources must be offset to create a comfortable, healthy
indoor environment. Discarded heat from one location may be useful elsewhere
in the building, so it is logical (often cost-effective, too) to recover energy that
might otherwise be wasted … particularly in a time of rising energy costs.
This manual discusses various technologies for and applications of air-to-air
energy recovery in HVAC systems. ■
SYS-APM003-EN
1

Why Recover Energy?
The answer to the question posed by the title of this section is twofold, at least
in the context of an HVAC system: economics and compliance with local codes.
Economics
The energy impact of increased
ventilation rates is one of several issues
addressed in the U.S. EPA study, Energy
Cost and IAQ Performance of
Ventilation Systems and Controls (EPA402-S-01-001, January 2000). ■
Efforts to improve indoor air quality (IAQ) typically include bringing more
outdoor air into the building … a practice that increases monthly utility costs by
increasing the amount of heating, cooling, humidifying, and dehumidifying
performed by the HVAC system. A study conducted by the U.S. Environmental
Protection Agency (EPA) quantifies the economic effect of increased ventilation
rates, which were defined in the 1989 version of ASHRAE Standard 62,
Ventilation for Acceptable Air Quality. Depending on the building type, system
type, and climate, the EPA study found that HVAC energy costs increased by
2 percent to 67 percent, while required capacities for peak cooling increased by
15 percent to 40 percent.
These statistics underscore the importance of identifying opportunities to
reduce consumption, particularly when the cost and availability of energy are of
increasing concern. Correctly implemented, energy recovery offsets the
operating-cost penalty that results from bringing more outdoor air into the
building. Recovering enough energy may even allow the use of smaller
mechanical heating and cooling equipment.
Compliance with Codes
ANSI/ASHRAE/IESNA Standard
90.1–2007 is available from ASHRAE’s
online bookstore at www.ashrae.org.
The companion user’s manual explains
the intent and application of the
standard; it also includes a complete set
of compliance forms and worksheets. ■
Setting aside the logical and practical arguments for recovering energy from
other areas of the building, local codes may actually mandate the practice.
In the United States, the primary standard related to energy consumption in
commercial buildings is ANSI/ASHRAE/IESNA Standard 90.1–2007, Energy
Standard for Buildings Except Low-Rise Residential Buildings. Standard 90.1
sets minimum design requirements that encourage energy efficiency
throughout the building. Affected systems include lighting, motors, HVAC,
service water heating, and the building envelope.
Section 6.5.6.1 of that standard requires the use of exhaust-air energy recovery
when an individual fan system meets both of the following conditions:
■
Design supply airflow equals or exceeds 5,000 cfm (2.4 m ³/s)
and
■
2
Minimum outdoor airflow equals or exceeds 70 percent of the design supply
airflow
SYS-APM003-EN

Why Recover Energy?
This requirement generally applies to two types of systems: dedicated
(100-percent) outdoor-air systems and mixed-air systems that serve spaces
with high minimum requirements for outdoor airflow (for example, densely
occupied spaces). Of course, exhaust-air energy recovery may make economic
sense for systems that deliver 40-percent or less outdoor airflow by permitting
equipment downsizing and consuming less new energy.
Standard 90.1 mandates a minimum effectiveness of 50 percent when
exhaust-air energy is required. (The inset on p. 4 lists the exceptions to this
requirement; effectiveness is discussed on pp. 6 – 7.) As defined by the standard,
Fifty percent energy recovery effectiveness shall mean a
change in the enthalpy of the outdoor air supply equal to 50%
of the difference between the outdoor air and return air at
design conditions.
The publications listed below discuss
energy recovery from refrigerant in
direct-expansion refrigeration systems
and from condenser water in watercooled chilled water systems. They also
offer suggestions for system
optimization:
■
Refrigerant Heat Recovery, Trane
applications engineering manual 
SYS-AM-5
■
Multiple-Chiller-System Design
and Control, Trane applications
engineering manual SYS-APM001EN
■
Waterside Heat Recovery in HVAC
Systems, Trane applications
engineering manual SYS-APM005EN
■
Chiller Heat Recovery,
ASHRAE application guide ISBN
883413-99-0 ■
Further, the exhaust-air energy-recovery system must be installed with bypass
dampers or other controls that permit airside economizer operation when such
economizers are prescribed by Section 6.5.1.1 of the standard.
Section 6.5.2 of Standard 90.1 also relates to the use of energy recovery; it
limits coincident heating and cooling of the same air stream. According to
Section 6.5.2.3, if space humidity is directly regulated — through the use of
humidistats, for example — controls must prevent reheating, mixing of hot and
cold air streams, or other means of simultaneously heating and cooling the
same air. (The inset on p. 4 lists the exceptions to this requirement.) Exception E
permits reheat if at least 75 percent of the energy used to reheat the air is
recovered on site.
Limiting the use of “new” energy for reheat will almost certainly prompt HVAC
design professionals to recover heat from the cooling process when reheat is
required, particularly in dehumidification applications. Potential sources of
recoverable heat include:
■
Condenser water in a water-cooled, chilled water system
■
Hot refrigerant in a refrigeration system
■
Another air stream or another location in the same air stream
The rest of this manual describes how air-to-air heat exchangers can be used to
transfer energy within or between air streams. ■
SYS-APM003-EN
3

Why Recover Energy?
ANSI/ASHRAE/IESNA Standard 90.1–2007 on Exhaust-Air Energy Recovery …
6.5.6.1 Exhaust Air Energy Recovery.
Individual fan systems that have both a
design supply air capacity of 5,000 cfm
(2.4 m³ /s) or greater and have a minimum
outdoor air supply of 70% or greater of the
design supply air quantity shall have an
energy recovery system with at least 50%
recovery effectiveness. Fifty-percent
energy recovery effectiveness shall mean a
change in the enthalpy of the outdoor air
supply equal to 50% of the difference
between the outdoor air and return air at
design conditions. Provision shall be made
to bypass or control the heat recovery
system to permit air economizer operation
as required by 6.5.1.1.
Exceptions to 6.5.6.1:
a) Laboratory systems meeting 6.5.7.2.
b) Systems serving spaces that are not
cooled and that are heated to less than
60°F (16°C).
c) Systems exhausting toxic, flammable,
paint or corrosive fumes or dust.
d) Commercial kitchen hoods used for
collecting and removing grease vapors
and smoke.
e) Where more than 60% of the outdoor
air heating energy is provided from
site-recovered or site solar energy.
f) Heating systems in climate zones 1
through 3.
g) Cooling systems in climate zones 3c, 4c,
5b, 5c, 6b, 7 and 8.
h) Where the largest exhaust source is
less than 75% of the design outdoor
airflow.
4
i) Systems requiring dehumidification
that employ energy recovery in series
with the cooling coil.
Note: Interpretation 90.1-2001-7, and
the 90.1 Users Manual, provide further
explanantion of the meaning of 50%
energy recovery effectiveness. Both are
available at www.ashrae.org.
… and on Dehumidification
6.5.2.3 Dehumidification. Where
humidistatic controls are provided, such
controls shall prevent reheating, mixing of
hot and cold air streams, or other means of
simultaneous heating and cooling of the
same air stream.
Exceptions to 6.5.2.3:
a) The system is capable of reducing
supply air volume to 50% or less of
the design airflow rate or the
minimum rate specified in 6.2 of
ASHRAE Standard 62.1, whichever
is larger, before simultaneous
heating and cooling takes place.
b) The individual fan cooling unit has a
design cooling capacity of 
80,000 Btu/h (23 kW) or less and is
capable of unloading to 50% capacity
before simultaneous heating and
cooling takes place.
providing mechanical cooling.
d) Systems serving spaces where
specific humidity levels are required
to satisfy process needs, such as
computer rooms, museums, surgical
suites, and buildings with
refrigerating systems, such as
supermarkets, refrigerated
warehouses, and ice arenas. This
exception also applies to other
applications for which fan volume
controls in accordance with
Exception (a) are proven to be
impractical to the enforcement
agency.
e) At least 75% of the energy for
reheating or for providing warm air
in mixing systems is provided from a
site-recovered (including condenser
heat) or site solar energy source.
f) Systems where the heat added to the
air stream is the result of the use of a
desiccant system and 75% of the heat
added by the desiccant system is
removed by a heat exchanger, either
before or after the desiccant system
with energy recovery. ■
c) The individual mechanical cooling
unit has a design cooling capacity of
40,000 Btu/h (12 kW) or less. An
individual mechanical cooling unit is
a single system composed of a fan or
fans and a cooling coil capable of
SYS-APM003-EN
Applications for
Air-to-Air Energy Recovery
Air-to-air energy recovery describes the transfer of sensible heat, or sensible
plus latent heat, between two or more air streams, or between two locations
within the same air stream. When applied in HVAC systems, air-to-air energyrecovery devices perform one of two principal functions:
■
Precondition outdoor air brought into the building for ventilation
■
Temper, or reheat, supply air for independent control of space conditions
(dry-bulb temperature, humidity)
By including two separate, air-to-air energy-recovery devices, one for each
function, an HVAC system can simultaneously provide preconditioning and
tempering. (See “Applications that Combine OA Preconditioning and SA
Tempering,” pp. 84 – 88.)
Basic Concepts
Let’s briefly review three fundamentals of energy recovery before discussing its
application in the design of comfort systems: sensible versus total energy
recovery, effectiveness ratio, and balanced versus unbalanced airflow.
Sensible versus Total Energy Recovery
Sensible-energy recovery refers to the transfer of sensible heat only. Adding
sensible heat raises the dry-bulb temperature of an air stream; removing
sensible heat lowers the dry-bulb temperature. Removing sensible heat may
cause moisture to condense from the warmer air stream, but that moisture
never transfers to the colder air stream.
Common examples of sensible-energy-recovery devices include coil loops,
fixed-plate heat exchangers, heat pipes, and sensible-energy, rotary heat
exchangers (also known as sensible-energy wheels).
Total-energy recovery describes the transfer of both sensible and latent heat.
Latent heat is the energy stored in the water vapor of a moist air stream. Adding
latent heat raises the dew-point temperature (moisture content) of an air
stream; removing latent heat lowers the dew-point temperature.
Common examples of total-energy recovery devices include total-energy rotary
heat exchangers (also known as total-energy wheels) and fixed-membrane heat
exchangers.
(“Energy-Recovery Technologies,” pp. 38 – 64, further describes the sensibleand total-energy-recovery devices identified above.)
SYS-APM003-EN
5
Applications for
Air-to-Air Energy Recovery
Fundamental Equations for Air-to-Air Energy Recovery
Historically, manufacturers of air-to-air
energy-recovery equipment used various
methods to calculate the effectiveness of
their devices. ARI Standard 1060 – 2005,
Performance Rating of Air-to-Air Heat
Exchangers for Energy Recovery
Ventilation Equipment, normalizes the
calculation of effectiveness for rotary heat
exchangers, heat pipes, and fixed-plate
heat exchangers by defining the
“effectiveness ratio” equation shown at
right.
The only way to assure equitable
comparisons of airside energy-recovery
equipment is to require certification of
performance in accordance with ARI 1060.
For more information about the
standard, see “ARI Standard 1060,” 
pp. 60 – 64.
Note: ARI Standard 1060 does not apply
to coil loops. ARI Standard 410 – 2001,
Forced-Circulation Air-Cooling and AirHeating Coils, defines the rating
requirements for individual coils. To
calculate coil-loop effectiveness, use the
certified coil performance in the
“effectiveness ratio” equation.
Effectiveness ratio. The following
equation compares the amount of energy
that is actually transferred with the
maximum amount that could be
transferred between the two air streams:
·
ms   x1 – x2 
 = ----------------------------------------·
m min   x 1 – x 3 
where,

= sensible, latent, or total
effectiveness
·
ms
= mass flow rate, lb/hr (kg/s),
through supply side of heatexchange device
·
me
= mass flow rate, lb/hr (kg/s),
through exhaust side of heatexchange device
·
m min = smaller mass flow rate (supply,
·
·
m s ; or exhaust, m e )
x1
= entering supply air: dry bulb, °F
(°C); humidity ratio, lb (water) / lb
(dry air) (kg/kg); or enthalpy, 
Btu/lb (kJ/kg)
x2
= leaving supply air: dry bulb,
humidity ratio, or enthalpy
x3
= entering exhaust air: dry bulb,
humidity ratio, or enthalpy
x4
= leaving exhaust air: dry bulb,
humidity ratio, or enthalpy
Sensible-heat transfer is determined by
the sensible effectiveness of the device, the
minimum airflow rate, and the difference
between the entering dry-bulb
temperatures of the two air streams:
60min
Q s =     C p  ------------------  V min   T 1 – T 3 
1hr
 Q s =     C p  V min   T 1 – T 3  
where,
= sensible heat actually transferred,
Qs
Btu/hr (kW)
6

= sensible effectiveness of device

= density of air, lb/ft³ (kg/m³)
Cp
= specific heat of air, Btu/lb • °F
(kJ/kg • °K)
V min = smaller airflow (supply or exhaust
side), cfm (m³/s)
T1
= entering supply-air dry bulb, °F
(°C)
T3
= entering exhaust-air dry bulb, °F
(°C)
Note: At “standard” conditions, the
product of density, specific heat, and the
time conversion equals 1.085 (1.21). The
minutes-per-hour factor is only needed to
calculate Q s in I-P units.
Total-heat transfer is determined by the
total effectiveness of the device, the
minimum airflow rate, and the difference
between the entering enthalpies of the two
air streams:
60min
Q T =     ------------------  V min   h 1 – h 3 
1hr
 Q T =     V min   h 1 – h 3  
where,
= total (sensible plus latent) heat
actually transferred, Btu/hr (kW)
QT

= total effectiveness of device

= density of air, lb/ft³ (kg/m³)
V min = smaller airflow (supply or exhaust
side), cfm (m³/s)
h1
= entering supply-air enthalpy, 
Btu/lb (kJ/kg)
h3
= entering exhaust-air enthalpy,
Btu/lb (kJ/kg)
Note: At “standard” conditions, the
product of density and the time conversion
equals 4.5 (1.2). The minutes-per-hour
factor is only needed to calculate Q T in I-P
units. ■
SYS-APM003-EN
Applications for
Air-to-Air Energy Recovery
Effectiveness Ratio
Figure 1. Air conditions at an
energy-recovery device
Effectiveness, , quantifies the performance of the heat-exchange device when
it is used for energy recovery. Mathematically, the effectiveness ratio represents
the amount of energy (sensible, latent, or total) that the device actually
transfers divided by the maximum energy transfer possible between two air
streams; see Figure 1.
For a given heat exchanger geometry, effectiveness is determined at test
conditions using various airflow rates. ANSI/ASHRAE Standard 84 –1991,
Method of Testing Air-to-Air Heat Exchangers, provides the equation (shown in
the inset on p. 6) that describes the effectiveness of an air-to-air energyrecovery device. The equation can be used to determine sensible, latent, or
total effectiveness based on measurements obtained through laboratory tests.
Note that the minimum mass flow rate is the smaller of the mass flow rates
through either side of the heat exchanger.
The amount of sensible heat transferred depends on the sensible effectiveness
of the device, the minimum airflow rate, and the difference between the drybulb air temperatures entering the supply and exhaust sides of the heat
exchanger. The heat-transfer equations on p. 6 express the mathematical
relationships between these variables.
Similarly, the amount of total energy transferred depends on the total
effectiveness of the device, the minimum airflow rate, and the difference
between the enthalpies of the air streams entering the supply and exhaust sides
of the device. Again, minimum airflow is the smaller of the two airflows,
whether supply or exhaust.
Balanced versus Unbalanced Airflows
Airflows are considered “unbalanced” when the rate of flow through one side
of the heat exchanger differs from the rate of flow through the other side.
Laboratory tests suggest that unbalanced airflows increase the effectiveness
of the device. In actual operation, however, the device transfers less heat
overall because the amount of heat transferred depends on the smaller of the
two airflows. This irony is best explained using an example.
Figure 2 (p. 8) illustrates the relationship between airflows and the effectiveness
of a generic sensible-energy-recovery device. The horizontal axis represents
exhaust-side airflow divided by supply-side airflow. A value of 1.0 indicates
SYS-APM003-EN
7
Applications for
Air-to-Air Energy Recovery
sensible effectiveness
Figure 2. Example of effectiveness for a sensible-energy-recovery device
Figure 3. Balanced (equal) airflows
exhaust airflow / supply airflow
balanced airflows; a value less than 1.0 indicates that the exhaust airflow is less
than the supply airflow.
Q s = 0.66  1.085  10 000 cfm   96°F – 78°F 
Q s = 128 898 Btu/hr
 Q s = 0.66  1.21  4.7 m³/s   35.7°C – 25.6°C  
 Q s = 37.9 kW 
Figure 4. Unbalanced (unequal) airflows
Figure 3 shows how the sensible-energy-recovery device performs when
operated with balanced exhaust and supply airflows, that is, with 10,000 cfm
(4.7 m³/s) of air flowing through each side of the device. Sensible effectiveness
at this balanced condition is 66 percent (Figure 2). Using that value to solve the
sensible-heat-transfer equation indicates that the device will transfer
128,898 Btu/hr (37.9 kW) of sensible heat from the supply air stream to the
exhaust air stream at standard air conditions.
Figure 4 demonstrates how unbalanced airflows affect the performance of the
same energy-recovery device. In this case, the supply airflow is 10,000 cfm
(4.7 m³/s), but the exhaust airflow is only 7,000 cfm (3.3 m³/s). Although the
imbalance increases the sensible effectiveness of the device to 79 percent, it
now transfers 16 percent less sensible heat from the supply air stream to the
exhaust air stream — that is, only 108,000 Btu/hr (31.9 kW).
Balanced airflow is difficult to achieve in actual installations: exfiltration and
local exhausts usually result in a system-level exhaust airflow that is less than
the outdoor airflow brought into the building. Despite this challenge, equalizing
outdoor and exhaust airflows not only affords the greatest reductions in cooling
and heating capacities but also increases energy savings.
Q s = 0.79  1.085  7 000 cfm   96°F – 78°F 
Q s = 108 000 Btu/hr
Note: In most applications, an exhaust airflow that is at least 70 percent of the
outdoor airflow yields an attractive payback.
 Q s = 0.79  1.21  3.3 m³/s   35.7°C – 25.6°C  
 Q s = 31.9 kW 
8
SYS-APM003-EN
Applications for
Air-to-Air Energy Recovery
Outdoor-Air Preconditioning
Buildings are ventilated with outdoor air to prevent the buildup of contaminants
that are generated indoors. In most climates, this outdoor air imposes a sizable
load on the HVAC equipment.
Outdoor-air preconditioning is the process of cooling, dehumidifying, heating,
and/or humidifying the outdoor air entering the system. Using air-to-air energy
recovery (sometimes called “exhaust-air energy recovery”) to perform this
function can significantly reduce the load on HVAC equipment as well as the
energy consumed by that equipment. Energy recovery may even be required to
comply with ASHRAE Standard 90.1–2007 (Section 6.5.6) and other energy
codes.
Figure 5. OA-preconditioning arrangements
sensible
heat plus
moisture
total-energy
recovery
sensible
heat only
sensible-energy
recovery
Figure 5 represents air-to-air energy-recovery arrangements that precondition
outdoor air (OA) by exchanging energy with the exhaust air (EA) stream. During
the cooling season, when it is hot and humid outside, the total-energy-recovery
device precools and “pre-dries” (dehumidifies) the outdoor air by transferring
both sensible heat and moisture to the exhaust air stream. During the heating
season, when outdoor conditions are cold and dry, the same arrangement
preheats and prehumidifies the outdoor air by removing both sensible heat and
moisture from the exhaust air stream and releasing it into the supply air stream.
Substituting a sensible-energy recovery device limits energy transfer to
sensible heat; moisture (latent heat) is not recovered. As the examples that
follow demonstrate, either type of recovered energy — sensible or total — can
effectively precondition outdoor air. First cost and potential energy savings
typically determine which type of recovery best suits a particular application.
Sensible versus Total Energy Recovery
A Scenario to Aid Comparison
An energy-recovery device installed in a central air handler preconditions
10,000 cfm (4.7 m³/s) of outdoor air. Local exhausts for the rest rooms remove
air from the building, as does the exfiltration caused by maintaining an indoor
pressure that is slightly positive relative to outdoors. With only 7,000 cfm
(3.3 m³/s) exhausted centrally, unbalanced airflows exist at the energy-recovery
device. Table 1, p. 10, summarizes the design airflows and thermal conditions
for this scenario.
The rest of this section compares the energy-recovery performance of this
system in two climates — Jacksonville, Florida (predominantly cooling) and
Minneapolis, Minnesota (predominantly heating) — and with different energyrecovery devices: a coil loop that recovers only sensible energy and a
total-energy wheel that recovers both sensible and latent energy.
SYS-APM003-EN
9
Precondition
Outdoor Air
Applications for
Air-to-Air Energy Recovery
Table 1. Design parameters for examples of OA preconditioning (Figures 6 – 9)
Cooling (ASHRAE 0.4%)
Location1
Jacksonville, Fla.
96°F DB (35.7°C)
76°F WB (24.5°C)
31°F DB (-0.8°C)
Minneapolis, Minn.
91°F DB (32.8°C)
73°F WB (22.7°C)
-16°F DB (-26.5°C)
78°F DB (25.6°C)
63.5°F WB (17.5°C)
70°F DB (21.1°C)
54 °F WB (12.2 °C)
Return air from the conditioned space
Airflows
Heating (ASHRAE 99.6%)
Outdoor
10,000 cfm (4.7 m³/s)
Central exhaust
7,000 cfm (3.3 m³/s)
Local exhaust (rest rooms)
2,500 cfm (1.2 m³/s)
Exfiltration through building envelope
500 cfm (0.2 m³/s)
1 Cooling design conditions for these locations represent the ASHRAE 0.4% design condition for sensible cooling, that
is, the peak dry-bulb temperature and mean coincident wet-bulb temperature. Heating design conditions represent the
ASHRAE 99.6% design condition.
OA Preconditioning with Recovered Sensible Energy
For this comparison, the sensible-energy recovery device in Figure 6 represents
a coil loop with an effectiveness of 57 percent. During the cooling season, the
coil loop preconditions the outdoor air stream by transferring sensible heat to
the exhaust air stream. During the heating season, it warms the entering
outdoor air with sensible heat recovered from the exhaust air.
Let’s first look at the design conditions for Jacksonville, Florida, where cooling
predominates. The ASHRAE 0.4-percent condition for sensible cooling design
(that is, peak dry-bulb temperature and mean coincident wet-bulb temperature)
is 96°F dry bulb and 76°F wet bulb (35.7°C DB, 24.5°C WB). At this condition,
Figure 6. OA preconditioning with sensible-energy recovery in Jacksonville, Florida
10
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Precondition
Outdoor Air
Applications for
Air-to-Air Energy Recovery
Figure 7. OA preconditioning with sensible-energy recovery in Minneapolis, Minnesota
Cooling-season savings in Jacksonville, Fla.:
Q s = 1.085  10 000 cfm   96°F – 88.8°F 
Q s = 78 120 Btu/hr = 6.5 tons
 Q s = 1.21  4.7 m³/s   35.7°C – 31.6°C  
 Q s = 23.3 kW 
Heating-season savings in Jacksonville, Fla.:
Q s = 1.085  10 000 cfm   47.9°F – 31°F 
Q s = 183 365 Btu/hr = 183 MBh
 Q s = 1.21  4.7 m³/s   8.8°C –  – 0.8 °C   
 Q s = 54.6 kW 
Cooling-season savings in Minneapolis, Minn.:
Q s = 1.085  10 000 cfm   91°F – 85.9°F 
Q s = 55 335 Btu/hr = 4.6 tons
 Q s = 1.21  4.7 m³/s   32.8°C – 29.9°C  
 Q s = 16.5 kW 
Heating-season savings in Minneapolis, Minn.:
Q s = 1.085  10 000 cfm   6.6°F –  – 16 °F  
Q s = 245 210 Btu/hr = 245 MBh
 Q s = 1.21  4.7 m³/s   – 14.1 °C –  – 26.5 °C   
 Q s = 70.5 kW 
SYS-APM003-EN
based on data from a coil selection program, the coil loop precools the outdoor
air to 88.8°F DB (31.6°C). It also reduces the required cooling capacity by
6.5 tons (23.3 kW), which means lower operating costs and permits downsizing
of the cooling plant.
Figure 6 also shows the winter performance of the coil loop. The ASHRAE
99.6-percent condition for heating design in Jacksonville is 31°F DB (-0.8°C).
This time the coil loop transfers sensible heat from the exhaust air stream to the
outdoor air stream, warming it to 47.9°F (8.8°C). The recovered energy reduces
the design heating capacity by 183 MBh (54.6 kW) and, in turn, reduces
operating costs and permits downsizing of the heating plant.
How well does the same coil loop recover sensible energy in a climate where
heating predominates? Figure 7 depicts the performance of our example HVAC
system in Minneapolis, Minnesota. At the sensible-cooling-design condition of
91°F DB and 73°F WB (32.8°C DB, 22.7°C WB), the coil loop precools the outdoor
air to 85.9°F DB (29.9°C) and reduces the required cooling capacity by 4.6 tons
(16.5 kW). At the heating design condition of -16°F (-26.5°C), recovered energy
preheats the outdoor air to 6.6°F (-14.1°C), which saves 245 MBh (70.5 kW).
Notice that the air that passes through the exhaust side of the coil loop reaches
the saturation curve (Figure 7), indicating that moisture from the exhaust air
stream is condensing on the coil surface. The small amount of latent heat
removed from the exhaust air contributes to the total quantity of heat that
transfers to the outdoor air stream; however, only sensible (not latent) heat is
transferred. Condensation on the exhaust side of the energy-recovery device
can pose a problem in cold climates. (See “Frost Prevention,” pp. 18–19.)
11
Precondition
Outdoor Air
Applications for
Air-to-Air Energy Recovery
OA Preconditioning with Recovered Total Energy
Figure 8 and Figure 9 illustrate the effect of recovering total energy in the
scenario described on p. 9. For this example, the energy-recovery device is
a total-energy wheel with an effectiveness of 82 percent, given unbalanced
airflows of 10,000 cfm (4.7 m³/s) on the supply side of the wheel and 7,000 cfm
(3.3 m³/s) on the exhaust side. During the cooling season, the wheel transfers
sensible and latent heat (moisture) from the outdoor air stream to the exhaust
air stream, precooling and dehumidifying the incoming outdoor air. Conversely,
the wheel warms and humidifies the outdoor air stream during the heating
season, using sensible heat and moisture recovered from the exhaust air
stream.
Cooling-season savings in Jacksonville, Fla.:
Q T = 4.5  10 000 cfm   39.3 – 33.3 Btu/lb 
Q T = 270 000 Btu/hr = 22.5 tons
 Q T = 1.2  4.7 m³/s   91.4 – 77.5 kJ/kg  
 Q T = 78.4 kW 
Heating-season savings in Jacksonville, Fla.:
Q s = 1.085  10 000 cfm   53.3°F – 31°F 
Q s = 241 955 Btu/hr = 242 MBh
 Q s = 1.21  4.7 m³/s   11.8°C –  – 0.8 °C   
 Q s = 71.7 kW 
Q L = 0.7  10 000 cfm   29.3 – 20.2 grains/lb 
Q L = 63 700 Btu/hr = 63.7 MBh
 Q L = 3 010  4.7 m³/s   0.0042 – 0.0029 kg/kg  
 Q L = 18.4 kW 
The selection program for the wheel indicates that, at Jacksonville’s
cooling design condition (Table 1, p. 10), the wheel will cool and dehumidify the
outdoor air to 85.7°F DB and 69.3°F WB (29.8°C DB, 20.7°C WB). See Figure 8.
Transferring moisture as well as sensible heat lowers operating costs and
reduces the required cooling capacity — this time by 22.5 tons (78.4 kW).
At Jacksonville’s heating design condition, the sensible energy recovered from
the exhaust air stream raises the dry-bulb temperature of the entering outdoor
air to 53.3°F DB (11.8°C). Preheating the outdoor air in this fashion reduces the
required heating capacity by 242 MBh (71.7 kW). The total-energy wheel also
humidifies the entering outdoor air with moisture recovered from the exhaust
air stream, increasing the moisture content from 20.2 grains/lb to 29.3 grains/lb
(0.0029 kg/kg to 0.0042 kg/kg). If the HVAC system includes a mechanical
humidifier, the capacity required at design can be reduced by 63.7 MBh
(18.4 kW).
Figure 8. OA preconditioning with total energy recovery in Jacksonville, Florida
12
SYS-APM003-EN
Precondition
Outdoor Air
Applications for
Air-to-Air Energy Recovery
Figure 9. OA preconditioning with total energy recovery in Minneapolis, Minnesota
Cooling-season savings in Minneapolis, Minn.:
Q T = 4.5  10 000 cfm   36.5 – 32.1 Btu/lb 
Q T = 198 000 Btu/hr = 16.5 tons
 Q T = 1.2  4.7 m³/s   84.9 – 74.7 kJ/kg  
 Q T = 57.5 kW 
Heating-season savings in Minneapolis, Minn.:
Q s = 1.085  10 000 cfm   40.1°F – 1°F 
Q s = 424 235 Btu/hr = 424 MBh
 Q s = 1.21  4.7 m³/s   4.5°C –  – 17.2 °C   
 Q s = 123.4 kW 
Q L = 0.7  10 000 cfm   21.0 – 2.0 grains/lb 
Q L = 133 000 Btu/hr = 133.0 MBh
 Q L = 3 010  4.7 m³/s   0.003 – 0.0003 kg/kg  
 Q L = 38.2 kW 
Figure 9 illustrates how the same total-energy wheel performs in Minneapolis
at design conditions. During the summer, the wheel cools and dehumidifies the
outdoor air to 83.6°F DB, 67.8°F WB (28.7°C DB, 19.9°C WB), which lowers the
required cooling capacity by 16.5 tons (57.5 kW). During the winter, the wheel
heats the outdoor air to 40.1°F (4.5°C) DB, which saves 424 MBh (123.4 kW) in
sensible heating capacity. The wheel also humidifies the entering outdoor air,
raising its moisture content from 2 grains/lb to 21 grains/lb (0.0003 kg/kg to
0.003 kg/kg). If the HVAC system includes mechanical humidification, then
operating the total-energy wheel will reduce the required humidification
capacity by 133 MBh (38.2 kW).
Note: We added a preheat coil upstream of the wheel for our Minneapolis
example. The preheat coil warms the entering outdoor air to about 1°F (-17.2°C)
to prevent frost from forming on the exhaust-side surface of the wheel. See
“Frost Prevention,” pp. 18 –19 and pp. 55 – 56, for more information.
Effect on First Cost
The previous examples demonstrate the effects of preconditioning outdoor air
with energy recovered from the exhaust air stream. Although the benefits are
appealing, they are not without cost. Depending on size and complexity, adding
an energy-recovery device can significantly increase the initial investment in the
air handler.
Additional exhaust ductwork. Except for coil loops, air-to-air energyrecovery devices require adjacent air streams. Routing most of the exhaust air
SYS-APM003-EN
13
Precondition
Outdoor Air
Applications for
Air-to-Air Energy Recovery
to a central location so that the path is adjacent to that of the entering outdoor
air may require more ductwork (and space) than an application without energy
recovery. Even if a coil loop is used, minimizing the distance between the air
streams will not only reduce piping- and glycol-related costs but also permit a
smaller pump and expansion tank.
Increased fan motor sizes. Adding an air-to-air energy-recovery device to
precondition outdoor air increases the static pressure drop in both the outdoorand exhaust-air paths, requiring larger fan motors. It may even create the need
for a powered, central exhaust fan where one was not required before. The
configuration of the fans will determine the extent to which their motor sizes —
and energy consumption — will increase.
In a mixed-air system, the impact on the supply fan depends on the pressure
drop through the outdoor-air path versus that of the return-air path. If the static
pressure drop through the return-air path is greater, the supply fan need not
overcome the entire static pressure drop created by the energy-recovery device.
Adding a third (and even a fourth) fan could make it unnecessary to increase
the sizes of the supply and exhaust fan motors. The energy costs of this
approach would be nearly the same as for a two-fan system, although the
installation and maintenance costs would almost certainly be higher.
Heating- and cooling-plant downsizing. Whether the climate is
predominantly cooling (Jacksonville, Florida) or predominantly heating
(Minneapolis, Minnesota), the first-cost savings from even a modest reduction
in HVAC capacity can at least partially subsidize the investment in the energyrecovery device, associated ductwork, and larger fan motors.
The decision to use an air-to-air energy-recovery device for outdoor-air
preconditioning requires careful consideration … and not for solely economic
reasons. Downsizing the HVAC equipment, particularly in packaged directexpansion (DX) applications, avoids other system- and building-related
problems that can result from overcapacity. Providing too much available
cooling can cause compressors to short-cycle and create swings in supply-air
temperature that not only make occupants uncomfortable but also result in
inadequate humidity control. Failure to maintain appropriate humidity levels
can cause microbial growth to flourish inside the building, adversely affecting
occupant health and accelerating the deterioration of structural components
and furnishings.
Table 2 summarizes the plant-downsizing potential provided by the sensibleand total-energy recovery devices in the previous examples. Recall that
unbalanced airflows through an energy-recovery device increase effectiveness
but lessen the amount of heat transferred overall. Table 3 illustrates the
additional first-cost savings made possible by routing the rest-room exhaust
through the energy-recovery device — which reduces the airflow imbalance
from 3,000 cfm to 500 cfm (1.4 m³/s to 0.2 m³/s) — rather than expelling it locally.
The net savings in first cost usually makes it beneficial to route as much
14
SYS-APM003-EN
Precondition
Outdoor Air
Applications for
Air-to-Air Energy Recovery
exhaust air to the energy-recovery device as possible, despite the extra expense
incurred for rest-room-exhaust ductwork and exhaust-fan power.
The economics of using an air-to-air energy recovery device to precondition
outdoor air often hinges on downsizing both plants, cooling and heating. Some
engineers hesitate to reduce the capacity of the heating plant for installations in
cold climates even though the energy-recovery device makes the extra capacity
redundant. (The importance of redundancy depends on the specific
application.) Their reluctance often reflects concern that the failure of the
energy-recovery device, usually a total-energy wheel, will leave the system
without sufficient capacity to heat the building. That concern is unwarranted
because the total-energy wheel only reduces the heating load of the outdoor air
brought into the building for ventilation. If a mechanical failure occurs, the
system operator can choose to temporarily bring less outdoor air into the
building until the wheel is repaired.
This observation raises another important point: An energy-recovery device
that preconditions outdoor air is no less critical than the boiler or the belt on a
supply fan. If the wheel breaks, repair it promptly.
Table 2. Capacity reductions attributable to OA preconditioning (unbalanced airflows)
Capacity Reduction
Due to Recovered Energy1
Jacksonville, Florida
Minneapolis, Minnesota
Sensible
Total
Sensible
Total
Cooling plant
6.5 tons
(23.3 kW)
22.5 tons
(78.4 kW)
4.6 tons
(16.5 kW)
16.5 tons
(57.5 kW)
Heating plant
183.0 MBh
(54.6 kW)
242.0 MBh
(71.7 kW)
245.0 MBh
(70.5 kW)
424.0 MBh
(123.4 kW)
Humidification system
n/a
63.7 MBh
(18.4 kW)
n/a
133.0 MBh
(38.2 kW)
1 Capacity reductions are based on the effectiveness of the energy-recovery device, which results from the unbalanced
airflows of 10,000 cfm (4.7 m³/s) outdoor air and 7,000 cfm (3.3 m³/s) exhaust air.
Table 3. Capacity reductions attributable to OA preconditioning (“balanced” airflows)
Capacity Reduction
Due to Recovered Energy1
Jacksonville, Florida
Minneapolis, Minnesota
Sensible
Total
Sensible
Total
Cooling plant
7.3 tons
(26.3 kW)
25.1 tons
(88.3 kW)
5.2 tons
(18.3 kW)
19.9 tons
(69.7 kW)
Heating plant
187.7 MBh
(56.5 kW)
289.7 MBh
(84.9 kW)
299.5 MBh
(87.9 kW)
517.5 MBh
(151.3 kW)
Humidification system
n/a
78.4 MBh
(23.7 kW)
n/a
161.0 MBh
(46.9 kW)
1 Capacity reductions are based on the effectiveness of the energy-recovery device, which results from the nearly
balanced airflows of 10,000 cfm (4.7 m³/s) outdoor air and 9,500 cfm (4.5 m³/s) exhaust air.
SYS-APM003-EN
15
Precondition
Outdoor Air
Applications for
Air-to-Air Energy Recovery
Effect on Operating Costs
Less energy consumed for heating and cooling. Typically, the “real”
money in using air-to-air energy recovery for outdoor-air preconditioning
accrues from the first-cost “subsidy” rather than lower operating costs. Still, if
the first-cost subsidy from downsizing the cooling and heating plants does not
entirely offset the initial investment in the energy-recovery device, operating
cost savings may quickly make up the difference.
Energy simulation software makes it easier to predict and evaluate how various
energy-recovery devices will perform in a particular application. Such an
analysis can predict the length of the payback period based on the building
location, type of building, type of HVAC system, system control modes, quantity
of outdoor air, airside economizer operation, and local utility rates. Interaction
between different system operating modes and the airside economizer often
requires the use of a whole-building economic analysis (performed with the
help of DOE-2, TRACE™, System Analyzer™, or similar software).
More energy consumed by fans. Whenever air passes through the air-to-air
energy recovery device, the static pressure drop of the system increases and
the fans, in turn, consume more energy. The magnitude of the pressure drop
through the device and the configuration of the fans determine how much
additional energy is consumed. The operating cost savings provided by
recovered energy must exceed the increased cost of operating the fans in order
to justify the cost of the energy-recovery device. A year-round, project-specific
analysis must be performed to determine the overall effect on the energy
consumption of the HVAC system.
Proper airside economizer control. In many climates, the operation of a
mixed-air system’s airside economizer can provide the benefits of “free”
cooling for much of the year. While the economizer operates, air-to-air energy
recovery offers no additional benefit. In fact, unless it is turned off, the energyrecovery device actually increases the cooling load by adding heat to the supply
air stream. Achieving maximum energy savings and minimizing the payback
period for energy recovery therefore depends on proper economizer operation
as well as on proper integration of the energy-recovery device into the system
control modes. (For a detailed discussion of these control modes, see “SystemLevel Control of Recovered Energy,” pp. 65 – 89.)
When the HVAC system includes an airside economizer, size the air-to-air
energy-recovery device to precondition only the minimum outdoor airflow
required for ventilation … not the maximum airflow expected during
economizer operation. (Avoiding oversizing also minimizes the first cost of the
device.)
To accommodate economizer operation when the energy-recovery device is
idle, add bypass dampers (Figure 10) to reduce the airside pressure drop and
16
SYS-APM003-EN
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Outdoor Air
Applications for
Air-to-Air Energy Recovery
Figure 10. Central air handler with economizer and energy recovery
Bypass dampers divert
airflow around the energy wheel
during economizer operation.
minimize the energy consumption of the fans. Alternatively, provide two
separate paths for outdoor air: one for “ventilation” air and the other for
“economizer” air.
Note: Section 6.5.6.1 of ASHRAE Standard 90.1–2007 requires bypass, or some
other control, to permit airside economizer operation when an economizer is
required by Section 6.5.1.1.
Need for capacity control. Although it is often overlooked, capacity control
is a key aspect of designing an HVAC system that uses air-to-air energy recovery
to precondition outdoor air. Regardless of whether sensible or total energy is
recovered, modulating the capacity of the energy-recovery device prevents it
from overheating the supply air when weather conditions are cold but not
extreme. Consider the following example.
Assume that a total-energy wheel preconditions outdoor air in a mixed-air,
variable-air-volume (VAV) system that supplies air at a constant 55°F DB
(12.8°C); see Figure 11. When it is 40°F DB (4.4°C) outside, the system delivers a
Figure 11. Unmodulated energy-recovery capacity for OA preconditioning
SYS-APM003-EN
17
Precondition
Outdoor Air
Applications for
Air-to-Air Energy Recovery
Figure 12. Modulated energy-recovery capacity for OA preconditioning
supply airflow of 18,000 cfm (8.5 m³/s). With the wheel turned off, the resulting
mixed-air condition would be 53.3°F (11.8°C). Activating the wheel transfers
sensible heat (and moisture) from the warm exhaust air to the cool entering
outdoor air, avoiding the use of mechanical heating. However, if allowed to
operate at full capacity, the wheel would heat the entering outdoor air to 64.6°F
(18.1°C) and, after mixing with return air from the space, produce a mixed-air
condition of 67°F (19.4°C). Mechanical or economizer cooling is then necessary
to obtain the supply-air target of 55°F (12.8°C).
Regulating the capacity of the wheel — in this case, by modulating the exhaustside dampers (Figure 12) —can prevent overheating. Operating the wheel at
less than full capacity allows it to warm the entering outdoor air to only 43°F
(6.1°C), which yields 55°F (12.8°C) mixed air without the help of mechanical
heating or cooling.
Note: Capacity control for energy-recovery devices is important during heating
operation, but is seldom required during the cooling mode. Specific control
methods are discussed in “Energy-Recovery Technologies,” pp. 38 – 59.
Other Considerations
Although it is important to optimize life-cycle costs, other issues must be
considered to effectively precondition outdoor air using air-to-air energy
recovery.
Frost prevention. Any air-to-air energy-recovery device that preconditions
outdoor air is subject to frost buildup during very cold weather. That’s because
the moisture that condenses on the exhaust-side of the device (recall Figure 7,
p. 11) freezes, eventually blocking airflow, if the surface temperature of the
device falls below 32°F (0°C). Device effectiveness, entering conditions, and
18
SYS-APM003-EN
Precondition
Outdoor Air
Applications for
Air-to-Air Energy Recovery
Figure 13. Psychrometric comparison of frost thresholds for energy-recovery devices
Saturation (moisture
condensation) occurs
along this curve
airflows determine the outdoor temperature at which frost forms; but the
threshold is much lower for total-energy-recovery devices than for devices that
recover only sensible energy. Figure 13 illustrates why ...
When exhaust air passes through a total-energy-recovery device, the transfer of
moisture to the outdoor air stream (psychrometrically) prevents the leaving
exhaust-air condition from reaching the saturation curve. Sensible-energyrecovery devices do not remove moisture from the exhaust air stream, so the
leaving exhaust-air condition reaches saturation (and frosts) at a warmer
outdoor condition.
Note: Although the latent heat of condensation is slight, it contributes to the
amount of heat that a sensible-energy-recovery device transfers to the outdoor
air stream.
Frost can be avoided in one of two ways:
■
Reduce the heat-transfer capacity of the energy-recovery device. This
practice raises the surface temperature of the device, but limits the extent to
which the heating plant can be downsized.
■
Preheat the outdoor (or exhaust) air before it enters the energy-recovery
device. Permitting full-capacity energy recovery at the coldest outdoor
conditions maximizes the potential downsizing of the heating plant.
“Air-to-Air Energy Recovery Technologies” (pp. 38 – 59) discusses specific frost
prevention methods for various air-to-air energy-recovery devices.
Proximity of outdoor-air intake to building exhaust. Local codes often
impose a minimum separation distance between the outdoor-air intake and the
building exhaust. In applications that route the exhaust air to an indoor air
handler for energy recovery, the outdoor- and exhaust-air ductwork can usually
be arranged to provide the minimum separation distance. Meeting this
requirement poses a greater design challenge in applications with outdoor
air handlers.
Note: Table 5-1 of ASHRAE Standard 62.1-2007 includes minimum separation
distance requirements for the location of an outdoor-air intake. For example,
SYS-APM003-EN
19
Applications for
Air-to-Air Energy Recovery
the standard requires positioning the outdoor-air intake at least 1 ft (0.3 m)
above the ground or roof, and at least 15 ft (5 m) from any source of
significantly-contaminated (Class 3) exhaust.
Acceptable cross-leakage. Many types of air-to-air energy-recovery devices
permit some degree of cross-leakage, although the amount of leakage is
usually less than 5 percent (even for wheels) in most applications. (“EnergyRecovery Technologies,” pp. 38 – 59, discusses specific methods of reducing
cross-leakage for various types of air-to-air energy-recovery devices.)
Cross-leakage between the exhaust- and supply-air streams is seldom
problematic in mixed-air systems: most of the air that returns from the space is
recirculated as supply air anyway. Local codes that permit the recirculation of
exhaust air from rest rooms usually require sufficient dilution with fresh
outdoor air.
Note: Section 5.17 of ASHRAE Standard 62.1-2007 limits the amount of crossleakage for different classes of air. As an example, the standard limits the crossleakage of exhaust air from restrooms (Class 2) to no more than 10 percent of
the resulting supply airflow (that is, EATR cannot be greater than 10 percent;
see "Exhaust Air Transfer Ratio," page 62).
Figure 14. Supply-air (SA) tempering
Supply-Air Tempering
Supply-air tempering adds sensible heat to the air downstream of the cooling
coil so that the dry-bulb temperature and humidity of the conditioned space can
be controlled independently.
For a more detailed discussion of the
use of supply-air tempering to control
space humidity, refer to the Engineers
Newsletter titled “Dehumidify with
Constant-Volume Systems” (ENEWS29/4). You can find it at www.trane.com/
engineersnewletters.
The Trane applications engineering
manual, Dehumidification in HVAC
Systems (SYS-APM004-EN), also
addresses this subject. It is available
from the Trane bookstore at
www.trane.com/bookstore. ■
20
In the simple schematic depicted in Figure 14, the cooling coil dehumidifies the
supply air, lowering the dew point sufficiently to maintain the desired space
humidity. If the dehumidified supply air is so cold that it would overcool the
space, then it must be reheated (or “tempered”) before delivery to prevent the
dry-bulb temperature in the space from falling below the setpoint.
Only systems that directly control the space humidity or supply-air dew point
require supply-air tempering. Common applications include:
■
Constant-volume, mixed-air systems that directly control space humidity
■
Dedicated (100-percent) outdoor-air systems that deliver dry (low dew-point)
air at a “space-neutral” dry-bulb temperature
Supply-air tempering can be accomplished using either new energy —
electricity, hot water, steam or gas, for example — or recovered energy from
another part of the system. Moisture transfer is seldom desirable when
tempering supply air, so energy recovery is typically limited to sensible heat.
SYS-APM003-EN
Temper
Supply Air
Applications for
Air-to-Air Energy Recovery
Note: To comply with ASHRAE Standard 90.1–2007 (Section 6.5.2) and related
energy codes, site-recovered energy may be required for applications that
include supply-air tempering. Standard 90.1 (see p. 4) limits the use of reheat in
a previously cooled air stream unless at least 75 percent of the reheat energy is
recovered.
The rest of this section examines the effect of using air-to-air heat exchangers to
temper the supply air in a constant-volume mixed-air system and in a dedicated
outdoor-air system.
Supply-Air Tempering in Mixed-Air Systems
Table 4. Design parameters for examples
of SA tempering in a mixed-air system
(Figures 15–20)
Cooling
(ASHRAE 0.4%)
Sensible
Outdoor
conditions1
Indoor
setpoints
Airflows
96°F DB (35.7°C)
76°F WB (24.5°C)
Latent
84°F DB (28.8°C)
76°F DP (24.6°C)
Dry-bulb
temperature
74°F DB (23.3°C)
Relative
humidity
(max limit)
60% 2
Supply
1,500 cfm (0.7 m³/s)
Outdoor
450 cfm (0.2 m³/s)
1 Cooling design conditions represent the peak dry-
bulb temperature and mean coincident wet-bulb
temperature for sensible cooling, and the peak dew
point and mean coincident dry-bulb temperature for
latent cooling, in Jacksonville, Florida.
To demonstrate the potential savings of tempering supply air with recovered
energy, let’s examine the mixed-air HVAC system designed for a year-round
school in Jacksonville, Florida. See Figure 15. The system consists of a
constant-volume air handler that delivers 1,500 cfm (0.7 m³/s) of supply air to a
single classroom. Outdoor air for ventilation accounts for 450 cfm (0.2 m³/s) of
the supply air.
At the ASHRAE 0.4-percent “sensible-design” condition — that is, peak drybulb temperature of 96°F and mean coincident wet-bulb temperature of 76°F
(35.7°C DB, 24.5°C WB) — the system delivers 55.7°F (13.2°C) supply air to offset
the space sensible load. Table 4 summarizes the thermal conditions and design
airflows for this scenario.
At part-load conditions, the constant-volume system responds to the
decreasing space sensible load by supplying warmer air to the space. When the
relative humidity in the space exceeds the 60-percent limit, modulation of the
cooling coil’s capacity maintains the space relative humidity below the
maximum limit, while modulation of the reheat coil’s capacity maintains the
dry-bulb temperature at setpoint.
2 The maximum limit for indoor humidity control used in
this example is 60 percent.
SYS-APM003-EN
Figure 15. SA tempering in a constant-volume, mixed-air HVAC system
21
Temper
Supply Air
Applications for
Air-to-Air Energy Recovery
Figure 16. Mixed-air HVAC System with SA tempering
Note: A common alternative to modulating the capacity of the cooling coil is to
operate it at maximum capacity when dehumidifying. This control strategy
quickly lowers space humidity, and then returns the system to the normal
cooling mode.
Tempering energy needed at latent design:
Q s = 1.085  1 500 cfm   63°F – 59°F 
Q s = 6 510 Btu/hr
 Q s = 1.21  0.7 m³/s   17.2°C – 15°C  
 Q s = 1.86 kW 
Figure 16 plots the psychrometric conditions of the outdoor air, supply air, and
return air at the ASHRAE 0.4-percent design conditions. For Jacksonville, the
“latent-design” condition is a peak dew point of 76°F and a mean coincident
dry-bulb temperature of 84°F (24.6°C DP, 28.8°C DB). The sensible load in the
space is lower at this condition than it is at the sensible-design condition.
Therefore, the temperature of the supply air must be warmer — 63°F (17.2°C) in
this example — to match the sensible cooling capacity with the sensible load in
the space and avoid overcooling. To accomplish this, the cooling coil first
dehumidifies the supply air to 59°F (15°C) in order to maintain a relative
humidity of 60 percent in the space. The reheat coil then raises the dry-bulb
temperature of the supply air to 63°F (17.2°C). Without the use of energy
recovery, tempering the supply air requires 6,510 Btu/hr (1.86 kW) of new
energy.
Air-to-air recovery for supply-air tempering can be accomplished using
either of two configurations: series or parallel. The next sections compare their
respective performance.
Series Configuration
Figure 17 shows an air-to-air, sensible-energy-recovery device applied in a
series, or “wraparound,” configuration within a constant-volume, mixed-air
system. (Although the example depicts a coil loop, any sensible-energyrecovery device could be used.) The device removes heat from the air upstream
of the cooling coil and releases it in the supply air downstream of the coil.
22
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Temper
Supply Air
Applications for
Air-to-Air Energy Recovery
Figure 17. SA tempering: “Series” transfer in a constant-volume, mixed-air system
Although it is described as energy “recovery,” this arrangement does not
“recover” heat; instead, it transfers heat from one location to another within the
same air stream.
At the part-load, latent-design condition for our example classroom, mixed air
enters the upstream coil of the coil loop at 77°F DB and 69°F WB (25°C DB,
20.6°C WB). The coil loop removes sensible heat from the mixed air, cooling it
to 73.2°F DB (22.9°C DB), and then transfers that heat to the other side of the
coil loop, downstream of the cooling coil. As shown in Figure 18, the cooling
coil must dehumidify the air to 59°F (15°C) in order to keep the relative humidity
in the space below the 60-percent limit. The downstream coil of the coil loop
then warms the supply air to the desired dry-bulb temperature of 63°F (17.2°C).
Using air-to-air energy recovery not only saves the 6,510 Btu/hr (1.86 kW) of
heating energy needed for tempering, but also reduces the load on the cooling
Figure 18. SA tempering: “Series” transfer in a constant-volume, mixed-air system
SYS-APM003-EN
23
Temper
Supply Air
Applications for
Air-to-Air Energy Recovery
Cooling-energy savings at latent design:
coil by precooling the mixed air before it enters the cooling coil. Reducing the
enthalpy of the mixed air to 32.2 Btu/lb (74.9 kJ/kg) saves 6,075 Btu/hr (1.76 kW)
of cooling energy.
Q T = 4.5  1 500 cfm   33.1 – 32.2 Btu/lb 
Q T = 6 075 Btu/hr
 Q T = 1.2  0.7 m³/s   77.0 – 74.9 kJ/kg  
 Q T = 1.76 kW 
Notice that in a mixed-air system, the use of an air-to-air energy-recovery device
in a series configuration does not reduce the installed capacity of the cooling
plant. At the sensible-design condition, the 55.7°F (13.2°C) air that is supplied to
the space does not require tempering. Without heat transfer, no precooling
occurs. Therefore, the cooling coil and cooling plant must be sized to handle the
entire cooling load at the sensible-design condition.
Parallel Configuration
Figure 19 shows the same constant-volume, mixed-air system … but this time
the air-to-air, sensible-energy-recovery device is applied in a parallel
configuration. (Again, although this example depicts a coil loop, any sensibleenergy-recovery device can be used.) The device recovers sensible heat from
the return air stream and releases it in the supply air downstream of the cooling
coil, raising its temperature.
At the part-load, latent-design condition, air returns from the space and
enters the return-side coil of the coil loop at 74°F DB and 64.5°F WB (23.3°C DB,
18.1°C WB). The coil loop removes sensible heat from the return air stream,
cooling it to 70.1°F DB (21.2°C DB), and then transfers this heat to the supplyside coil of the coil loop, downstream of the cooling coil. See Figure 20. At this
condition, the air leaving the cooling coil must be 59°F (15°C) to maintain the
relative humidity in the space below the 60-percent limit. The supply-side coil of
the coil loop then heats the supply air to the desired dry-bulb temperature of
63°F (17.2°C).
Like the series configuration, the parallel configuration also reduces the load
on the cooling coil while saving the 6,510 Btu/hr (1.86 kW) of heating energy
Figure 19. SA tempering: “Parallel” recovery in a constant-volume, mixed-air system
24
SYS-APM003-EN
Temper
Supply Air
Applications for
Air-to-Air Energy Recovery
Figure 20. SA tempering: “Parallel” recovery in a constant-volume, mixed-air system
Cooling-energy savings at latent design:
Q T = 4.5  1 500 cfm   33.1 – 32.5 Btu/lb 
Q T = 4 050 Btu/hr
 Q T = 1.2  0.7 m³/s   77.0 – 75.6 kJ/kg  
 Q T = 1.18 kW 
needed for tempering. By removing sensible heat from the return air, the
coil loop effectively lowers the enthalpy of the mixed air from 33.1 Btu/lb to
32.5 Btu/lb (77.0 kJ/kg to 75.6 kJ/kg). The resulting savings is 4,050 Btu/hr
(1.18 kW) of cooling energy.
Applying an air-to-air energy recovery device in a parallel (rather than series)
arrangement within a mixed-air system still does not reduce the installed
capacity of the cooling plant. The supply air does not require tempering at the
sensible-design condition, so no precooling occurs. Both the cooling coil and
the cooling plant must therefore be sized to handle the entire cooling load at the
sensible-design condition.
Series versus Parallel
In mixed-air systems, both series and parallel configurations reduce the heating
energy required for tempering. Although the series configuration saves more
cooling energy than the parallel configuration, neither permits downsizing of
the cooling or heating plant.
With the series configuration, outdoor conditions largely determine the
condition of the mixed air that enters the energy-recovery device. As a result,
the amount of heat available for transfer varies, but can be larger than the
constant quantity of heat that the parallel configuration recovers from the
return air stream. In other words, the series configuration benefits when the
outdoor air is warmer than the return air; the parallel configuration benefits
when the outdoor air is cooler than the return air.
Three criteria ultimately determine which energy-recovery arrangement is the
right choice for a given project …
SYS-APM003-EN
25
Temper
Supply Air
Applications for
Air-to-Air Energy Recovery
■
First cost
■
Savings in cooling and heating energy
■
Additional fan energy to overcome the static pressure drop of the energyrecovery device
Supply-Air Tempering in Dedicated Outdoor-Air Systems
Dedicated outdoor-air systems should
dehumidify the outdoor air to a low dew
point, typically 45°F to 55°F DP (7.2°C
to 12.8°C). This allows the dedicated
outdoor-air system to offset the space
latent loads as well as the ventilation
load. Dehumidifying to a low dew point
provides two benefits:
■
■
Smaller space loads, which permits
downsizing of the constant-volume
terminal units
Virtually no condensation in the
space 
(The cooling coil in each terminal
operates dry because it handles only
the sensible load.)
The Trane applications engineering
manual, Dehumidification in HVAC
Systems (SYS-APM004-EN), addresses
dedicated outdoor-air systems in detail.
It is available from the Trane bookstore
at www.trane.com/bookstore. ■
Dedicated outdoor-air systems that are designed to deliver dry (low dew-point),
“neutral-temperature,” conditioned air also need tempering and can benefit by
using air-to-air sensible-energy recovery to provide it. (An alternative approach
is to deliver the dehumidified outdoor air cold — that is, not reheated to neutral,
see pp.65-66.) Systems of this type typically deliver the conditioned outdoor air
to ceiling plenums (when used with fan-coils, for example) or directly to
occupied spaces (as in water-source heat pump applications, for example).
Sensible-energy recovery may decrease the first cost and operating costs of the
system when compared to using new energy for tempering. It may also enable
compliance with energy standards and local codes.
To demonstrate the potential for energy savings, let’s revisit the example
classroom in Jacksonville, Florida. This time, a dedicated outdoor-air handler
(or makeup-air unit) delivers conditioned outdoor air, CA, directly to several
classrooms at a “neutral,” dry condition of 71°F DB, 53°F DP (21.7°C DB, 11.7°C
DP). It conditions 450 cfm (0.2 m³/s) of outdoor air for each classroom. The
capacity of the cooling coil modulates to maintain the conditioned-air dew point
at setpoint; similarly, the capacity of the reheat coil modulates to maintain the
conditioned-air dry-bulb temperature at setpoint. Separate, constant-volume
terminals, one in each classroom, handle the space load. See Figure 21.
Note: An alternative to maintaining a fixed dew point is to reset the
conditioned-air dew point to maintain the humidity in the “worst-case” space
below the maximum limit. In other words, the capacity of the cooling coil
modulates to make the conditioned outdoor air dry enough to keep the most
humid room below 60-percent relative humidity.
For Jacksonville, the ASHRAE 0.4-percent peak-enthalpy design condition —that
is, 42.4 Btu/lb (98.6 kJ/kg) — represents a peak wet-bulb temperature of 79°F
(26.2°C WB) and a mean coincident dry-bulb temperature of 91°F (32.6°C DB). In
a dedicated outdoor-air system that maintains a constant conditioned-air dew
point, this is the condition at which the cooling-coil load is the highest because
the enthalpy of the outdoor air is the highest.
26
SYS-APM003-EN
Temper
Supply Air
Applications for
Air-to-Air Energy Recovery
Figure 21. SA tempering in a dedicated outdoor-air system
Energy required for OA dehumidification:
Q T = 4.5  450 cfm   42.4 – 22.1 Btu/lb 
Q T = 41 108 Btu/hr
 Q T = 1.2  0.2 m³/s   98.6 – 51.4 kJ/kg  
 Q T = 11.33 kW 
The cooling coil dehumidifies the outdoor air to 53.5°F DB, 53°F DP (11.9°C DB,
11.7°C DP), which corresponds to an enthalpy of 22.1 Btu/lb (51.4 kJ/kg). See
Figure 22. The reheat coil then warms this air to the “neutral” dry-bulb
temperature of 71°F (21.7°C). Without the use of energy recovery, the system
would consume 41,108 Btu/hr (11.33 kW) of new energy for dehumidification
and 8,544 Btu/hr (2.37 kW) for tempering.
Energy required for tempering:
Q s = 1.085  450 cfm   71.0°F – 53.5°F 
Q s = 8 544 Btu/hr
 Q s = 1.21  0.2 m³/s   21.7°C – 11.9°C  
 Q s = 2.37 kW 
Now, let’s examine the impact of using an air-to-air sensible-energy-
recovery device to provide the heat for tempering. As for constant-volume,
mixed-air systems, sensible-energy recovery can be applied in either of two
configurations to temper the conditioned air in a dedicated outdoor-air system:
series and parallel.
Figure 22. Dedicated outdoor-air system with SA tempering
SYS-APM003-EN
27
Temper
Supply Air
Applications for
Air-to-Air Energy Recovery
Series Configuration
Figure 23 shows an air-to-air, sensible-energy-recovery device that is applied in
a series, or “wraparound,” configuration within a dedicated outdoor-air system.
(Although the example depicts a fixed-plate heat exchanger, any sensibleenergy-recovery device could be used.) The device removes sensible heat from
the air upstream of the cooling coil and releases it in the air downstream of that
coil. Rather than “recover” heat, the series arrangement transfers heat from
one location to another within the same air stream.
At the peak-enthalpy condition for our example classroom, the upstream side of
the fixed-plate heat exchanger removes heat from the entering outdoor air,
cooling it to 78.4°F DB, 74.6°F WB (25.8°C DB, 23.7°C WB). Because the air
passing through the heat exchanger becomes saturated (Figure 24), that device
removes both sensible heat and moisture from the outdoor air stream. The
latent heat of condensation that is generated as moisture condenses from the
air adds to the overall amount of heat transferred; however, only sensible
heat —not moisture — moves to the other side of the heat exchanger.
The cooling coil dehumidifies the air to a dew point of 53°F (11.7°C) and all of
the heat (sensible and latent) removed from the outdoor air upstream of the
cooling coil is transferred (as sensible heat only) to temper the air downstream
of that coil. The resulting dry-bulb temperature of the supply air is 71°F (21.7°C).
Using the air-to-air energy-recovery device as described here not only saves
the 8,544 Btu/hr (2.37 kW) of heating energy needed for tempering, but also
decreases the load on the cooling coil by precooling the outdoor air upstream
Figure 23. SA tempering: “Series” transfer in a dedicated outdoor-air system
28
SYS-APM003-EN
Temper
Supply Air
Applications for
Air-to-Air Energy Recovery
Figure 24. SA tempering: “Series” transfer in a dedicated outdoor-air system
Cooling-energy savings:
Q T = 4.5  450 cfm   42.4 – 38.2 Btu/lb 
Q T = 8 544 Btu/hr
 Q T = 1.2  0.2 m³/s   98.6 – 88.9 kJ/kg  
 Q T = 2.33 kW 
of the coil. Reducing the enthalpy of the outdoor air to 38.2 Btu/lb (88.9 kJ/kg)
saves 8,544 Btu/hr (2.33 kW) of cooling energy.
Unlike its application in a mixed-air system, series transfer of sensible energy in
a dedicated outdoor-air system enables a reduction in the installed capacity of
the cooling plant. By precooling the outdoor air, the energy-recovery device
lessens the load on the cooling coil — in this case, by 8,544 Btu/hr (2.33 kW). Of
course, delivering the air at a “neutral” condition means that tempering is
required whenever the cooling coil is dehumidifying, even at design conditions.
When it is cold outside and the dedicated outdoor-air system must heat the
entering outdoor air, the cooling coil is turned off. At such times, a seriesconfigured, air-to-air energy-recovery device provides no benefit.
Parallel Configuration
Figure 25, p. 30, shows the same dedicated outdoor-air system … but this
time the air-to-air sensible-energy-recovery device is applied in a parallel
configuration. (Again, although a fixed-plate heat exchanger is shown in this
example, any sensible-energy-recovery device can be used.) The device
recovers sensible heat from the exhaust air stream and releases it downstream
of the cooling coil to temper the conditioned outdoor air.
At the peak-enthalpy condition for this same classroom example, the
outdoor air is cooled and dehumidified to 53°F DP (11.7°C DP) by the cooling
coil. The fixed-plate heat exchanger recovers heat from the exhaust air stream
and uses it to temper the air downstream of the cooling coil, warming it to
66.9°F DB (19.4°C DB). See Figure 26 on p. 30. The supplemental reheat coil
further warms the conditioned air to the desired 71°F DB (21.7°C DB).
SYS-APM003-EN
29
Temper
Supply Air
Applications for
Air-to-Air Energy Recovery
Figure 25. SA tempering: “Parallel” recovery in a dedicated outdoor-air system
Reheat-energy savings:
Q s = 1.085  450 cfm   66.9°F – 53.5°F 
Q s = 6 543 Btu/hr
 Q s = 1.21  0.2 m³/s   19.4°C – 11.9°C  
 Q s = 1.82 kW 
The air-to-air energy recovery device in the parallel configuration only saves
6,543 Btu/hr (1.82 kW) of heating energy needed for tempering at this condition.
This is because the temperature of the air entering the warm side of the heat
exchanger is cooler (78°F versus 91°F [25.6°C versus 32.6°C]) than in the series
configuration.
Also, unlike the series configuration, the parallel configuration neither reduces
the load on the cooling coil, nor permits a smaller cooling plant. Why? Because
the heat that is recovered from the exhaust air stream was going to be rejected
from the building anyway. Cooling the exhaust air stream does not reduce the
load on the cooling coil.
The parallel configuration differs from the series configuration in another
important way. When it is cold outside and the dedicated outdoor-air system
Figure 26. SA tempering: “Parallel” recovery in a dedicated outdoor-air system
30
SYS-APM003-EN
Temper
Supply Air
Applications for
Air-to-Air Energy Recovery
Figure 27. SA tempering: “Parallel” recovery in a dedicated outdoor-air system
Heating-energy savings:
Q s = 1.085  450 cfm   52.3°F – 31°F 
Q s = 10 400 Btu/hr
 Q s = 1.21  0.2 m³/s   11.3°C –  – 0.8 °C   
 Q s = 2.93 kW 
must heat the entering outdoor air, the parallel configuration lets the energyrecovery device use heat from the exhaust air stream to warm the entering
outdoor air. To demonstrate, consider the winter design condition, which is
31°F DB (-0.8°C DB), for the example classroom in Jacksonville. The heat
exchanger removes sensible heat from the 70°F (21.1°C) exhaust air and uses it
to warm the entering outdoor air to 52.3°F (11.3°C); see Figure 27. The main
heating coil further heats the air to the desired conditioned-air temperature. In
this scenario, the parallel configuration saves 10,400 Btu/hr (2.93 kW) of heating
energy, cutting operating costs and allowing the heating plant to be downsized.
Applying an air-to-air energy-recovery device in a parallel configuration lets it
temper supply air and precondition entering outdoor air, although it cannot
perform both functions simultaneously. The device tempers the conditioned
outdoor air whenever dehumidification is required and preconditions the
outdoor air during the heating season.
Note: In cold climates, some method of frost prevention may be required. Also,
if a chilled-water cooling coil is used, some method to protect that coil from
freezing may be required because the energy-recovery device adds heat
downstream of the cooling coil.
Series versus Parallel
In dedicated outdoor-air systems that deliver neutral-temperature air, both
series and parallel configurations reduce the new energy that is required for
tempering. When it is hot outside, the series configuration can transfer more
heat than the parallel arrangement. However, because it is less dependent on
outside conditions — and because the return-air condition is relatively stable —
the parallel configuration can recover a nearly constant amount heat.
SYS-APM003-EN
31
Temper
Supply Air
Applications for
Air-to-Air Energy Recovery
The series configuration saves cooling energy, even allowing the cooling plant
to be downsized. By contrast, the parallel configuration can be used in the
heating mode, which saves heating energy and allows the heating plant to be
downsized.
Because it warms the ventilation
(outdoor) air, a dedicated OA system
that delivers neutral air requires more
cooling-plant capacity than a
comparable system that supplies cold
air. Although series-configured energy
recovery in a neutral conditioned-air
system permits the cooling plant to be
downsized with respect to a system with
no heat recovery, the cooling plant will
still be larger than for a dedicated
outdoor-air system that supplies cold
air. ■
The parallel configuration requires that most of the building exhaust be routed
back to a central location, while the series configuration does not require this
additional expense. This additional expense drives most applications to use the
series configuration, unless the system includes an additional energy-recovery
device to perform outdoor-air preconditioning. An example of this “dualfunction” energy-recovery system is discussed on pp. 84 – 88.
As in mixed-air applications, three criteria ultimately determine which energyrecovery arrangement — series or parallel — is the right choice for a given
project:
■
First cost
■
Savings in cooling and heating energy
■
Additional fan energy to overcome the static pressure drop of the energyrecovery device
Note: One innovative design combines both configurations using a coil loop
with three coils. When the cooling coil is active, the coil loop operates in the
series configuration to save both cooling energy and reheat energy. When the
cooling coil is off and the heating coil is active, the coil loop switches and
operates in the parallel configuration to save heating energy. For further
discussion of this concept, see pp. 42 – 43.
Effect on First Cost
The previous examples demonstrate the effects of tempering supply air with
energy recovered from the exhaust air stream or from upstream of the cooling
coil. Energy savings must be balanced with the cost of adding the energyrecovery device to the air-handling system. Depending on the size and
complexity of the system, the initial investment may be significantly higher.
Heating and cooling-plant downsizing. Table 5 summarizes the plantcapacity reductions obtained in the preceding examples of supply-air
tempering. When used to temper supply air in a mixed-air system, air-to-air
energy recovery does not reduce the capacity of either the cooling plant or the
heating plant.
Using air-to-air energy recovery may permit a smaller cooling or heating plant
in dedicated outdoor-air systems that deliver neutral air. If applied in the series
configuration, the cooling plant can be downsized because the energy-recovery
device precools the air upstream of the cooling coil whenever the cooling coil is
dehumidifying — even at design conditions. If the air-to-air energy-recovery
32
SYS-APM003-EN
Temper
Supply Air
Applications for
Air-to-Air Energy Recovery
Table 5. Capacity reductions attributable to supply-air tempering
Energy-Recovery Configuration
Reduction in Mechanical-Plant Capacity
Series
Parallel
Mixed-air system
Cooling


Heating


Cooling
8,544 Btu/hr (2.33 kW)

Heating

10,400 Btu/hr (2.93 kW)
Dedicated OA system
(neutral air)
device is applied in a parallel configuration, the heating plant can be downsized.
During the heating mode, the energy-recovery device recovers sensible heat
from the exhaust air stream and uses it to warm the entering outdoor air.
Of course, when investigating the economic benefit of air-to-air energy recovery
for tempering supply air, you must also consider its impact on the first cost of
other system components.
Increased fan motor sizes. As noted earlier in this chapter, adding an air-toair energy-recovery device to the air stream increases the static pressure loss in
the air-distribution system. The configuration of the energy-recovery device and
the fans will determine the extent of the pressure loss and the resulting fanpower requirements. Providing the additional power necessary may require
larger fan motors.
Additional exhaust ductwork. If applied in a parallel configuration to
temper conditioned outdoor air, most energy-recovery devices used in
dedicated outdoor-air systems require adjacent outdoor- and exhaust-air
streams. This prerequisite means that most of the exhaust air must be routed
back to a central location, and, therefore, typically requires more ductwork than
the same system without energy recovery.
This generalization does not apply to coil loops. A coil loop can recover heat
from one or several exhaust air streams, none of which is adjacent to each other
nor to the outdoor air stream. Of course, when the two air streams are far apart,
the coil loop requires more piping and glycol as well as a larger pump and
expansion tank.
Effect on Operating Costs
Less energy consumed for heating and cooling. Using air-to-air sensibleenergy recovery can reduce energy consumption associated with supply-air
tempering. Given the complex interaction between system control modes and
the humidity in the space, simulation software is often required to estimate the
performance of various energy-recovery devices. Such an analysis can predict
the length of the payback period based on the building location, type of
SYS-APM003-EN
33
Temper
Supply Air
Applications for
Air-to-Air Energy Recovery
building, type of HVAC system, system control modes, quantity of outdoor air,
airside economizer operation, and local utility rates.
Need for supplemental heat. Applications that temper supply air using
recovered energy may require a supplemental heating coil downstream of the
energy-recovery device to achieve the desired dry-bulb temperature at all
conditions. The example in Figure 25 (p. 30) illustrates one such application: a
dedicated outdoor-air system with a parallel-configured energy-recovery
device. The same example system, using the series configuration, would
probably also need a supplemental heating coil to maintain the desired drybulb temperature at less severe outdoor conditions such as a mild, rainy day.
Let’s revisit another example — this time, the mixed-air system that applies a
coil loop in a series configuration (Figure 17, p. 23). As depicted in Figure 28,
the condition of the outdoor air is 69°F DB, 65°C WB (20.6°C DB, 18.3°C WB). To
satisfy the sensible load in the space and maintain the relative humidity below
60 percent, the supply air must be delivered at a dew point of 57.5°F (14.2°C).
Furthermore, the dry-bulb temperature can be no colder than 65°F (18.3°C) to
avoid overcooling the space.
Operating at full capacity, the coil loop transfers heat from the mixed air
upstream of the cooling coil to the air downstream of the coil, warming it to
62.8°F DB (17.1°C DB). Despite operating at full capacity, the coil loop cannot
transfer enough heat to achieve the desired supply-air temperature of 65°F DB
(18.3°C DB). To make up the shortfall, a supplemental heating coil is required
downstream of the coil loop. Most likely, this coil will also operate during the
heating mode when a heating load exists and the coil loop is turned off.
Recall that in mixed-air systems, the amount of heat that can be transferred
from the return air stream is more constant for parallel configurations of energy
recovery than it is for series configurations. Recall, too, that the parallel
configuration is less dependent on ambient conditions. When the outdoor air is
warmer than the return air, the series configuration has more heat to transfer
Figure 28. Mixed-air system with “series” heat transfer and supplemental heat
34
SYS-APM003-EN
Temper
Supply Air
Applications for
Air-to-Air Energy Recovery
than the parallel configuration. When the outdoor air is cooler than the return
air, the series configuration has less heat to transfer than the parallel
configuration.
The relationship between ambient conditions, energy-recovery configuration,
and available heat for transfer also exists in dedicated outdoor-air systems. A
device in the series configuration can transfer more heat when it is hot
outside — but less heat when the outdoor air is cooler than the exhaust air —
than a parallel-configured device. The parallel configuration is less dependent
on outdoor conditions because the temperature of the exhaust air stream
remains relatively constant.
More energy consumed by fans. As noted earlier, the static pressure drop
of the system will increase — causing the fans to consume more energy —
whenever air passes through the air-to-air energy-recovery device. The amount
of additional energy consumed depends on the static pressure drop through
the device. The operating cost savings provided by recovered energy must
exceed the increased cost of operating the fans in order to justify the
operational benefit of the energy-recovery device. A year-round, project-specific
analysis must be performed to determine the effect on the energy consumption
of the entire HVAC system.
Bypass dampers may be used to reduce the airside pressure loss when the
energy-recovery device is idle, thus reducing fan energy consumption. They are
particularly valuable when the energy-recovery device is applied in a series
configuration. Whenever the cooling coil is turned off, the bypass dampers can
save substantial fan energy.
Need for capacity control. An air-to-air energy-recovery device used for
supply-air tempering generally requires some method for modulating the
capacity of the device. Modulation prevents the device from overheating the
supply air during mild weather.
To illustrate, consider the mixed-air system represented psychrometrically in
Figure 29 on p. 36. The condition of the outdoor air is 90°F DB, 78°C WB (32.2°C
DB, 25.6°C WB). To satisfy the sensible load without overcooling the space, the
supply air must be delivered at a dry-bulb temperature of 61°F (16.1°C).
Operating at full capacity, the “series” coil loop cools the mixed air to 74.4°F DB
(23.6°C DB). The heat removed from the mixed air upstream of the cooling coil
is transferred to the air downstream of the coil, warming the supply air to 63.5°F
DB (17.5°C DB). Letting the coil loop operate at full capacity (run “wild”)
overheats the supply air, causing the space to be warmer than desired.
Modulating the capacity of the coil loop avoids this situation.
Note: Tempering is not needed when the relative humidity in the space is lower
than the maximum limit. To prevent heat transfer, the coil-loop pump should be
turned off.
SYS-APM003-EN
35
Temper
Supply Air
Applications for
Air-to-Air Energy Recovery
Figure 29. Unmodulated energy-recovery capacity for SA tempering (mixed-air system)
A mixed-air system that tempers supply air to control space humidity generally
requires very little reheat when it is warm outside and lots of reheat when the
weather is cool and damp (that is, at part load). The capacity of the energyrecovery device, whether arranged in a series or parallel configuration, must be
modulated to avoid transferring too much heat.
The Trane application manual,
Dehumidification in HVAC Systems
(SYS-APM004-EN), discusses dedicated
outdoor-air systems in detail, including
resetting leaving air conditions to
optimize system energy consumption. It
is available from the Trane bookstore at
www.trane.com/bookstore. ■
In a dedicated outdoor-air system that delivers “neutral” air at constant dewpoint and dry-bulb temperatures, the condition of the air leaving the cooling
coil will be the same whenever the coil operates. Because this dehumidified
outdoor air is then reheated to the same dry-bulb temperature, the required
amount of tempering (reheat) is also constant whenever the energy-recovery
device operates. In the parallel configuration, the source of heat to be recovered
(the exhaust air stream) is relatively consistent, and capacity control for the
energy-recovery device may not be required. In the series configuration,
however, the source of heat to be transferred (the outdoor air stream) definitely
fluctuates. Therefore, some method of controlling the capacity of the energyrecovery device will probably be needed to avoid overheating the conditioned
outdoor air at certain operating conditions.
Further, if the setpoints (either dew point, dry bulb, or both) of the air leaving
the dedicated outdoor-air unit are automatically reset based on actual space
conditions, a method of capacity control is required to optimize the energy use
of the HVAC system. Capacity control is needed whether the energy-recovery
device is arranged in the parallel or series configuration.
“Energy-Recovery Technologies,” pp. 38 – 59, discusses specific capacity control
methods for various air-to-air energy-recovery devices.
36
SYS-APM003-EN
Temper
Supply Air
Applications for
Air-to-Air Energy Recovery
Other Considerations
Several other issues must be addressed to optimize the life-cycle cost of a
mixed-air or dedicated outdoor-air system that tempers supply air using air-toair energy recovery.
Frost prevention. Although frost prevention is typically required for outdoorair preconditioning applications, it is seldom necessary in supply-air tempering
applications.
As noted earlier, when a parallel-configured energy-recovery device is applied
in a dedicated outdoor-air system, frost prevention may be required in cold
climates if the energy-recovery device is also used to precondition the outdoor
air during the heating season.
“Air-to-Air Energy Recovery Technologies” (pp. 38 – 59) discusses specific frost
prevention methods for various air-to-air energy-recovery devices.
Acceptable cross-leakage. Many types of air-to-air energy-recovery devices
permit some degree of cross-leakage. This is seldom of concern in mixed-air
systems because most of the air that returns from the space is recirculated as
supply air. Cross-leakage is also a non-issue when the series configuration is
used because both sides of the energy-recovery device are located in the same
air stream.
Only dedicated outdoor-air systems that apply air-to-air energy recovery in a
parallel configuration require special attention. Leakage between the two air
streams, however slight, may transfer exhaust-air contaminants to the
conditioned outdoor air stream. (“Energy-Recovery Technologies,” pp. 38 – 59,
discusses specific methods of reducing cross-leakage for various types of air-toair energy-recovery devices.) ■
SYS-APM003-EN
37

Energy-Recovery Technologies
Additional information on these and
other air-to-air, energy-recovery
technologies can be found in the “Air-toAir Energy Recovery” chapter of the
ASHRAE Handbook—HVAC Systems
and Equipment. To order a copy, visit
ASHRAE’s online bookstore at
www.ashrae.org. ■
Of the methods for recovering airside energy, coil loops, fixed-plate heat
exchangers, heat pipes, and rotary heat exchangers (or wheels) are the most
commonly used. As summarized in Table 6, coil loops and heat pipes recover
sensible energy; fixed-plate heat exchangers and wheels can recover either
sensible or total energy. This chapter reviews the typical performance,
advantages and disadvantages, methods of capacity control and frost
prevention, and cross-leakage characteristics for each of these airside energyrecovery technologies. See Table 14 (p. 64) for a comparative overview.
Table 6. Common air-to-air energy-recovery technologies
Sensible-Energy Recovery
For a concise comparison of the energyrecovery technologies discussed in this
chapter, see Table 14 on p. 64. ■
Total-Energy Recovery
■
Coil loops
■
Heat pipes
■
Fixed-plate heat exchangers
■
Fixed-membrane heat exchangers
■
Rotary heat exchangers
(also known as “sensible-energy wheels” or
“heat wheels”)
■
Rotary heat exchangers
(also known as “total-energy wheels” or
“enthalpy wheels”)
Coil Loop
Figure 30. Coil loop
A coil loop — also called a “coil runaround loop — consists of two or more
finned-tube coils that are piped together in a closed loop; see Figure 30. A small
pump circulates the working fluid (usually a solution of inhibited glycol and
water) through the two coils. An expansion tank and a means for modulating
capacity, either a three-way mixing valve or a variable-speed drive on the
pump, complete this energy-recovery device.
Coil loops are the most flexible of all air-to-air energy-recovery devices because
they can transfer energy between air streams that are physically separated by
some distance, which makes coil loops particularly advantageous in retrofit
situations. Also, a coil loop can recover energy from multiple exhaust-air
streams (using multiple exhaust-side coils). Of course, recovering energy from
multiple exhaust air streams requires additional coils, more piping and glycol,
and a larger pump.
Typical Performance
Coil-loop selections are typically based on a sensible effectiveness of 45 percent
to 65 percent, assuming balanced airflow, and an airside static-pressure loss of
0.3 to 1.0 in. wg (75 to 250 Pa) per coil. The flexibility of the coil loop lets the
38
SYS-APM003-EN

Coil Loop

Energy-Recovery Technologies
Figure 31. Sensible effectiveness versus face velocity in coil loops
designer vary the number of rows, spacing and type of fins, face velocity, and
fluid flow rate to optimize the device for a specific application.
Figure 31 and Figure 32 depict
representative coil-loop performance.
Given the design flexibility of coils, coil
loops can be selected for a broad range
of performance requirements. ■
Adding more rows and fins to the coils increases the sensible effectiveness of
the coil loop … but it also causes the fan(s) to consume more energy because it
increases the static pressure loss in the air distribution system. See Figure 31
and Figure 32. In other words, the cool-loop design with the highest
effectiveness may not yield the best net energy savings (that is, energy saved
through recovery minus additional fan and pump energy consumed).
Figure 32. Airside pressure drop versus face velocity in coil loops
SYS-APM003-EN
39

Coil Loop

Energy-Recovery Technologies
When a coil loop is used to precondition outdoor air, for example, its payback
usually can be improved by basing the coil selections on the lowest possible
fluid flow rate and face velocity. Although a higher fluid flow rate will increase
the sensible effectiveness of the coil loop, it will also necessitate a larger, more
expensive pump and larger piping—and will increase the energy consumption
of the pump.
■
To maximize net energy savings (best payback), select coils with fewer rows
(four or six) and wider fin spacing (120 fins/ft). This design strategy reduces
the pressure drop through the coil loop.
■
To maximize effectiveness and, therefore, the amount of heat recovered,
select coils with more rows (eight) and with closely spaced fins (144 fins/ft).
This design strategy generally offers the greatest downsizing potential for
cooling and heating plants.
Note: Turbulators inside the coil tubes improve heat transfer (and effectiveness)
without requiring an increased rate of fluid flow through the coil.
Position the pump upstream of the supply-side coil (Figure 30, p. 38) so that the
pump’s heat of compression is applied to the fluid entering that coil. Raising the
temperature difference between the fluid and the air flowing through the
supply-side coil improves the heat-recovery capacity of the loop during cold
weather.
For a coil loop that tempers supply air and is configured in a series
arrangement, use two-row coils whenever possible. Minimizing the number of
coil rows lessens any adverse effect on the energy consumption of the fan.
Capacity Control
Providing a means of capacity control — either a three-way mixing valve or a
variable-speed drive on the pump — prevents the coil loop from overheating the
supply air during mild weather.
A temperature sensor in the supply air stream, downstream of the supply-side
coil, monitors the leaving-air temperature. The mixing valve then appropriately
modulates the fluid flow rate through the supply-side coil. When less fluid flows
through the supply-side coil, the loop adds less heat to the supply air stream. (If
the pump is equipped with a variable-speed drive, then the capacity of the loop
is controlled by modulating the fluid flow rate through the entire coil loop.)
Both the mixing valve and the variable-speed drive can provide equally
effective capacity control. The variable-speed drive is easier to apply, however,
because the location of the three-way mixing valve is critical for proper
operation during the frost prevention mode. The variable-speed drive also
saves pumping energy; however, the savings potential is slight because the
pump already consumes only a modest amount of energy.
40
SYS-APM003-EN

Coil Loop

Energy-Recovery Technologies
Table 7. Considerations for recovering energy with coil loops
Advantages
■
Transfers energy between air streams that
are separated by distance, simplifying
retrofits
■
No cross-leakage between air streams
■
Flexible design/application: Coils can be
selected for the optimum amount of energy
transfer, making them less expensive than
other energy-recovery devices
■
Easily turned off when energy recovery is not
beneficial
■
Easy to control
■
Fits readily within the casing of a packaged
air handler
Disadvantages
■
Transfers only sensible heat
■
May require an expansion tank to
accommodate expansion and contraction of
heat-transfer fluid
■
Requires design and field installation of
piping, pump, expansion tank, and mixing
valve (or variable-speed drive)
■
Requires maintenance of the pump
Note: In outdoor-air preconditioning applications when the temperature outside
is warmer than the desired supply-air temperature but cooler than the exhaustair temperature, turn off the coil-loop pump to avoid transferring unwanted heat
into the supply air stream.
Applying a coil loop for outdoor-air preconditioning in a mixed-air system that
includes an airside economizer entails additional design considerations:
■
Size the coil loop to handle only the minimum ventilation airflow, not full
economizer airflow.
■
Use bypass dampers in both air streams to reduce fan energy consumption
when the coil loop is inactive.
■
Bypass dampers, if present, can also control capacity (and inhibit frost
formation on the exhaust-side coil, in some climates), thereby eliminating
the need for a mixing valve or variable-speed drive for the pump.
For further discussion of exhaust-air bypass for capacity control, refer to
“Rotary Heat Exchangers” (pp. 53 – 55).
Frost Prevention
In outdoor-air preconditioning applications, the same three-way mixing valve or
variable-speed drive that controls the heating capacity of the coil loop can be
used to prevent frost from forming on the exhaust-side coil. Frost prevention is
triggered by a temperature sensor that monitors the temperature of the fluid
entering the exhaust-side coil (Figure 30, p. 38). If the sensor detects a fluid
temperature that is colder than 30°F to 32°F (-1.1°C to 0°C), then the three-way
mixing valve redirects the warm fluid leaving the exhaust-side coil into the fluid
SYS-APM003-EN
41

Coil Loop

Energy-Recovery Technologies
returning from the supply-side coil. A variable-speed pump can provide the
same protective function by reducing the fluid flow rate through the entire loop.
In either case, the protective action of the mixing valve or the variable-speed
pump raises the temperature of the fluid entering the exhaust-side coil and
keeps the surface temperature of the coil above 32°F (0°C) to prevent frost from
forming. Both methods of frost prevention are easy to implement; however,
they also limit the amount of heat that can be recovered during the coldest
times of the year. Consequently, less opportunity exists to downsize heating
equipment. (Preheat can effectively prevent frost formation, too; see p. 56.)
Cross-Leakage
Coil loops are particularly popular for applications such as hospitals and
laboratories in which minimizing the risk of contamination is critical. The two
air streams can be physically separated from each other, and the working fluid
that transfers heat between them is isolated within a closed piping loop.
Although the coil loop, by itself, cannot cause cross-leakage, it is important to
identify other possible paths of contamination. If the coils of the loop are
housed within a single air handler, the air-handler casing may not be leakproof.
To reduce the risk of cross-leakage in this situation, arrange the supply and
exhaust fans so that the pressure in the exhaust side of the air handler is less
than the pressure on the supply side. (See Table 11, p. 58, for more information
about minimizing the risk of cross-leakage.)
Cross-contamination can also occur between the air streams if the exhaust-air
outlet and the outdoor-air intake are improperly positioned or if certain wind
directions can cause the exhaust air to reenter the building through the intake
opening.
Coil Loop with Three Coils
Figure 33. Coil loop with three coils
A coil loop need not be limited to only two coils. Figure 33 illustrates a design
that uses a single, three-coil loop to either precondition the outdoor air or
temper the supply air. (In this case, a variable-speed pump provides capacity
control and frost prevention. Alternatively, these functions could be performed
by several three-way mixing valves.) A two-position control valve determines
which function the coil loop will perform by “converting” the loop from a
parallel configuration to a series arrangement.
During dehumidification, when the cooling coil dries the supply air, the control
valve directs the working fluid through the coils arranged in series with the
cooling coil. The cooling coil dehumidifies the passing supply air. Meanwhile,
the coil loop removes sensible heat from the air upstream of the cooling coil
(reducing the cooling coil load) and transfers it downstream of the cooling coil,
where the recovered heat tempers the dehumidified supply air. When it
42
SYS-APM003-EN
Plate Heat
Exchanger

Energy-Recovery Technologies
functions in the series arrangement described here, this coil-loop design saves
both cooling energy and reheat energy.
When mechanical cooling is unnecessary and the temperature outside is cooler
than the desired supply-air temperature, the same coil loop heats the entering
outdoor air. The control valve redirects the working fluid through the parallel
arrangement of the coil loop, which transfers heat from the exhaust-air path to
the outdoor-air path, upstream of the cooling coil. (Adding recovered heat
upstream of a chilled-water cooling coil, as shown in Figure 33, may provide
adequate freeze protection for the coil.)
A coil-loop design with three coils provides more energy savings and a greater
opportunity to downsize equipment than a conventional two-coil design.
Depending on the application, it may also eliminate the need to protect the
cooling coil from freezing during cold weather. Together, these benefits may
economically justify the additional first cost of the “extra” coil and control
valve.
Fixed-Plate Heat Exchanger
Figure 34. Fixed-plate heat exchanger
As its name implies, a fixed-plate heat exchanger consists of alternate layers of
thermally conductive plates that are sealed to form separate passages for two
air streams; see Figure 34. The plates are constructed from aluminum or other
materials with a similarly high thermal conductivity. Unlike the coil loop, which
uses separate heat-exchanging devices (coils) to collect heat from one air
stream and release it into another, the fixed-plate heat exchanger “packages”
both functions within the same device.
Fixed-plate heat exchangers exist in various configurations, materials, and
capacities; of the available flow patterns, cross-flow is the most common. The
plates in most of these devices transfer only sensible heat between the two air
streams. An exception is the fixed-membrane heat exchanger, which can
transfer both sensible heat and moisture. Its “plates” are membranes of a
paper-like material that allows moisture, as well as sensible heat, to transfer
from one air stream to the other. Fixed-membrane heat exchangers are more
commonly used in residential applications because of their small capacity and
relatively high static-pressure drop, and because they are not self-cleaning.
Typical Performance
Fixed-plate heat exchangers compare favorably with other types of airside
energy-recovery devices because they transfer heat directly from one air stream
to another without an intermediate exchange medium. With balanced airflows,
sensible effectiveness typically ranges from 60 percent to 70 percent. (For a coil
loop to match the recovery capacity of a fixed-plate heat exchanger, it must be
SYS-APM003-EN
43
Plate Heat
Exchanger

Energy-Recovery Technologies
Figure 35. Sensible effectiveness versus face velocity in fixed-plate heat exchangers
equipped with eight-row coils.) In outdoor-air preconditioning applications, a
fixed-plate heat exchanger with a sensible effectiveness of 60 percent usually
affords the best balance between the initial cost of the device and the savings it
provides in operating cost.
Figure 35 and Figure 36 depict
representative performance for fixedplate heat exchangers. ARI Standard
1060 (p. 60) certifies the performance of
these devices, enabling accurate
comparisons between manufacturers
and with other energy-recovery
technologies. ■
Adding more plates to a fixed-plate heat exchanger increases the sensible
effectiveness of the device; however, it also increases the size and weight of the
device. See Figure 35 and Figure 36. More importantly, it increases the airside
pressure drop, which in turn increases the energy consumption of the fan(s).
The incremental improvement in actual performance seldom justifies the added
first cost of additional plates.
Fixed-plate heat exchangers are most commonly used in applications with
airflows less than 10,000 cfm (4.7 m³/s). Adding one of these devices will
increase the static-pressure drop by 0.6 to 1.4 in. wg (150 to 350 Pa) through
each side of the air distribution system.
Capacity Control
Face-and-bypass dampers (Figure 37) are most often used to control the
capacity of a fixed-plate heat exchanger. During mild weather, the face dampers
modulate closed and the linked bypass dampers open to reduce airflow
through the exhaust side of the heat exchanger. Positioning the dampers in this
way avoids overheating the supply air by reducing the amount of heat transfer
that occurs in the heat exchanger. Air therefore leaves the supply side of the
heat exchanger at a lower temperature.
44
SYS-APM003-EN
Plate Heat
Exchanger

Energy-Recovery Technologies
Figure 36. Airside pressure drop versus face velocity in fixed-plate heat exchangers
Bypassing air around the exhaust side of the heat exchanger forces all of the
entering outdoor air to pass through the heat exchanger, which minimizes
temperature stratification in the supply air stream.
Frost Prevention
Figure 37. Face-and-bypass dampers
Of all airside energy-recovery devices, the fixed-plate heat exchanger is the
most susceptible to frost because of its high sensible effectiveness and crossflow configuration. Frost prevention is only an issue if the heat exchanger is
used to precondition outdoor air; it is unnecessary for supply-air tempering.
As shown in Figure 38 (p. 46), frost is most likely to develop in the corner of the
heat exchanger where the cold entering outdoor air recovers heat from the
exhaust air on the leaving edge of the heat exchanger. In this corner, regardless
of the amount of moisture in the exhaust air stream, the exhaust air is in contact
with the coldest surface of the heat exchanger, which approximates the entering
outdoor-air condition. This means that frost will form when the outdoor air
drops below 32°F DB (0°C DB).
In some climates, the same face-and-bypass dampers that control capacity
can also provide frost protection. If the climate is too cold to rely on face-andbypass dampers, however, the heat exchanger should be equipped with
another frost-preventing device. Some heat-exchanger manufacturers add a
damper to divert exhaust air away from the cold corner when frost may occur;
others create temporary warm spots using a moving plate. Another frostprevention technique requires two heat exchangers and alternates their use
every 90 seconds. Finally, one can simply bypass all of the outdoor air around
the heat exchanger and use supplemental heat to melt the ice.
SYS-APM003-EN
45
Fixed-Plate
Heat

Energy-Recovery Technologies
Figure 38. “Cold spot” in a fixed-plate heat exchanger
RA
Figure 39. “Frost-avoidance” damper
Figure 39 shows an integral “frost-avoidance” damper that diverts air away
from the cold corner of the heat exchanger. A temperature sensor monitors the
temperature of the exhaust air leaving the cold corner of the heat exchanger.
When the monitored temperature falls below the frost threshold, the frostavoidance damper closes to divert the entering outdoor air away from that
corner. The temperature in the cold corner of the heat exchanger remains above
freezing, which allows the heat exchanger to operate at much lower outdoor
temperatures. If extremely cold outdoor temperatures are expected, add a
preheat coil to warm the entering outdoor air above the frost-threshold
condition.
Cross-Leakage
Little cross-leakage occurs between the air streams as they pass through a
fixed-plate heat exchanger, but such devices are not leakproof. High-quality
designs use an effective plate sealing system to minimize leakage between the
air streams. If the small amount of cross-leakage is of concern, arrange the
supply and exhaust fans so that the pressure on the exhaust side of the heat
exchanger is less than the pressure on the supply side. (See Table 11, p. 58, for
more information about minimizing the risk of cross-leakage.)
Remember, too, that cross-leakage can also occur through the air-handler
casing and between the building’s exhaust-air outlet and outdoor-air intake. If
these openings are improperly located, or under certain wind conditions,
exhaust air leaving the building may reenter through the outdoor-air intake.
46
SYS-APM003-EN

Heat Pipe

Energy-Recovery Technologies
Table 8. Considerations for recovering energy with fixed-plate heat exchangers
Advantages
Disadvantages
■
Relatively high sensible effectiveness
■
Transfers only sensible energy (heat)
■
Little cross-leakage between air streams
■
Requires adjacent air streams
■
Easy to clean
■
Relatively high frost threshold
■
Heavy
■
High first cost in large applications
Heat Pipe
Figure 40. Heat-pipe assembly
Adapted, by permission, from Innergytech Inc.
Figure 41. Cross section of heat-pipe tube
A heat pipe resembles a finned-tube coil; see Figure 40 and Figure 41. Each
of its large tubes is an independent, sealed container that is filled with a heattransfer fluid (typically a chemical refrigerant). A partition divides the air “side”
of the heat pipe into separate “evaporator” and “condenser” sections. The
capillary wick structure within the pipe walls induces refrigerant flow between
the “evaporator” and “condenser” without the help of a compressor or pump.
Here’s how …
Two air streams flow across each heat pipe: warm air flows across the
“evaporator” and cool air flows across the “condenser.” The refrigerant inside
the “evaporator” evaporates as it absorbs heat from the warm air. Given the
comparative coolness of the air flowing over the “condenser,” the refrigerant
vapor pressure is lower in that section of the heat pipe than in the “evaporator.”
The difference in vapor pressures causes the refrigerant vapor to migrate from
the “evaporator” to the “condenser.” Cool air flowing over the “condenser”
absorbs heat from the refrigerant vapor, which causes the refrigerant to
condense. The liquid refrigerant then “wicks” back to the “evaporator” section
of the heat pipe and the cycle repeats.
The direction of heat transfer reverses when the relative temperatures of the
two air streams switch. In an outdoor-air preconditioning application, for
example, when the outdoor air is warmer than the exhaust air, the “evaporator”
is in the outdoor air stream and the “condenser” is in the exhaust air stream.
When the outdoor air is cooler than the exhaust air, the “evaporator” is in the
exhaust air stream and the “condenser” is in the outdoor air stream.
Typical Performance
Heat pipes are typically selected for a sensible effectiveness of 30 to 55 percent,
with balanced airflow. Airside pressure drops range from 0.2 to 0.8 in. wg (50 Pa
to 200 Pa), based on the number of tube rows and the number of fins.
Adapted, by permission, from Innergytech Inc.
SYS-APM003-EN
Figure 42 and Figure 43 (p. 48) illustrate the effect of adding more rows to the
heat pipe. The sensible effectiveness of the device increases, as does the fan
47

Heat Pipe

Energy-Recovery Technologies
Figure 42. Sensible effectiveness versus face velocity for heat pipes
Figure 42 and Figure 43 depict
representative performance for heat
pipes. ARI Standard 1060 (p. 60)
certifies the performance of these
devices, enabling accurate comparisons
between manufacturers and with other
energy-recovery technologies. ■
energy required to overcome the additional static pressure drop. A heat pipe
that is selected for maximum effectiveness and, therefore, maximum heat
recovery will differ from a heat pipe that is selected for the best payback from
energy savings. Consider these guidelines when choosing a heat pipe for a
particular energy-recovery application:
■
To precondition outdoor air, select the heat pipe based on the lowest
possible face velocity. Four or six rows of tubes usually provide optimal
heat-pipe selections.
Figure 43. Airside pressure drop versus face velocity for heat pipes
48
SYS-APM003-EN

Heat Pipe

Energy-Recovery Technologies
■
To temper supply air, choose a two-row heat pipe if the device is applied in a
series (wrap-around) arrangement. Fewer rows lessens the impact on fan
energy consumption.
Capacity Control
Heat pipes, like other devices that recover sensible energy, require a means of
capacity control to avoid overheating the supply air when the weather is mild.
The extent to which capacity must be controlled — partial reduction versus
modulation and shutoff — determines which of two common methods is used:
tilt control or face-and-bypass dampers.
Tilt control partially reduces the transfer capacity of the heat pipe. Tilting the
heat pipe to elevate the evaporator end of the tubes above the condenser end
slows the rate at which the liquid refrigerant flows back to the “evaporator.”
The steeper the tilt angle, the slower the rate of flow and the less heat that can
be recovered and transferred to the air leaving the supply side of the heat pipe.
Given the finite extent of tilt (usually 4° to 6°), this method provides only a
limited range of capacity control; consequently, it may not prevent unnecessary
heat transfer in all applications and climates.
Figure 44 illustrates how tilt control can be accomplished. An actuator is
attached to one end of the heat pipe. To permit free movement throughout the
range of tilt, a pivot is provided at the center of the heat-pipe support and
flexible connectors attach the heat pipe to the ductwork.
Figure 44. Tilt-control mechanism for regulating heat-pipe capacity
pivot
Adapted from ASHRAE Handbook—2000 HVAC Systems and Equipment, Figure 20, page 44.17.
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Energy-Recovery Technologies
Face-and-bypass dampers provide an alternative means of capacity control.
During mild weather, the face dampers modulate closed and the linked bypass
dampers open to reduce airflow across one end of the heat pipe. Fully closing
the face dampers and fully opening the bypass dampers effectively turns off the
heat pipe.
Note: Some equipment designs use solenoid valves to regulate or stop the flow
of refrigerant in the heat pipe when little or no heat is desired.
Frost Prevention
In outdoor-air preconditioning applications, the tilt mechanism or face-andbypass dampers that control capacity also can be used to prevent frost
formation during cold weather. Reducing the capacity of the heat pipe increases
the temperature at which the exhaust air stream leaves the heat pipe, which
prevents frost from forming; however, it also reduces the opportunity to
downsize the heating plant. The exact temperature at which frost will form
depends on the amount of moisture contained in the exhaust air stream.
If extremely cold weather is expected, it may be necessary to add a preheat coil
to ensure that the entering outdoor air remains above the frost threshold.
Table 9. Considerations for recovering energy with heat pipes
Advantages
Disadvantages
■
Little cross-leakage between air streams
■
Contains refrigerant
■
Relatively low maintenance
■
Transfers only sensible energy (heat)
■
Can be packaged inside an air handler
■
Requires external face-and-bypass dampers
to prevent unwanted heat transfer
Cross-Leakage
The ARI 1060 certification program
(discussed on pp. 60 – 63) shows that heat
pipes are susceptible to cross-leakage.
Refer to the AHRI directory of certified
products at www.ahridirectory.org to find
the certified performance — including
cross-leakage — of specific, rated
products. ■
50
Despite the proximity required, a minimal amount of cross-leakage occurs
between the two air streams that pass through the heat pipe. A solid partition
separates the “evaporator” and “condenser” sections of the device, and the
working fluid that serves as the heat-transfer medium is sealed within the heatpipe tubes. Risk of cross-leakage can increase, however, if the integrity of the
seal around the partition degrades over the life of the device.
Beyond the heat-pipe assembly, two other potential paths of cross-leakage
must be considered: the casing of the air handler that typically houses the heat
pipe, and the exhaust- and outdoor-air openings in the building envelope. To
reduce the likelihood of cross-leakage via the air-handler casing, arrange the
supply and exhaust fans so that the pressure in the exhaust end of the air
handler is less than the pressure in the supply end. (For further details, see
Table 11, p. 58.)
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Wind passing over a building creates turbulence that, under specific conditions,
may force exhaust air to reenter the building through the outdoor-air intake. To
minimize this risk, determine the proper location for the building’s exhaust- and
outdoor-air openings based on atmospheric conditions and building geometry.
Rotary Heat Exchanger
Rotary heat exchangers, commonly called “wheels,” consist of a revolving
cylinder that rotates between two air streams. Heat transfer is accomplished as
the air streams pass through a matrix of channels that direct the air through the
wheel in a counterflow arrangement. The nature of the heat-transfer medium
determines whether the wheel will transfer only sensible heat or both sensible
heat and moisture.
The channels in a sensible-energy wheel (or “heat wheel”) are constructed of
aluminum, copper, stainless steel, or other thermally conductive materials that
do not absorb moisture. As the wheel rotates, the channels absorb sensible
heat from the warmer of the two air streams and reject it to the cooler air
stream.
A total-energy wheel (also known as an “enthalpy wheel” or “passive desiccant
wheel”) transfers both sensible heat and moisture. The matrix of channels in a
total-energy wheel usually consists of a fibrous material (such as paper), plastic,
or aluminum. To promote moisture transfer, the channel surfaces are coated
with a desiccant material, that is, a substance that has a strong affinity for water
vapor and an enormous internal surface area. While the wheel rotates through
Figure 45. Total-energy wheel and examples of heat-transfer media
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the two air streams, it absorbs sensible heat and moisture from the hot, humid
air stream and releases it into the cooler, drier air stream.
The differences between the temperature and moisture content of the two air
streams creates a vapor-pressure differential that provides the driving force
needed to transfer water vapor. In many climates, the ability to transfer
moisture is useful year-round, dehumidifying during the cooling season and
humidifying during the heating season.
Typical Performance
Figure 46 and Figure 47 depict
representative performance for totalenergy wheels. ARI Standard 1060
(p. 60) certifies the performance of these
devices, enabling accurate comparisons
between manufacturers and with other
energy-recovery technologies. ■
Heat-transfer capacity for a wheel is determined by the size of the channels,
rotational speed, wheel diameter, wheel depth, and, in the case of a totalenergy wheel, the amount of desiccant used. Wheels have good effectiveness
regardless of wheel channel shape. With balanced airflows, sensible-energy
wheels typically perform at a sensible effectiveness of 60 percent to 80 percent;
they also create a pressure drop that can range from 0.5 to 1.0 in. wg (125 Pa to
250 Pa). Total-energy wheels perform with a total effectiveness that typically
ranges from 65 percent to 80 percent (assuming balanced airflows) and creating
a pressure drop of 0.7 to 1.2 in. wg (175 to 300 Pa).
In outdoor-air preconditioning applications, a total-energy wheel with an
effectiveness of about 70 percent usually provides the optimal balance between
first cost and operating-cost savings. Increased effectiveness requires more
media and lower face velocities, necessitating a larger air handler; see Figure 46
and Figure 47. This incremental cost seldom justifies the performance
improvement.
Figure 46. Total effectiveness versus face velocity for total-energy wheels
52
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Figure 47. Airside pressure drop versus face velocity for total-energy wheels
Compared to other airside energy-recovery technologies, rotary heat
exchangers are “self-cleaning.” Small, dry particles pass through the wheel
channels; larger particles blow clear as the wheel rotates and the direction of
airflow reverses. The wheel may be manufactured in a single piece or as
multiple segments. Segmented wheels allow for easier removal for service
(cleaning).
Table 10. Considerations for recovering energy with rotary heat exchangers
Advantages
Disadvantages
■
Total-energy wheels transfer both sensible
heat and moisture (latent energy)
■
Sensible-energy wheels transfer only
sensible heat
■
High effectiveness
■
■
Can be packaged inside an air handler
May permit cross-leakage between air
streams
■
“Self-cleaning” with respect to dry particles
■
Belt, motor, and bearings require periodic
maintenance
Capacity Control
Modulating the capacity of a rotary heat exchanger is usually accomplished by
diverting a portion of the air around either side of the wheel, or by varying its
rotational speed using a variable-frequency drive (VFD) connected to the wheel
motor.
Figure 48 (p. 54) illustrates an air-handler configuration that places a
conventional damper parallel to the wheel. In this arrangement, only exhaust
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Figure 48. Air handler with a rotary heat exchanger and exhaust-air bypass dampers
air bypasses the wheel. Forcing all of the outdoor air to pass through the wheel
minimizes temperature stratification of the air leaving the wheel and may
eliminate the need to protect downstream hydronic coils from freezing.
After comparing the stability, reliability, cost, and flexibility of these control
methods, we recommend exhaust-air bypass for modulating the capacity of
rotary heat exchangers for two reasons:
■
Exhaust-air bypass provides a more linear unloading characteristic than a
VFD, which simplifies capacity control and results in more stable control of
the leaving air temperature.
As shown in Figure 49, a VFD must slow the rotation of the wheel to
50 percent of nominal speed to achieve a 10-percent reduction in sensible
effectiveness (capacity). Beyond that point, capacity drops off rapidly.
■
Exhaust-air bypass provides a wider range of capacity control. Typically, a
VFD can slow the motor to 33 percent of nominal speed (a 67-percent
reduction of the rotations per minute). At that rotational rate, however, the
sensible effectiveness (capacity) of the wheel is still 80 percent of nominal.
In mixed-air systems that include airside economizers, the same damper that
allows increased exhaust airflow during the economizer mode can also
modulate the capacity of the rotary heat exchanger.
Note: Base the size of the rotary heat exchanger on the minimum outdoor
airflow required for ventilation, not on full economizer airflow, in mixed-air
applications. This design strategy lowers the first cost of the heat exchanger
and, if bypass dampers are used, it avoids imposing the pressure drop of the
wheel during airside economizing.
54
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Figure 49. Typical response of capacity-control methods for rotary heat exchangers
Frost Prevention
Frost begins to form on a sensible wheel at a much higher outdoor temperature
than on a total-energy wheel. This is because a total-energy wheel removes
moisture from the exhaust air stream, which lowers the dew point and delays
when the exhaust air finally becomes saturated. (See Figure 13, p. 19.) Figure 50
illustrates how to predict the dry-bulb temperature at which frost will begin to
form on a total-energy wheel. On a psychrometric chart, plot the return air
condition (RA); then draw a line, tangent to the saturation curve, through this
point. The “frost threshold” — that is, the outdoor dry-bulb temperature at
which frost formation can begin — is the point where the tangent line touches
the saturation curve.
Notice that frost does not necessarily form on the wheel at outdoor dry-bulb
temperatures colder than 32°F (0°C). In Figure 50, for example, frost prevention
only becomes necessary when the temperature outside falls below 0°F (-17.8°C).
Figure 50. Determining the “frost threshold” temperature
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Together, the humidity ratio of the exhaust air and the dry-bulb temperature of
the outdoor air determine which of three frost-prevention methods —
preheating the entering air, reducing the wheel’s capacity, or turning off the
wheel — best suits a particular application.
Preheat entering outdoor (or exhaust) air. Raising the dry-bulb
temperature of the entering outdoor air prevents the surface temperature of the
wheel from becoming so cold that frost forms on the exhaust side. Unlike other
methods of frost prevention, it also means that the wheel can continue
operating at full capacity — recovering as much energy as possible. The preheat
coil is controlled to maintain the leaving-air temperature above the frost
threshold.
Note: For applications with electric preheat coils, adhere to the manufacturer’s
guidelines for installation and placement to assure proper airflow across the
heating elements and to minimize temperature stratification in the leaving air.
Speed Control and
Frost Prevention “Don’t Mix”
… At least not in total-energy-wheel
applications. As the rotation of a totalenergy wheel slows, both its latent and
sensible effectiveness (capacity)
decrease — but not at the same rate; see
Figure 49 (p. 55).
When the wheel begins to slow, its
ability to remove moisture (latent
effectiveness) from the exhaust air
stream decreases faster than its ability
to raise the dry-bulb temperature of the
exhaust air (sensible effectiveness). The
likelihood of frost actually increases
because the exhaust air reaches the
saturated condition more quickly.
Further slowing the wheel’s rotation
prevents frost, but it significantly
reduces heat transfer, too. With leavingair temperatures well below freezing on
the supply side of the wheel,
downstream hydronic coils often need
additional protection from freezing. ■
56
Some system designs preheat the exhaust-side air stream instead of the
supply-side air stream. Why? Because a hot water or steam coil can be used
without fear of coil freeze-up and without adding glycol to the hot water loop.
Although this design requires a slightly larger preheat coil to prevent frost, it
also increases the temperature differential between the two air streams, which
transfers more sensible heat to the outdoor air stream. (In specific applications,
preheating the exhaust-side air stream may eliminate the need for a heating
coil in the air handler.) Make sure that the air temperature leaving the preheat
coil does not exceed the maximum operating temperature of the selected totalenergy wheel.
Preheat is recommended for climates with extreme winter conditions, for
applications in which the exhaust-air humidity ratio is high during the winter, or
for systems that include mechanical humidification. When compared with other
methods of frost prevention, preheat provides the most energy recovery during
peak winter conditions.
Reduce the capacity of the wheel (outdoor-air bypass). Moisture
condenses on the exhaust side of the wheel when the exhaust air stream
becomes saturated and the wheel surface is colder than 32 °F (0 °C). Therefore,
another way to deter frost formation is to prevent the exhaust air from reaching
the saturation curve. Transferring less total energy — that is, reducing the
capacity of the wheel — yields cooler air leaving the supply side of the wheel
and warmer air leaving the exhaust side. However, this method also reduces
the opportunity to downsize the heating plant.
As described on pp. 53 – 54, air can be diverted around one side of the wheel to
reduce its capacity. Rather than place the bypass damper in the exhaust air
path, however, locate it in the outdoor air path to bypass part of the outdoor air
around the supply side of the wheel. This arrangement prevents the exhaust air
from reaching the saturated condition by becoming too cold, but it also diverts
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cold outdoor air around the wheel. In many climates, adding an air blender will
provide sufficient freeze protection for the hydronic coils downstream of the
wheel.
Note: When outdoor-air bypass is used for frost prevention, the same supplyside bypass damper also can provide capacity control. Outdoor-air bypass is
best used in mild climates where frost conditions occur during only a few hours
of the year. Freeze protection may still be required to adequately safeguard the
hydronic coils downstream of the wheel. Avoid using outdoor-air bypass in
systems that include mechanical humidification.
Turn off the wheel. The easiest and most reliable way to prevent frost is
to turn off the wheel when the outdoor temperature falls below the frost
threshold. This method eliminates the opportunity to downsize the heating
plant and requires full freeze protection for downstream coils, but in moderate
climates, it may be the most cost-effective option for mixed-air systems that
use a relatively low percentage of outdoor air.
Cross-Leakage
Leakage between the exhaust and supply air streams occurs in all rotary heat
exchangers, regardless of their respective designs. Because some amount of
cross-leakage is unavoidable, do not use wheels to precondition outdoor air for
occupied spaces when highly concentrated or hazardous contaminants exist in
the exhaust air stream. (An example of this type of application is a laboratory
fume hood that handles a carcinogenic substance or biohazard.) Instead, use a
coil loop, which has no potential for cross-leakage.
A small amount of cross-leakage of air that contains rest-room exhaust is often
acceptable in many applications, provided that it is sufficiently diluted with
fresh outdoor air. In these instances, rotary heat exchangers can be used to
recover energy from an air stream that includes rest-room exhaust.
Air leaks from one side of the wheel to another along three paths:
■
Through the seals, which have a typical leakage rate of 3 percent
to 5 percent
■
Through the matrix, which has a typical leakage rate of approximately
2 percent (If the matrix is fluted, leakage along this path is negligible.)
■
Via “carryover” as the wheel rotates, which represents a typical leakage rate
of approximately 2 percent
Each leakage path presents different application challenges and must be
controlled differently.
Generally, the amount and direction of cross-leakage through the seals and
matrix is controlled by managing the pressure difference between the supply
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Rotary Heat
Exchanger
and exhaust air streams. The configuration of the exhaust and supply fans with
respect to the rotary heat exchanger (blow-through versus draw-through)
affects this pressure difference; see Table 11. The best fan arrangement for a
particular application depends on the desired performance characteristics.
Table 11. Configuration versus cross-leakage through rotary heat exchangers
Fan–Rotary Heat Exchanger Arrangement
Differential Pressure Characteristics
Draw-through exhaust, blow-through supply
■ Creates a comparatively higher static
pressure in the supply path
■
Draw-through exhaust, draw-through supply
■ Direction of leakage depends on the static
pressure difference between the supply and
exhaust air streams
Blow-through exhaust, draw-through supply
■ Creates a comparatively higher static
pressure in the exhaust path
OA
58
■
Best arrangement to minimize air leakage
from exhaust to supply through seals and
matrix
■
Little cross-leakage through seals and matrix
in most applications
■
Design system so that static pressure in
exhaust path nearly equals static pressure in
supply path to minimize cross-leakage
■
Promotes good airflow across the wheel with
minimal leakage through seals and matrix
■
Design system so that static pressure in
exhaust path nearly equals static pressure in
supply path to minimize cross-leakage
■
Promotes significant leakage from exhaust
to supply through seals and matrix; avoid
using this arrangement
Air leaks from supply path to exhaust path
Blow-through exhaust, blow-through supply
■ Direction of leakage depends on the static
pressure difference between the supply and
exhaust air streams
■
Application Considerations
Air leaks from exhaust path to supply path
SA
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When it comes to controlling cross-leakage via “carryover,” fan arrangement
has no effect. Instead, wheel geometry and rotational speed determine how
much air the rotary heat exchanger will carry from one air stream to the other.
To minimize contamination between the air streams, some rotary heat
exchangers are provided with a purge section; see Figure 51. As the wheel
rotates, the channels in a 5°-to-10° arc pass through the purge section. While
there, the channels are flushed with outdoor air that is immediately diverted
into the exhaust air stream.
Although the purge section minimizes contaminant “carryover” into the supply
air stream, it does nothing to mitigate leakage through the seals or matrix. It
also reduces the effectiveness and associated energy savings of the wheel.
Without the purge, however, there is no other means to control the “carryover”
path of cross-leakage between the air streams.
Most airside energy-recovery applications can tolerate a modest amount of
cross-leakage. In such cases, properly arranged fans and a wheel with an
effective sealing system often make a purge section unnecessary. Do not use
rotary heat exchangers — with or without a purge — for applications that require
absolutely no cross-leakage.
As for each of the previously discussed energy-recovery technologies, the
discussion of cross-leakage paths for rotary heat exchangers must include
the potential path through the air-handler casing and between the building’s
exhaust outlet and outdoor-air intake. Improper placement of these openings
or adverse wind conditions may induce exhaust air to reenter the building
through the outdoor air intake.
Figure 51. Rotary heat exchanger with purge section
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ARI Standard 1060
ARI 1060 or ASHRAE 84?
ASHRAE Standard 84 defines the
standard method for testing a fixedplate heat exchanger, rotary heat
exchanger (wheel) or a heat pipe. It
defines the laboratory setup and
instrumentation requirements; it does
not define standard test or rating
conditions.
The companion rating and certification
standard is ARI 1060. This document
defines standard conditions
(temperature and humidity entering
both sides of the device) and reporting
requirements. It also includes a
certification process that allows a
manufacturer's results to be verified by
an independent third party.
A design engineer should specify
performance based on ARI 1060, which
results in certified performance. By
definition, this also means that the
performance was measured according to
the standardized methods for testing
such a device, which are defined in
ASHRAE Standard 84.■
The Air-Conditioning, Heating, and Refrigeration Institute (AHRI) is a
voluntary, nonprofit organization comprised of manufacturers of airconditioning, refrigeration, and heating products.
Two of AHRI’s most important functions are the development of
performance rating standards and the administration of performance
certification programs for eligible products. Participation in the
programs is voluntary and open to nonmembers of AHRI on an equal
basis.
Note: In 2007, the Air-Conditioning and Refrigeration Institute (ARI) and the Gas
Appliance Manufacturers Association (GAMA) merged to become AHRI.
More specifically for this discussion, ARI Standard 1060 –2005 defines
requirements for testing, rating, and marking these factory-made energyrecovery devices: heat-pipe heat exchangers, fixed-plate heat exchangers, and
rotary heat exchangers.
Run-around coil loops, however, are not addressed by Standard 1060 because
of the broad variability in design. Instead, ARI Standard 410 –2001, ForcedCirculation Air-Cooling and Air-Heating Coils, defines the testing, rating, and
marking requirements for individual coils. To determine the performance of a
particular run-around loop, simply apply the certified performance of the coils
in the “effectiveness ratio” equation. (See “Fundamental Equations for Air-toAir Energy Recovery” on p. 6.)
Note, too, that ARI Standard 1060 only pertains to applications in which the
energy-recovery device preconditions outdoor air; it does not govern supply-air
tempering. Furthermore, certification under this standard is limited solely to the
energy-recovery component. That is, the certified effectiveness of an energyrecovery device does not extend to its actual performance after it is installed in
a central air handler or packaged rooftop air conditioner.
Historically, manufacturers used various methods to calculate the effectiveness
of their air-to-air energy-recovery ventilation (AAERV) equipment, sometimes
overstating performance. By normalizing the effectiveness calculation, the ARI
Standard 1060 certification program:
60
■
Fosters confidence that a certified air-to-air energy-recovery component will
provide the specified performance
■
Permits equitable comparisons of energy-recovery devices from various
manufacturers
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Figure 52. Seal of certified performance
■
Accurately predicts leaving air conditions by rating sensible and latent
effectiveness separately (Technologies that transfer latent energy/moisture
are differentiated from those that do not.)
■
Documents cross-leakage data, which permits proper air-handler design and
application (including fan selection and ventilation control)
■
Permits more accurate sizing — or more appropriately, downsizing —of
heating and cooling plants
■
Eliminates the need for independent testing by a third party (ARI is one of
the industry’s most reputable third-part testing organizations.)
Ultimately, the only way to properly compare equipment that includes an air-toair energy-recovery device to precondition outdoor air is to require certification
under ARI Standard 1060. Equipment specifications should require ARI-certified
performance and the ARI 1060 seal of certification (Figure 52).
Standard Rating Conditions
ARI Standard 1060 – 2005, Performance
Rating of Air-to-Air Heat Exchangers for
Energy Recovery Ventilation Equipment
is available from the AHRI Web site
(www.arhinet.org). The directory of
AHRI-certified products is available at
www.ahridirectory.org. ASHRAE
Standard 84-1991 is available from
ASHRAE’s online bookstore at
www.ashrae.org. ■
ARI 1060 defines certified rating points that measure sensible, latent, and total
effectiveness at both 100 percent and 75 percent of rated airflow. These airflow
conditions represent the typical selection range for AAERV devices. The test
procedure for certification uses equal airflows through the supply and exhaust
sides of the device, as defined by ASHRAE Standard 84 –1991, Method of
Testing Air-to-Air Heat Exchangers. Effectiveness is then calculated (using the
“effectiveness ratio” equation found on p. 6) for the heating and cooling
conditions summarized in Table 12. The test also measures the airside pressure
drop at each rating condition.
A further requirement for ARI 1060 certification is a tracer-gas test that provides
two leakage measurements — the exhaust air transfer ratio (EATR) and the
outdoor air correction factor (OACF) — at three different deck-to-deck pressures.
Table 12. Standard rating conditions from ARI 1060 1
Condition
Entering supply air
Heating
Cooling
dry-bulb temperature
35°F
(1.7°C)
95°F
(35.0°C)
wet-bulb temperature
33°F
(0.6°C)
78°F
(26.0°C)
Entering exhaust air dry-bulb temperature
70°F
(21.0°C)
75°F
(24.0°C)
wet-bulb temperature
58°F
(14.0°C)
63°F
(17.0°C)
1 The standard rating conditions defined in Table 1 of ARI Standard 1060 – 2005 include rates for supply and exhaust
airflows, as well as pressure differentials for various performance tests.
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Exhaust Air Transfer Ratio
Figure 53. EATR quantifies cross-leakage
Expressed as a percentage of downstream supply airflow, the exhaust air
transfer ratio (EATR) helps to quantify cross-leakage from the exhaust air
stream to the supply air stream. In Figure 53, the static pressure of the air
leaving the supply side of the wheel is 1.0 in. wg (250 Pa) greater than the static
pressure of the air entering the exhaust side. If the rotary heat exchanger in this
example has an EATR of 0.6 percent, then 60.4 cfm (0.03 m3/s) leaks into the
supply air stream from the exhaust side of the wheel.
EATR
air leakage from exhaust to supply = V2  ----------------100
SP supply – SP exhaust = 1.0 in. wg  250 Pa 
If the predicted cross-leakage is too high for the specific application, it may be
possible to lower the EATR by rearranging the fans, adding a purge to the
wheel, or using a different energy-recovery device such as a coil loop. To
achieve the lowest EATR, use a blow-through fan on the supply side of the
wheel and a draw-through fan on the exhaust side.
Note: An arrangement that combines a blow-through supply fan with a blowthrough exhaust fan produces the least effect on fan energy consumption,
although its EATR is slightly higher.
Outdoor Air Correction Factor
Figure 54. OACF aids supply-fan sizing
The outdoor air correction factor (OACF) is defined as the supply-side airflow
upstream of the wheel (V1 ) divided by the supply-side airflow downstream of
the wheel (V2 ). This value is important for sizing the supply and exhaust fans
and is usually small unless the pressure difference between the air streams is
greater than 3.0 in. wg (750 Pa).
If the system includes an outdoor-airflow monitoring station or a flowmeasuring damper, its setpoint can be adjusted to compensate for the OACF
and thereby assure that the proper amount of outdoor air flows into the wheel
so that the required amount of outdoor air leaves the wheel.
Using EATR and OACF for System Design
The arrangement of the supply and exhaust fans, relative to the energyrecovery device, determines how the EATR and OACF values are used to
calculate the necessary capacity of each fan. Using the example shown in
Figure 54, the draw-through exhaust fan must be sized to deliver 11,167 cfm
(5.27 m³ /s). To assure that 10,000 cfm (4.7 m³ /s) of fresh outdoor air enters this
example system, use a setpoint of 11,167 cfm (5.27 m³ /s) for an airflow-
62
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monitoring station or a flow-measuring damper (such as the Trane Traq™
damper) positioned upstream of the wheel:
Voa  OACF
OA setpoint = ------------------------------- 1 – EATR

---------------
100 
Table 13 shows the equations for calculating the airflow at each station.
Table 13. Fan sizing calculations
Fan Airflows at Energy-Recovery Device
Calculations for Fan Airflow 1
Supply-side airflow V2 :
Voa
EATR
V2 = Voa + V2  -----------------  = -------------------------------
100  
1 – EATR
----------------- 


100
Cross-leakage airflow V3  2 :
Voa
V3  2 = V2 – Voa = -------------------------------- – Voa
 1 – EATR
----------------- 

100 
Exhaust-side airflow V3 :
Voa
V3 = Vea + V3  2 = Vea + --------------------------------– Voa
 1 – EATR

---------------
100 
Supply-side airflow V1 :
Voa  OACF
V1 = V2  OACF = -------------------------------- 1 – EATR
----------------- 

100 
Cross-leakage airflow V1  4 :
Voa  OACF
V1  4 = V1 – Voa = -------------------------------- – Voa
 1 – EATR
----------------- 

100 
Exhaust-side airflow V4 :
Voa  OACF
V4 = Vea + V1  4 = Vea + --------------------------------- – Voa
 1 – EATR
----------------- 

100 
1
Voa represents the required outdoor airflow for proper ventilation, and Vea represents the system-level exhaust airflow
required for proper building pressurization.
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Table 14. Comparative overview of air-to-air energy-recovery technologies
Energy-Recovery Technology
Coil Loop
Fixed-Plate
Heat Exchanger
Heat Pipe
Rotary Heat Exchanger
effectiveness
45 – 65% sensible
60 – 70% sensible
30 – 55% sensible
60 – 80% sensible,
65 – 80% total
pressure drop
through each side of
the energy-recovery
device
0.3 – 1.0 in. wg
(75 – 250 Pa)
0.6 – 1.4 in. wg
(150 – 350 Pa)
0.2 – 0.8 in. wg
(50 – 200 Pa)
0.5 – 1.0 in. wg sensible,
0.7 – 1.2 in. wg total
3-way mixing valve
●
Characteristic
Typical performance 1
Method of capacity
modulation
(125 – 250 Pa sensible,
175 – 300 Pa total)
●
dampers
(face-and-bypass)
●
(face-and-bypass)
●
●
(pump motor)
(wheel motor)
speed control
Frost prevention
techniques 2
●
(exhaust-side bypass)
●
(partial modulation)
tilt control
modest
Susceptibility to frost 2
●
(face-and-bypass)
highest
modest
high (sensible),
lowest (total)
■
3-way mixing valve
■
dampers
(face-and-bypass)
■
(diverting)
■
(face-and-bypass)
■
(supply-side bypass)
■
tilt control
■
preheat coil
■
■
■
■
speed control
(pump motor)
Likelihood of crossleakage between
air streams 3
none
slight
slight
modest
1 Per ARI Standard 1060, performance of energy-recovery devices — effectiveness and pressure drop — is based on equal airflows through the exhaust and supply sides of the device.
2 Pertains only to applications in which the energy-recovery device is used to precondition outdoor air.
3 Assessment of cross-leakage potential pertains only to the energy-recovery device. Be sure to account for other potential paths of leakage such as the air-handler casing or short-
circuiting of building exhaust into the outdoor-air intake.
64
SYS-APM003-EN
System-Level Control
of Recovered Energy
Ventilation systems can be classified according to the composition of the
supply air that is eventually delivered to the conditioned space: that is,
dedicated outdoor air (100-percent outdoor air) or mixed air (outdoor air plus
recirculated air). Air-to-air energy recovery can be added to either type of
ventilation system.
For more information about dedicated
OA systems, refer to the Trane
Engineers Newsletter titled “Dedicated
Ventilation Systems” (ENEWS-30/3).
You can find it at www.trane.com/
engineersnewsletter.
The Trane applications engineering
manual, Dehumidification in HVAC
Systems (SYS-APM004-EN) also
includes a detailed discussion of
dedicated outdoor-air systems. It is
available from the Trane bookstore at
www.trane.com/bookstore. ■
Dedicated outdoor-air systems devote an air handler exclusively to heating,
cooling, humidifying, or dehumidifying all outdoor air (OA) brought into the
building for ventilation. The conditioned outdoor air is then delivered directly to
the occupied spaces (Figure 55) or to the outdoor/mixed-air intakes of other
HVAC units (Figure 56). In either configuration, the dedicated OA system may
be designed to deliver the conditioned air (CA) at either a “neutral” or cold
temperature.
A dedicated OA system that provides “neutral” air first dehumidifies the
outdoor air to a dew point that is drier than the space; it then reheats the air to
approximately room temperature, usually between 70°F and 72°F (21°C and
22°C). Often, the dedicated OA unit delivers neutral air directly to the occupied
spaces, where fan-coils (for example) handle the local loads. Alternatively, the
conditioned outdoor air can be ducted either to local units or into a plenum
within proximity of the local units.
Dedicated OA systems that are designed to provide cold air cool and
dehumidify the outdoor air — typically achieving 45°F to 55°F DB (7°C to
13°C DB) — but do not reheat it. Again, systems of this type can deliver the
conditioned outdoor air directly to the occupied spaces or to mixing boxes of
local air-handling units such as blower-coils, small air handlers, or packaged
rooftop air conditioners.
A dedicated OA system that delivers cold air eliminates the heating energy
associated with tempering; it also requires less cooling plant capacity than a
Figure 55. Conditioned (outdoor) air, CA,
delivered to occupied spaces
SYS-APM003-EN
Figure 56. Conditioned (outdoor) air, CA,
delivered to local air-handling units
65
System-Level Control
of Recovered Energy
similar system that provides neutral air because there is no tempering (reheat)
energy to offset. When delivered directly to the space, the cold ventilation air —
which has comparatively lower dry-bulb and (usually) dew-point
temperatures — reduces the sensible and latent cooling loads in the space.
When delivered to local units, the cold ventilation air lowers the enthalpy of the
resulting mixed air and, in turn, reduces the cooling capacity required from the
coils in the local mixed-air units.
Mixed-air ventilation systems, as the name implies, use an air handler to
condition a mixture of outdoor and recirculated return air, and then supply the
mixed air to each space. A constant-volume mixed-air system supplies the
same quantity of air, usually to a single space (or zone). The temperature of the
supply air is modulated to respond to the changing sensible cooling load in the
space.
A variable-air-volume (VAV) mixed-air system delivers supply air at a constant
temperature, typically 45°F to 55°F DB (7°C to 13°C DB), to multiple VAV
terminals. The VAV terminals respond to changing sensible cooling loads in
each space (or zone) by modulating the airflow appropriately.
What about ventilation plus energy recovery? Recall from p. 5 that the addition
of an energy-recovery device typically serves one of two purposes: it either
preconditions outdoor air brought into the building for ventilation, or tempers
(reheats) supply air to permit independent control of the dry-bulb temperature
and humidity in the conditioned space. A system can perform both functions if
it includes two energy-recovery devices; see “Applications that Combine
OA Preconditioning and SA Tempering” on pp. 84 – 88.
Together, the composition and temperature of the supply air define four types
of ventilation systems:
■
Dedicated OA systems that deliver “neutral”-temperature air (relative to
room conditions).
■
Dedicated outdoor-air systems that deliver cold air
■
Mixed-air systems that modulate the temperature of a constant volume of
supply air
■
Mixed-air systems that supply varying volumes of constant-temperature air
Each of these ventilation systems can benefit from using air-to-air energy
recovery to precondition the entering outdoor air. Budget constraints will
determine whether a total- or sensible-energy recovery device best suits a
particular application.
When tempering supply air, however, air-to-air energy recovery only benefits
systems that directly control the supply-air dew point or the humidity in the
66
SYS-APM003-EN
System-Level Control
of Recovered Energy
space … and the benefits only exist during mechanical cooling operation.
Sensible-energy recovery can provide recovered heat in compliance with
energy codes and standards; when compared with alternatives that use “new”
energy for tempering, it may also reduce system operating costs. (Total-energy
recovery is seldom used for supply-air tempering because it transfers moisture
into the dehumidified supply air.)
Integrating the operation of the energy-recovery device into the control
sequences for the air handler and the HVAC system is critical to realize the
anticipated cost savings and reduce the payback period. The overall control
strategy must also account for the psychrometric differences between total- and
sensible-energy recovery.
The rest of this section outlines typical system control sequences for dedicated
OA and mixed-air systems that apply energy recovery for OA preconditioning
and supply-air tempering; see Table 15. Each system control sequence provides
profiles of the psychrometric conditions and operating characteristics that
distinguish modes of control. Using total-energy recovery rather than sensibleenergy recovery requires a slightly different control strategy to account for the
transfer of latent heat (moisture).
Note: System configurations and control sequences other than those described
in this section may require definition of additional and/or different operating
modes.
Table 15. Airside applications for energy recovery
Ventilation System
Application of Recovered Energy
Composition of
Supply Air
Supply-Air
Temperature
Outdoor-Air
Preconditioning
Supply-Air Tempering
Dedicated outdoor air
neutral
total (Figure 58, p. 69) or
sensible (Figure 59, p. 70)
sensible — series or
parallel (Figure 71, p. 81)
cold
total (Figure 61, p. 72) or
sensible (Figure 62, p. 73)
 1
total (Figure 64, p. 75) or
sensible (Figure 65, p. 76)
sensible — series or
parallel (Figure 16, p. 83)
total (Figure 67, p. 78) or
sensible (Figure 68, p. 79)
 2
modulated
Mixed air
(constant volume)
(outdoor air plus
recirculated return air) constant
(variable air volume)
1 Dedicated OA systems that deliver cold, dry ventilation air do not include a reheat function,
2 Mixed-air VAV systems, which deliver constant-temperature, cold supply air to local VAV terminals, do not require
supply-air tempering at the central air handler. Although certain part-load conditions may necessitate local reheating
of the supply-air within a particular space, recovering energy locally for that purpose is seldom economically feasible.
SYS-APM003-EN
67
Precondition
Outdoor Air
System-Level Control
of Recovered Energy
Dedicated OA System for Neutral-Temperature Air
Figure 57 illustrates a typical, dedicated OA system that delivers neutraltemperature conditioned air (CA) directly to each space. Fan-coils placed within
the spaces handle the sensible load. The dedicated OA unit comprises:
Does Section 6.5.2.3 of ASHRAE
Standard 90.1 – 2007 (see inset on p. 4)
prohibit “new energy” reheat in
dedicated OA units that deliver neutraltemperature air for ventilation
purposes?
■
Coils for cooling and heating
■
Constant-volume supply fan
■
Total-energy wheel (shown) or coil loop to precondition the outdoor air
(A bypass damper on the exhaust side of the wheel — or a mixing valve in
the coil loop — modulates capacity based on psychrometric conditions.)
No, Section 6.5.2 only limits new-energy
reheat by defining exceptions —that is,
specific circumstances — in which its use
is permitted.
■
Preheat coil for frost prevention (depending on the climate)
Typically, the quantity of air supplied
by the dedicated OA unit equals the
minimum ventilation airflow required
by ASHRAE Standard 62.1. Exception
A exempts this type of unit/system from
Section 6.5.2.3 because the airflow it
supplies cannot be further reduced
without violating the minimum limit for
proper ventilation. (This exception is
addressed in Example 6-YY of the user’s
manual that accompanies Standard
90.1-2007) Therefore, Standard 90.1
typically will permit the use of new
energy for reheat in most applications of
dedicated OA units.
Figure 58 and Figure 59 (p. 70) outline typical system control sequences that
apply total- and sensible-energy recovery, respectively, to precondition the
entering outdoor air.
Economics will determine whether total- or sensible-energy recovery is the best
choice for OA preconditioning in a particular application. A total-energyrecovery device offers the greatest potential to downsize mechanical cooling
and heating equipment … but a sensible-energy-recovery device may be a better
choice in dry climates where mechanical dehumidification is unnecessary or if
cross-leakage between air streams is a concern.
Figure 57. Dedicated OA system for neutral-temperature air: Total-energy recovery for
OA preconditioning
Examples presented later in this
chapter (pp. 80 – 81 and pp. 84 – 88) show
how an air-to-air, sensible-energyrecovery device can provide reheat. ■
68
SYS-APM003-EN
Precondition
Outdoor Air
System-Level Control
of Recovered Energy
Figure 58. Control sequence for a
dedicated OA system: 
Neutral-temperature air; total-energy
recovery for OA preconditioning
Psychrometric Conditions 1 Control Mode
Control Actions
Comments
1
hoa  hra
DPToa  target DPTca
■
Rotate the total-energy wheel
■
■
Close the exhaust-side bypass damper
All recovered energy lessens the mechanical
cooling load
■
Modulate the cooling coil and reheat coil
■
Closing the bypass damper lets the wheel operate
at full capacity for maximum recovery
■
Cooling coil enforces the CA dew-point limit;
reheat coil satisfies the CA dry-bulb target
■
Wheel capacity modulates to enforce the CA dewpoint limit; cooling coil satisfies the CA dry-bulb
target
■
Stopping the wheel avoids recovering unwanted
heat, assuming that the target CA dew-point is a
maximum limit 3
■
Diverting the exhaust air around the wheel
minimizes fan energy consumption
■
Cooling coil satisfies the CA dry-bulb target
■
Stopping the wheel avoids recovering unwanted
heat, which would increase the mechanical
cooling load
■
Cooling coil enforces the CA dew-point limit;
reheat coil satisfies the CA dry-bulb target
2
DBToa  DBTra
DPToa  target DPTca
3
DBTca  DBToa  DBTra
DPToa  target DPTca
4
hoa  hra
DPToa  target DPTca
Full recovery,
partial cooling,
partial tempering
Partial recovery,
partial cooling
No recovery,
partial cooling
No recovery,
partial cooling,
partial tempering
5
DBToa  DBTca
DPToa  target DPTca
Partial recovery
only
6
DBToa  DBTcritical
DPTca  target DPTca
Full recovery,
supplemental
heating
7
DBToa  DBTfrost threshold
Full recovery,
supplemental
heating, frost
prevention
■
Rotate the total-energy wheel
■
Modulate the exhaust-side bypass damper to
control wheel capacity
■
Modulate the cooling coil
■
Stop the total-energy wheel
■
Fully open the exhaust-side bypass damper 2
■
Modulate the cooling coil
■
Stop the total-energy wheel
■
Fully open the exhaust-side bypass damper 2
■
Modulate the cooling and reheat coils
■
Rotate the total-energy wheel
■
■
Modulate the exhaust-side bypass damper to
control wheel capacity
All recovered energy lessens the mechanical
heating load
■
Do not continuously operate the wheel at full
capacity to avoid overheating/overhumidifying 4
■
Rotate the total-energy wheel
■
■
Close the exhaust-side bypass damper
■
Modulate the heating coil as needed
Heating coil supplements the wheel capacity when
outdoor conditions reach the critical temperature,
satisfying the CA dry-bulb target 5
■
Modulate the preheat coil
■
■
Rotate the total-energy wheel
Preheating the entering OA prevents frost from
forming on the wheel, maximizing recovery
■
Close the exhaust-side bypass damper
■
■
Modulate the heating coil as needed
Heating coil supplements the heat recovered by
the wheel, satisfying the CA dry-bulb target
1 ca = conditioned outdoor air, DBT = dry-bulb temperature, DPT = dew-point temperature, h = enthalpy, oa = outdoor air, ra = return air
2 To respond to the pressure change caused by opening the bypass damper, provide for active control of building pressurization (by modulating the exhaust fan, for example).
3 In some applications, psychrometric conditions represented in Region 3 may entail mechanical humidification. To maintain the target dew point, add moisture to the air stream by
modulating the wheel capacity. Modulate the cooling coil to remove the sensible heat recovered by the wheel and maintain the desired conditioned-air dry-bulb temperature.
4 If the return-air dew point is high, it may be necessary to further reduce the capacity of the wheel to avoid overhumidifying the conditioned air. In that case, modulate the wheel
capacity to maintain the target conditioned-air dew point.
5 The “critical temperature” threshold is defined by the conditioned- and return-air dry-bulb temperatures, and the effectiveness of the energy-recovery device.
SYS-APM003-EN
69
Precondition
Outdoor Air
System-Level Control
of Recovered Energy
Figure 59. Control sequence for a
dedicated OA system: 
Neutral-temperature air; sensible-energy
recovery for OA preconditioning
Psychrometric Conditions 1 Control Mode
Control Actions
Comments
1
DBToa  DBTra
DPToa  target DPTca
■
Start the coil-loop pump
■
All recovered energy lessens the cooling load
■
Fully open the mixing valve (or operate the
pump at full speed)
■
■
Modulate the cooling and reheat coils
Opening the mixing valve (or full-speed pump
operation) lets the coil loop operate at full capacity
for maximum recovery
■
Cooling coil satisfies the CA dew-point target;
reheat coil satisfies the CA dry-bulb target
2
DBToa  DBTra
DPToa  target DPTca
3
DBTca  DBToa  DBTra
DPToa  target DPTca
4
DBToa  DBTra
DPToa  target DPTca
5
DBToa  DBTca
DPToa  target DPTca
6
DBToa  DBTcritical
DPToa  target DPTca
7
DBToa  DBTfrost threshold
Full recovery,
partial cooling,
partial tempering
Full recovery,
partial cooling 2
No recovery,
partial cooling 2
No recovery,
partial cooling,
partial tempering
Partial recovery
only 2
Full recovery,
supplemental
heating 2
Full recovery,
supplemental
heating, frost
prevention 2
■
Start the coil-loop pump
■
All recovered energy lessens the cooling load
■
Fully open the mixing valve (or operate the
pump at full speed)
■
Cooling coil satisfies the CA dry-bulb target
■
Modulate the cooling coil
■
Stop the coil-loop pump
■
Dehumidification is unnecessary
■
Modulate the cooling coil
■
Stopping the coil-loop pump avoids recovering
unwanted heat / increasing the cooling load
■
Cooling coil satisfies the CA dry-bulb target
■
Stopping the coil-loop pump avoids recovering
unwanted heat / increasing the cooling load
■
Cooling coil dehumidifies the entering outdoor air,
satisfies the desired CA dew point
■
Reheat coil maintains the CA dry-bulb target
■
Stop the coil-loop pump
■
Modulate the cooling and reheat coils
■
■
■ All recovered energy lessens the mechanical
heating load
Actuate the mixing valve (or vary pump speed)
■ Modulate the coil-loop capacity to avoid
to control coil-loop capacity
overheating the supply air
Start the coil-loop pump
■
Supplemental heating is unnecessary because the
heating load is less than coil-loop capacity
■
Start the coil-loop pump
■
■
Fully open the mixing valve (or operate the
pump at full speed)
Operating the coil loop at full capacity recovers as
much heat as possible from the exhaust air
■
■
Modulate the heating coil as needed
Below the critical temperature, the heating coil
provides the additional heat needed to satisfy the
CA dry-bulb target 3
■
Start the coil-loop pump
■
■
Actuate the mixing valve (or vary pump speed)
■
Modulate the heating coil as needed
Modulating the coil-loop capacity to prevent frost
formation on the exhaust side of the loop reduces
the amount of recovered energy for heating
■
Heating coil supplements the heat recovered by
the coil loop, satisfying the CA dry-bulb target
1 ca = conditioned air, DBT = dry-bulb temperature, DPT = dew-point temperature, oa = outdoor air, ra = return air
2 Certain applications may require mechanical humidification when outdoor conditions correspond to Regions 2, 4, 5, 6, or 7.
3 The “critical temperature” threshold is defined by the conditioned- and return-air dry-bulb temperatures, and the effectiveness of the energy-recovery device.
70
SYS-APM003-EN
Precondition
Outdoor Air
System-Level Control
of Recovered Energy
Dedicated OA Systems for Cold Air
Figure 60 illustrates a typical, dedicated OA system that delivers cold
conditioned air (CA) to the mixing boxes in multiple blower-coils, which in turn,
deliver the supply air to the occupied spaces. The dedicated OA unit comprises:
■
Coils for cooling and heating
■
Constant-volume supply fan
■
Total-energy wheel (shown) or fixed-plate heat exchanger to precondition
the outdoor air (A bypass damper on the exhaust-side of the wheel, or faceand-bypass dampers on the heat exchanger, modulate capacity based on
psychrometric conditions.)
■
Preheat coil for frost prevention (depending on the climate)
Figure 61 (p. 72) and Figure 62 (p. 73) outline typical system control sequences
that apply total- and sensible-energy recovery, respectively, to precondition the
entering outdoor air.
Figure 60. Dedicated OA system for cold air: Total-energy recovery for OA preconditioning
SYS-APM003-EN
71
Precondition
Outdoor Air
System-Level Control
of Recovered Energy
Figure 61. Control sequence for a dedicated
OA system: Cold air; total-energy recovery
for OA preconditioning
Psychrometric Conditions 1 Control Mode
Control Actions
Comments
1
hoa  hra
■
Rotate the total-energy wheel
■
■
Close the exhaust-side bypass damper
All recovered energy lessens the mechanical
cooling load
■
Modulate the cooling coil
■
Closing the bypass damper lets the wheel operate
at full capacity for maximum recovery
■
Cooling coil satisfies the CA dew-point target
■
Modulating the bypass damper lets the wheel
recover as much as energy as possible, while
maintaining the CA dew point at/below the target
Full recovery,
partial cooling
2
hoa  hra
DBToa  DBTra
DPToa  target DPTca
Partial recovery,
partial cooling
3
DBTca  DBToa  DBTra
DPToa  target DPTca
No recovery,
partial cooling
4
hoa  hra
DPToa  target DPTca
No recovery,
partial cooling
5
DBToa  DBTca
DPToa  target DPTca
Partial recovery
only
6
DBToa  DBTcritical
Full recovery,
supplemental
heating
7
DBToa  DBTfrost threshold
Full recovery,
supplemental
heating, frost
prevention
■
Rotate the total-energy wheel
■
Modulate the exhaust-side bypass damper to
control wheel capacity
■
Modulate the cooling coil
■
Cooling coil satisfies the CA dry-bulb target
■
Stop the total-energy wheel
■
■
Fully open the exhaust-side bypass damper 2
Operating the wheel would increase the cooling
load rather than decrease it 3
■
Modulate the cooling coil
■
Diverting the exhaust air around the wheel
minimizes energy consumption of the exhaust fan
■
Cooling coil satisfies the CA dry-bulb target
■
When outdoor conditions are cool and damp,
operating the wheel would transfer unwanted
sensible heat and possibly moisture to the
conditioned air
■
Stop the total-energy wheel
■
Fully open the exhaust-side bypass damper 2
■
Modulate the cooling coil
■
Cooling coil satisfies the CA dew-point target
■
Rotate the total-energy wheel
■
■
Modulate the exhaust-side bypass damper to
control wheel capacity
All recovered energy lessens the mechanical
heating load
■
Wheel capacity is controlled to achieve the desired
CA dry-bulb temperature without overheating. Do
not continuously operate the wheel at full
capacity: recooling overheated outdoor air will
create a mechanical cooling load
■
Rotate the total-energy wheel
■
■
Close the bypass damper
■
Modulate the heating coil as needed
When outdoor conditions reach the critical
temperature, operating the wheel at full “heating”
capacity cannot satisfy the CA dry-bulb target 4
■
Heating coil supplements the heat recovered by
the wheel, satisfying the CA dry-bulb target
■
Preheating the entering OA prevents frost from
forming on the wheel, maximizing recovery
■
Heating coil supplements the heat recovered by
the wheel, satisfying the CA dry-bulb target
■
Modulate the preheat coil
■
Rotate the total-energy wheel
■
Close the bypass damper
■
Modulate the heating coil as needed
1 ca = conditioned air, DBT = dry-bulb temperature, DPT = dew-point temperature, h = enthalpy, oa = outdoor air, ra = return air
2 To respond to the pressure change caused by opening the bypass damper, provide for active control of building pressurization (by modulating the exhaust fan, for example).
3 In some applications, psychrometric conditions represented in Region 3 may entail mechanical humidification. To maintain the target dew point, add moisture to the air stream by
modulating the wheel capacity. Modulate the cooling coil to remove the sensible heat recovered by the wheel and maintain the desired conditioned-air dry-bulb temperature.
4 The “critical temperature” threshold is defined by the conditioned- and return-air dry-bulb temperatures, and the effectiveness of the energy-recovery device.
72
SYS-APM003-EN
Precondition
Outdoor Air
System-Level Control
of Recovered Energy
Figure 62. Control sequence for a
dedicated OA system: Cold air;
sensible-energy recovery for
OA preconditioning
Psychrometric Conditions 1 Control Mode
Control Actions 2
Comments
1
DBToa > DBTra
■
Close the bypass damper
■
■
Modulate the cooling coil
All recovered energy lessens the mechanical
cooling load
■
Closing the bypass damper lets the fixed-plate
heat exchanger operate at full capacity for
maximum recovery of cooling energy
■
Cooling coil satisfies the CA dry-bulb (or dewpoint) target
■
Opening the bypass damper diverts all exhaust air
around the fixed-plate heat exchanger to prevent
recovery of unwanted heat
■
Cooling coil satisfies the CA dry-bulb (or dewpoint) target
■
All recovered energy lessens the mechanical
heating load
■
Modulate the capacity of the fixed-plate heat
exchanger to avoid overheating the supply air
■
Supplemental heating is unnecessary because the
heating load is less than heat-exchanger capacity
■
Closing the bypass damper directs all exhaust air
through the heat exchanger to recover as much
heat as possible
■
Below the critical temperature, the heating coil
provides the additional heat needed to satisfy the
CA dry-bulb target 3
■
Closing the bypass damper lets the heat
exchanger operate at full capacity
■
Closing the “frost-avoidance” damper on the
supply side of the heat exchanger prevents frost
from forming on the exhaust side 4
■
Heating coil supplements the heat recovered by
the coil loop, satisfying the CA dry-bulb target
2
DBTca  DBToa  DBTra
Full recovery,
partial cooling
No recovery,
partial cooling
Partial recovery
3
DBTcritical  DBToa  DBTca only
4
DBToa  DBTcritical
5
DBToa  DBTfrost threshold
Full recovery,
supplemental
heating
Full recovery,
supplemental
heating, frost
prevention
■
Open the bypass damper
■
Modulate the cooling coil
■
Modulate the exhaust-side face-and-bypass
dampers to control the capacity of the fixedplate heat exchanger
■
Close the bypass damper
■
Modulate the heating coil as needed
■
Close the bypass damper
■
Close the “frost-avoidance” damper
■
Modulate the heating coil as needed
1 ca = conditioned air, DBT = dry-bulb temperature, DPT = dew-point temperature, oa = outdoor air, ra = return air
2 Certain applications may require mechanical humidification when DPToa
 target DPTca.
3 The “critical temperature” threshold is defined by the conditioned- and return-air dry-bulb temperatures, and the effectiveness of the energy-recovery device.
4 If equipped with a “frost-avoidance” damper, a fixed-plate heat exchanger can operate at outdoor temperatures as cold as -20 °F (-29 °C). Where colder conditions are expected, use
a preheat coil for frost prevention.
SYS-APM003-EN
73
Precondition
Outdoor Air
System-Level Control
of Recovered Energy
CV-MA Systems for Modulated-Temperature Supply Air
Figure 63 illustrates a typical constant-volume, mixed-air (CV-MA) system that
modulates the supply-air temperature to meet the changing load in a single
space. The air handler comprises:
■
Coils for cooling and heating
■
Constant-volume supply fan
■
Total-energy wheel with exhaust-side bypass damper (shown) or coil loop
with mixing valve/variable-speed pump for modulated energy recovery
■
Supply-side bypass damper to divert airflow around the energy-recovery
device during economizer operation (when outdoor airflow equals full
supply airflow)
■
Dry-bulb economizer (fixed “ON” point)
■
Preheat coil for frost prevention
Figure 64 and Figure 65 (p. 76) outline typical control sequences that apply
total- and sensible-energy recovery, respectively, to precondition the entering
outdoor air.
Note: Diverting airflow around the energy-recovery device during economizer
operation saves fan energy by “subtracting” the related static pressure drop. As
an additional benefit, the energy-recovery device can be sized to handle only
ventilation airflow rather than economizer (full supply) airflow, reducing the
first cost.
Figure 63. CV-MA system for modulated-temperature air: Total-energy recovery for
OA preconditioning
74
SYS-APM003-EN
Precondition
Outdoor Air
System-Level Control
of Recovered Energy
Figure 64. Control sequence for a
CV-MA system: 
Modulated-temperature SA; total-energy
recovery for OA preconditioning
Psychrometric Conditions 1
Control Mode
Control Actions
Comments
1
hoa  hra 2
Full recovery,
partial cooling
■
Rotate the total-energy wheel
■
■
Close both bypass dampers
All recovered energy lessens the mechanical
cooling load
■
Modulate the cooling coil
■
Closing the bypass dampers lets the wheel
operate at full capacity for maximum recovery
■
Cooling coil satisfies the space dry-bulb setpoint
■
Operating the wheel would increase the
mechanical cooling load
■
Diverting the air around the wheel minimizes fan
energy consumption
■
Cooling coil satisfies the space setpoint
■
Operating the wheel would increase the
mechanical cooling load
■
Cooling coil supplements the economizer to
satisfy the space setpoint
■
Economizer handles the entire cooling load
■
Supplemental heating is unnecessary because the
heating load is less than wheel capacity
■
Modulating the exhaust-side bypass damper
prevents overheating
■
For systems without economizers, implement this
mode when DBTma  DBTsa 4
■
Closing both bypass dampers lets the wheel
operate at full capacity
2
DBToa  DBTecon ON
hoa  hra
No recovery,
partial cooling
3
No recovery,
DBTecon ON  DBToa  DBTsa economizer,
hoa  hra
partial cooling
4
DBToa  DBTsa
Vot  minimum setpoint
No recovery,
economizer only
5
Vot  minimum setpoint
DBToa  DBTcritical
Partial recovery
only
6
DBToa  DBTcritical
7
DBToa  DBTfrost threshold
■
Stop the total-energy wheel 3
■
Fully open both bypass dampers
■
Modulate the cooling coil
■
Stop the total-energy wheel 3
■
Fully open both bypass dampers
■
Fully open the economizer dampers 4
■
Modulate the cooling coil
■
Stop the total-energy wheel 3
■
Fully open both bypass dampers
■
Modulate the outdoor- (economizer) and
return-air dampers 4
■
Rotate the total-energy wheel
■
Close the supply-side bypass damper
■
Modulate the exhaust-side bypass damper to
control the wheel capacity 3
Full recovery,
supplemental
heating
■
Rotate the total-energy wheel
■
Close both bypass dampers
■
Modulate the heating coil as needed
■
Below the critical temperature, the heating coil
adds enough heat to satisfy the space setpoint 5
Full recovery,
supplemental
heating, frost
prevention
■
Modulate the preheat coil
■
■
Rotate the total-energy wheel
Preheating the entering OA prevents frost from
forming on the wheel, maximizing recovery
■
Close both bypass dampers
■
■
Modulate the heating coil as needed
Heating coil supplements the heat recovered by
the wheel, satisfying the space setpoint
1 DBT = dry-bulb temperature, econ ON = economizer “ON” point, h = enthalpy, ma = mixed air, oa = outdoor air, ra = return air, sa = supply air, Vot = total (system) outdoor airflow
 DBTra. Basing operation on dry-bulb temperature rather
than enthalpy may miss many hours of energy recovery; also, it may yield hours of wheel operation that increase the mechanical cooling load.
2 If dry-bulb (rather than enthalpy) control is used, the Region 1 control mode governs operation of the wheel when DBToa
3 Do not continuously operate the total-energy wheel at full capacity when outdoor conditions correspond to Regions 2, 3, 4, or 5. Recooling overheated air increases energy
consumption at the cooling plant.
4 In a system without an economizer, omit the control modes for Regions 3 and 4. Implement Region 5 operation when DBTma < DBTsa.
5 The “critical temperature” threshold is defined by the conditioned- and return-air dry-bulb temperatures, and the effectiveness of the energy-recovery device.
SYS-APM003-EN
75
Precondition
Outdoor Air
System-Level Control
of Recovered Energy
Figure 65. Control sequence for a
CV-MA system: Modulated-temperature
SA; sensible-energy recovery for
OA preconditioning
Psychrometric Conditions 1
Control Mode
Control Actions
Comments
1
DBToa  DBTra
Full recovery,
partial cooling
■
Start the coil-loop pump
■
All recovered energy lessens the cooling load
■
Close both bypass dampers
■
■
Fully open the mixing valve
■
Modulate the cooling coil
Opening the mixing valve (or full-speed pump
operation) lets the coil loop operate at full capacity
for maximum recovery
■
Cooling coil satisfies the space dry-bulb setpoint
■
Stopping the coil-loop pump avoids recovering
unwanted heat , increasing the cooling load
■
Opening the bypass dampers saves fan energy by
diverting airflow around the coil-loop coils
■
Cooling coil satisfies the space setpoint
■
Stopping the coil-loop pump avoids recovering
unwanted heat, increasing the cooling load
■
Cooling coil supplements the economizer to
satisfy the space setpoint
■
Stopping the coil-loop pump avoids recovering
unwanted heat, increasing the cooling load
■
Opening the bypass dampers saves fan energy by
diverting airflow around the coil-loop coils
2
No recovery,
DBTecon ON  DBToa  DBTra partial cooling
3
DBToa  DBTecon ON
DPToa  DBTsa
No recovery,
economizer,
partial cooling 2
■
Stop the coil-loop pump
■
Fully open both bypass dampers
■
Modulate the cooling coil
■
Stop the coil-loop pump
■
Fully open both bypass dampers
■
Fully open the OA (economizer) dampers
■
Modulate the cooling coil as needed
4
DBToa  DBTsa
Vot  minimum setpoint
No recovery,
■ Stop the coil-loop pump
economizer only 2 ■ Fully open both bypass dampers
5
Vot  minimum setpoint
DBToa  DBTcritical
Partial recovery
only 2
6
DBToa  DBTcritical
Full recovery,
supplemental
heating
7
DBToa  DBTfrost threshold
■
Modulate the outdoor- (economizer) and
return-air dampers
■
Economizer satisfies the space setpoint
■
Start the coil-loop pump
■
All recovered energy lessens the heating load
■
Close both bypass dampers
■
■
Start the coil-loop pump
■
■
Close both bypass dampers
Operating the coil loop at full capacity recovers as
much heat as possible from the exhaust air
■
Fully open the mixing valve
■
■
Modulate the heating coil as needed
Below the critical temperature, the heating coil
adds enough heat to satisfy the space setpoint 4
■
Start the coil-loop pump
■
■
Close both bypass dampers
■
Actuate the mixing valve (or vary pump speed)
Modulating the coil-loop capacity to prevent frost
formation on the exhaust side of the loop reduces
the amount of recovered energy for heating
■
Modulate the heating coil as needed
■
Heating coil supplements the heat recovered by
the coil loop, satisfying the space setpoint
■
Full recovery,
supplemental
heating, frost
prevention
Modulate the coil-loop capacity to avoid
overheating the supply air
Actuate the mixing valve (or vary pump speed)
■ Supplemental heat is unnecessary because the
to control coil-loop capacity 3
heating load is less than coil-loop capacity
1 DBT = dry-bulb temperature, econ ON = economizer “ON” point, ma = mixed air, oa = outdoor air, ra = return air, sa = supply air, Vot = total (system) outdoor airflow
2 If the system does not include an economizer, omit the control modes for Regions 3 and 4. Implement Region 5 operation when DBTma
 DBTsa.
3 Alternatively, the exhaust-side bypass damper can be modulated to control the capacity of the coil loop.
4 The “critical temperature” threshold is defined by the conditioned- and return-air dry-bulb temperatures, and the effectiveness of the energy-recovery device.
76
SYS-APM003-EN
Precondition
Outdoor Air
System-Level Control
of Recovered Energy
VAV-MA Systems for Constant-Temperature Supply Air
Figure 66 illustrates a typical mixed-air (MA) system that delivers a variable
volume of cold supply air to local, variable-air-volume (VAV) terminals. The air
handler comprises:
Trane engineering bulletin
CLCH-PRB009-EN, Recirculation
Energy Recovery Unit Controls, provides
more detailed control recommendations
for VAV systems. ■
■
Coils for cooling and heating
■
Supply fan with variable-speed drive
■
Total-energy wheel with exhaust-side bypass damper (shown) or coil loop
with mixing valve/variable-speed pump for modulated energy recovery
■
Supply-side bypass damper to divert airflow around the energy-recovery
device during economizer operation (when outdoor airflow equals full
supply airflow)
■
Economizer (comparative enthalpy control for total-energy recovery; fixed
dry-bulb control for sensible-energy recovery)
■
Pressure-independent, flow-measuring damper to maintain the minimum
required outdoor airflow
■
Preheat coil for frost prevention (depending on climate)
Figure 67 (p. 78) and Figure 68 (p. 79) outline typical control sequences that
apply total- and sensible-energy recovery, respectively, for OA preconditioning.
Note: Diverting airflow around the energy-recovery device during economizer
operation saves fan energy by “avoiding” the related static pressure drop. Size
the energy-recovery device to handle only ventilation airflow rather than
economizer (full supply) airflow, which will reduce the first cost.
Figure 66. VAV system for constant-temperature air: Total-energy recovery for
OA preconditioning
SYS-APM003-EN
77
Precondition
Outdoor Air
System-Level Control
of Recovered Energy
Figure 67. Control sequence for a VAV
system: Constant-temperature SA;
total-energy recovery for OA
preconditioning
Psychrometric Conditions 1 Control Mode
Control Actions
Comments
1
hoa  hra 2
■
Rotate the total-energy wheel
■
All recovered energy lessens the cooling load
■
Close both bypass dampers
■
■
Modulate the cooling coil
Closing the bypass dampers lets the wheel
operate at full capacity for maximum recovery
■
Cooling coil satisfies the SA dry-bulb setpoint
■
Stopping the wheel avoids transferring unwanted
heat and moisture, increasing the cooling load
2
DBToa  DBTsa
hoa  hra
Full recovery,
partial cooling
No recovery,
economizer,
partial cooling 3, 4
3
DBToa  DBTsa
Vot  minimum setpoint
No recovery,
economizer
only 3, 4
4
Vot  minimum setpoint
DBToa  DBTcritical
Partial recovery
only 3, 5
5
DBToa  DBTcritical
6
DBToa  DBTfrost threshold
■
Stop the total-energy wheel
■
Fully open both bypass dampers
■
Fully open the economizer dampers
■
Opening the bypass dampers saves fan energy
■
Modulate the cooling coil
■
Cooling coil supplements the economizer to
satisfy the SA dry-bulb setpoint
■
Stop the total-energy wheel
■
■
Fully open both bypass dampers
Economizer satisfies the SA dry-bulb setpoint,
handling the entire cooling load
■
Modulate the outdoor- (economizer) and
return-air dampers
■
Opening the bypass dampers saves fan energy
■
Supplemental heating is unnecessary because the
heating load is less than wheel capacity
■
Modulating the exhaust-side bypass damper
avoids overheating the supply air
■
For systems without economizers, implement this
mode when DBTma  DBTsa
■
Closing both bypass dampers lets the wheel
operate at full capacity
■
Rotate the total-energy wheel
■
Close the supply-side bypass damper
■
Modulate the exhaust-side bypass damper to
control wheel capacity
Full recovery,
supplemental
heating
■
Rotate the total-energy wheel
■
Close both bypass dampers
■
Modulate the heating coil as needed 6
■
Below the critical temperature, the heating coil
adds enough heat to satisfy the SA setpoint 7
Full recovery,
supplemental
heating, frost
prevention
■
Modulate the preheat coil
■
■
Rotate the total-energy wheel
Preheating the entering outdoor air prevents frost
from forming on the wheel, maximizing recovery
■
Close both bypass dampers
■
■
Modulate the heating coil as needed 6
Heating coil supplements the heat recovered by
the wheel, satisfying the SA dry-bulb setpoint
1 DBT = dry-bulb temperature, h = enthalpy, ma = mixed air, oa = outdoor air, ra = return air, sa = supply air, Vot = total (system) outdoor airflow
 DBTra. Basing operation on dry-bulb temperature rather
than enthalpy may miss many hours of energy recovery; also, it may yield hours of wheel operation that increase the mechanical cooling load.
2 If dry-bulb (rather than enthalpy) control is used, the Region 1 control mode governs operation of the wheel when DBToa
3 Do not continuously operate the total-energy wheel at full capacity when outdoor conditions correspond to Region 2, 3, or 4. Overheating the outdoor air stream increases the cooling
load and the energy consumption of the cooling plant.
4 If the system includes a dry-bulb (rather than comparative enthalpy) economizer, the economizer will not activate in the Region 2 control mode until DBToa
 DBTecon ON. If the system
does not include an economizer, omit the control mode for Region 3; implement Region 4 operation when DBTma < DBTsa.
5 In mixed-air VAV systems, a total-energy wheel may provide free heat at outdoor temperatures as cold as those in Region 6, effectively extending Region 4 operation through Region 5
conditions (eliminating the Region 5 control mode).
6 If the VAV terminals include sufficient heating capacity to handle the morning warm-up function, the wheel may recover enough heat (in some climates) to eliminate the need for a
centralized heating coil.
7 The “critical temperature” threshold is defined by the conditioned- and return-air dry-bulb temperatures, and the effectiveness of the energy-recovery device.
78
SYS-APM003-EN
Precondition
Outdoor Air
System-Level Control
of Recovered Energy
Figure 68. Control sequence for a
VAV system: Constant-temperature SA;
sensible-energy recovery for OA
preconditioning
Psychrometric Conditions 1 Control Mode
Control Actions
Comments
1
DBToa  DBTra
■
Start the coil-loop pump
■
All recovered energy lessens the cooling load
■
Close both bypass dampers
■
■
Fully open the mixing valve (or operate the
pump at full speed)
Operating the coil loop at full capacity (bypass
dampers closed; mixing valve open / highest pump
speed) maximizes the recovery of cooling energy
■
Modulate the cooling coil
■
Cooling coil satisfies the SA dry-bulb setpoint
■
Stop the coil-loop pump
■
■
Fully open both bypass dampers
Stopping the coil-loop pump avoids recovering
unwanted heat , increasing the cooling load
■
Modulate the cooling coil
■
Opening the bypass dampers saves fan energy
■
Cooling coil satisfies the SA dry-bulb setpoint
■
Stopping the coil-loop pump avoids recovering
unwanted heat , increasing the cooling load
2
DBToa  DBTra
DBToa  DBTecon ON
3
DBToa  DBTecon ON
DPToa  DBTsa
4
DBToa  DBTsa
Vot  minimum setpoint
5
Vot  minimum setpoint
DBToa  DBTcritical
6
DBToa  DBTcritical
7
DBToa  DBTfrost threshold
Full recovery,
partial cooling
No recovery,
partial cooling
No recovery,
economizer,
partial cooling 2
No recovery,
economizer only 2
Partial recovery
only 2
Full recovery,
supplemental
heating
Full recovery,
supplemental
heating, frost
prevention
■
Stop the coil-loop pump
■
Fully open both bypass dampers
■
Fully open the OA (economizer) damper
■
Opening the bypass dampers saves fan energy
■
Modulate the cooling coil as needed
■
Cooling coil supplements the economizer to
satisfy the SA dry-bulb setpoint
■
Stop the coil-loop pump
■
■
Fully open both bypass dampers
Stopping the coil-loop pump avoids recovering
unwanted heat , increasing the cooling load
■
Modulate the outdoor- (economizer) and
return-air dampers
■
Opening the bypass dampers saves fan energy
■
Economizer satisfies the SA dry-bulb setpoint
■
Start the coil-loop pump
■
All recovered energy lessens the heating load
■
Close both bypass dampers
■
■
Modulate the coil-loop capacity (mixing valve,
pump speed, or exhaust-side damper) 3
Modulate the coil-loop capacity to avoid
overheating the supply air / creating a cooling load
■
Supplemental heat is unnecessary because the
heating load is less than coil-loop capacity
■
Start the coil-loop pump
■
■
Close both bypass dampers
Operating the coil loop at full capacity (wide-open
mixing valve, for example) maximizes recovery
■
Fully open the mixing valve
■
■
Modulate the heating coil as needed
Below the critical temperature, the heating coil
adds enough heat to satisfy the SA setpoint 4
■
Start the coil-loop pump
■
■
Close both bypass dampers
■
Actuate the mixing valve (or vary pump speed)
Modulating the coil-loop capacity to prevent frost
formation on the exhaust side of the loop reduces
the amount of recovered energy for heating
■
Modulate the heating coil as needed
■
Heating coil supplements the heat recovered by
the coil loop, satisfying the SA dry-bulb setpoint
1 DBT = dry-bulb temperature, econ ON = economizer “ON” point, ma = mixed air, oa = outdoor air, ra = return air, sa = supply air, Vot = total (system) outdoor airflow
2 If the system does not include an economizer, omit the control modes for Regions 3 and 4. Implement Region 5 operation when DBTma
 DBTsa.
3 Alternatively, the exhaust-side bypass damper can be modulated to control the capacity of the coil loop.
4 The “critical temperature” threshold is defined by the conditioned- and return-air dry-bulb temperatures, and the effectiveness of the energy-recovery device.
SYS-APM003-EN
79
Temper
Supply Air
System-Level Control
of Recovered Energy
Dedicated OA System for Neutral-Temperature Air
Figure 69. Air handler
with fixed-plate heat exchanger
Does Section 6.5.2.3 of ASHRAE
Standard 90.1 – 2007 (see inset on p. 4)
prohibit “new energy” reheat in
dedicated OA units that deliver neutraltemperature air for ventilation
purposes?
Recall from the beginning of this section (p. 66) that recovering sensible energy
for supply-air tempering can benefit systems that directly control the supply-air
dew point or the humidity in the space. Figure 69 shows an example of an air
handler that tempers conditioned air with energy recovered by the series
arrangement of a fixed-plate heat exchanger. In Figure 70, the air handler
becomes part of a dedicated OA system that delivers neutral-temperature air
directly to the occupied spaces. Fan-coils installed in occupied spaces handle
the local sensible loads. The central air handler, which serves as a dedicated OA
unit, comprises:
■
Coils for cooling and heating
■
Constant-volume supply fan
■
Fixed-plate heat exchanger (arranged in a series or “wraparound”
configuration) with integral face-and-bypass dampers to modulate capacity
Figure 71 outlines a typical control sequence for this system.
Figure 70. Dedicated OA system for neutral-temperature air: Sensible-energy recovery
for tempering
No, Section 6.5.2 only limits new-energy
reheat by defining exceptions —that is,
specific circumstances — in which its use
is permitted.
Typically, the quantity of air supplied
by the dedicated OA unit equals the
minimum ventilation airflow required
by ASHRAE Standard 62.1. Exception
A exempts this type of unit/system from
Section 6.5.2.3 because the airflow it
supplies cannot be further reduced
without violating the minimum limit for
proper ventilation. (This exception is
addressed in Example 6-YY of the user’s
manual that accompanies Standard
90.1-2007) Therefore, Standard 90.1
typically will permit the use of new
energy for reheat in most applications of
dedicated OA units.
Examples presented later in this
chapter (pp. 80 – 81 and pp. 84 – 88) show
how an air-to-air, sensible-energyrecovery device can provide reheat. ■
80
SYS-APM003-EN
Temper
Supply Air
System-Level Control
of Recovered Energy
Figure 71. Control sequence for a dedicated
OA system: Neutral-temperature air;
sensible-energy recovery for tempering
Psychrometric
Conditions 1
1
DPToa  DPTca setpoint
2
DPToa  DPTca setpoint
DBToa  DBTca setpoint
3
DPToa  DPTca setpoint
DBToa  DBTca setpoint
Control Mode
Control Actions
Comments
Dehumidification,
recovery for tempering,
supplemental tempering
as needed
■
Modulate the cooling coil
■
■
Modulate the face-and-bypass dampers
■
Modulate the heating coil as needed
Sensible heat transferred from the outdoor
air (upstream of the cooling coil) to the air
(downstream of the cooling coil) lessens the
cooling and tempering (reheat) loads
■
Cooling coil dehumidifies the entering
outdoor air to maintain the desired leavingcoil dew-point temperature
■
Heat exchanger satisfies the CA dry-bulb
setpoint. Closing the bypass dampers lets
the fixed-plate heat exchanger operate at full
capacity
■
Heating coil provides the additional heat
needed (if any) to satisfy the CA dry-bulb
setpoint
■
Dehumidification and tempering are
unnecessary
■
Cooling coil maintains the desired leavingcoil dry-bulb temperature
■
Opening the bypass dampers saves fan
energy and prevents unwanted heat transfer
by diverting airflow around the fixed-plate
heat exchanger
■
Dehumidification and sensible cooling are
unnecessary
■
Opening the bypass dampers saves fan
energy and prevents unwanted heat transfer
■
Heating coil satisfies the CA dry-bulb target
No dehumidification,
no recovery,
partial sensible cooling
No dehumidification,
no recovery,
partial heating 2
■
Modulate the cooling coil
■
Open the bypass dampers
■
Turn off the cooling coil
■
Open the bypass dampers
■
Modulate the heating coil as needed
1 ca = conditioned air, DBT = dry-bulb temperature, DPT = dew-point temperature, oa = outdoor air, sa = supply air
2 If the fixed-plate heat exchanger is configured for parallel (rather than series) energy recovery, it can provide sensible heating when outdoor conditions correspond to Region 3.
Control actions for parallel energy-recovery during Region 3 are:
■
Modulate the face-and-bypass dampers (energy-recovery capacity) to prevent the heat exchanger from overheating the conditioned air.
■
If the capacity of the heat exchanger cannot warm the outdoor air to the desired temperature, modulate the heating coil to provide supplemental heat.
■
In cold climates, Region 3 operation may also require frost prevention if the heat exchanger is used to precondition the outdoor air.
SYS-APM003-EN
81
Temper
Supply Air
System-Level Control
of Recovered Energy
CV-MA Systems for Modulated-Temperature Supply Air
Constant-volume, mixed-air (CV-MA) systems that modulate the supply-air
temperature must provide tempering if they directly control the dry-bulb
temperature and the humidity in the space. The CV-MA system represented in
Figure 72 modulates the temperature of the supply air to meet the changing
load in a single space. The system also directly controls space humidity by
overcooling (to dehumidify) and then tempering (reheating) the supply air.
The air handler comprises:
■
Coils for cooling and heating
■
Constant-volume supply fan
■
Coil loop, arranged in a series (“wraparound”) configuration, for supply-air
tempering (A mixing valve provides capacity control.)
Table 16 outlines a typical CV-MA control sequence.
Note: Although the reheat coil uses new energy, compliance with ASHRAE
Standard 90.1 – 2007 can be achieved by sizing the coil loop to provide at least
75 percent of the required tempering (reheat) energy.
Figure 72. CV-MA system for modulated-temperature SA: Sensible-energy recovery for
SA tempering
82
SYS-APM003-EN
Temper
Supply Air
System-Level Control
of Recovered Energy
Table 16. Control sequence for a CV-MA system: Modulated-temperature SA; sensible-energy recovery for SA tempering
Psychrometric
Conditions 1
Control Mode
1
Partial cooling,
RHspace  RHmax limit
no recovery for tempering
DBTspace  DBTcooling setpoint
2
RHspace  RHmax limit
Dehumidification,
partial recovery for tempering,
supplemental tempering as
needed
3
Partial heating,
RHspace  RHmax limit
no recovery for tempering 3
DBTspace  DBTheating setpoint
Control Actions
Comments
■
Stop the coil-loop pump
■
Standard cooling mode
■
Modulate the cooling coil
■
Cooling coil satisfies the space dry-bulb
setpoint
■
Stopping the coil-loop pump avoids
transferring unwanted heat / overheating the
space
■
Modulate the cooling coil
■
Dehumidification mode 2
■
Start the coil-loop pump
■
■
Actuate the mixing valve (or vary pump
speed) to control coil-loop capacity
■
Modulate the heating coil as needed
Cooling coil overcools and dehumidifies the
supply air, enforcing the maximum relativehumidity limit (60% RH, for example) in the
space
■
Coil loop transfers sensible heat from the
outdoor air (upstream of the cooling coil) to
the overcooled supply air (downstream of
the cooling coil)
■
Modulating the coil-loop capacity maintains
the space dry-bulb setpoint
■
Heating coil supplements the heat recovered
by the coil-loop to satisfy the space setpoint
■
Turn off the cooling coil
■
Standard heating mode
■
Stop the coil-loop pump
■
■
Modulate the heating coil as needed
Stopping the coil-loop pump prevents
unwanted heat transfer
■
Heating coil satisfies the space setpoint
1 DBT = dry-bulb temperature, RH = relative humidity
2 Supply-air tempering and, therefore, air-to-air energy recovery only occur when space conditions correspond to Region 2.
3 Regardless of whether sensible energy is transferred using a series arrangement (as in the example CV-MA system described on p. 82) or recovered using a parallel arrangement,
the control actions for Region 3 operation are identical.
SYS-APM003-EN
83
System-Level Control
of Recovered Energy
Applications that Combine
OA Preconditioning and SA Tempering
Depending on the performance criteria, certain applications may benefit from
a system design that simultaneously preconditions outdoor air and tempers
supply air using two energy-recovery devices.
Dual-Function, Dedicated OA System
for Neutral-Temperature Air
Dedicated outdoor-air systems that deliver neutral-temperature conditioned air
are excellent candidates for dual-purpose energy recovery. The example in
Figure 73 shows a dedicated OA unit that not only handles the sensible and
latent loads of the outdoor air stream, but also sufficiently dehumidifies (lowers
the dew point of) the outdoor air to satisfy local latent loads. Fan-coils installed
in each space only handle the local sensible loads.
The dedicated OA unit comprises:
■
Preheat coil for frost prevention (depending on climate)
■
Total-energy wheel to precondition the entering outdoor air (Exhaust-side
bypass dampers provide capacity control.)
■
Cooling coil for dehumidification
■
Fixed-plate heat exchanger, in a parallel energy-recovery arrangement, for
supply-air tempering (Face-and-bypass dampers provide capacity control.)
Figure 73. Dual-function, dedicated OA system for neutral-temperature air
84
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Figure 74. Dedicated OA unit with two energy-recovery devices
■
Heating coil for supplemental tempering
■
Constant-volume supply fan
The elevation view in Figure 74 illustrates one possible physical arrangement of
these components within an air-handler casing.
Benefits of Dual-Function Energy Recovery
Plotting psychrometric performance reveals the benefits of dual-function
energy recovery for a dedicated OA system that delivers neutral-temperature
conditioned air. The example in Figure 75 uses the design conditions for
Jacksonville, Florida.
Cooling design. The total-energy wheel cools and dehumidifies the entering
outdoor air to 70.5°F DB, 62.7°F WB (21.4°C DB, 17.1°C WB). As the wheel
rotates, sensible heat and moisture collected from the outdoor air transfer to
Figure 75. Psychrometric performance for a dedicated OA system with dual-function
energy recovery (neutral-temperature CA)
HX = fixed-plate heat exchanger
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System-Level Control
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the exhaust air stream, which was precooled by the fixed-plate heat exchanger.
Precooling the exhaust air increases the amount of transferred heat.
Cooling-energy savings:
Q T = 4.5  10 000 cfm   39.3 – 28.3 Btu/lb 
Q T = 495 000 Btu/hr = 41.2 tons
 Q T = 1.2  4.7 m³/s   91.4 – 65.8 kJ/kg  
 Q T = 144 kW 
Reheat-energy savings:
Q s = 1.085  10 000 cfm   67.0°F – 53.5°F 
Q s = 146 475 Btu/hr = 146.5 MBh
 Q s = 1.21  4.7 m³/s   19.4°C – 11.9°C  
 Q s = 42.7 kW 
Heating-energy savings:
Q s = 1.085  10 000 cfm   62.8°F – 31°F 
Q s = 345 030 Btu/hr = 345 MBh
 Q s = 1.21  4.7 m³/s   17.1°C –  – 0.8 °C   
 Q s = 102 kW 
Meanwhile, the cooling coil dehumidifies the air to the desired conditioned-air
dew point. When the dehumidified air passes through the fixed-plate heat
exchanger, the sensible heat recovered from the exhaust air stream transfers to
the conditioned air stream. If the fixed-plate heat exchanger cannot warm the
conditioned air to the “neutral” target, 71°F DB (21.7°C DB) in this case, the
heating coil provides the additional tempering needed.
In this example, dual-function energy recovery reduces the mechanical cooling
load by 41.2 tons (144 kW), a 64-percent reduction. It also reduces the “newenergy” reheat load by 146.5 MBh (42.7 kW), a 77-percent reduction.
Heating design. Dehumidification and, therefore, supply-air tempering are
unnecessary for Jacksonville’s heating design condition, so the fixed-plate heat
exchanger is not used (that is, the bypass damper opens). Instead, the totalenergy wheel heats and humidifies the entering outdoor air and the heating coil
provides the additional sensible heat needed to reach the target conditioned-air
temperature, a neutral 70°F (21.1°C) in this case.
In this example, the total-energy wheel reduces the sensible load on the heating
coil by 345 MBh (102 kW), an 82-percent reduction.
Note: Because the fixed-plate heat exchanger in this example is arranged for
parallel energy recovery, it could potentially transfer heat from the exhaust air
stream; however, we discourage this practice. Removing heat from the exhaust
air stream reduces the capacity of the total-energy wheel and raises the
threshold temperature for frost prevention. The high effectiveness of a totalenergy wheel may allow it to recover more heat, overall, without operation of
the fixed-plate heat exchanger.
Control Sequence
Figure 76 outlines a typical control sequence for a “dual-function” dedicated
outdoor-air system (Figure 73, p. 84), which uses air-to-air energy recovery for
both outdoor-air preconditioning and supply-air tempering.
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Figure 76. Control sequence for a
dedicated OA system with dual-function
energy recovery
Psychrometric Conditions 1 Control Mode
Control Actions
Comments
1
h oa  h ra
DPToa  DPTca target
Full recovery for
■
preconditioning,
■
partial cooling,
■
partial recovery for
■
tempering,
supplemental tempering
Rotate the wheel (close the bypass damper) ■
Modulate the cooling coil
■
Modulate the face-and-bypass dampers
■
Modulate the reheat coil as needed
2
DBToa  DBTra
DPToa  DPTca target
Partial recovery for
preconditioning,
partial cooling,
no recovery for
tempering
■
Rotate the wheel (modulate the bypass
damper)
■ Modulate the cooling coil
■ Open the heat-exchanger bypass damper
■
3
DBTca  DBToa  DBTra
DPToa  DPTca target
No recovery for
preconditioning, partial
cooling, no recovery for
tempering 2
■
■
Wheel operation would increase the cooling
load
■ Cooling coil satisfies the CA dry-bulb target
■ Bypassing the wheel and heat exchanger saves
fan energy
4
h oa  h ra
DPToa  DPTca target
No recovery for
■
preconditioning,
■
partial cooling,
■
partial recovery for
■
tempering,
supplemental tempering
Stop the wheel (open the bypass damper) 3 ■ Wheel operation would increase the cooling
load
Modulate the cooling coil
■ Cooling coil maintains the CA dew point at/
Modulate the face-and-bypass dampers
below the target
Modulate the reheat coil as needed
■ Heat exchanger satisfies the CA dry-bulb target,
with help of the reheat coil, if necessary
5
Partial recovery for
DBTcritical  DBToa  DBTca preconditioning 4
DPToa  DPTca target
Stop the total-energy wheel (open the
bypass damper to save fan energy) 3
■ Modulate the cooling coil
■ Open the heat-exchanger bypass damper
■
Rotate the wheel (modulate the bypass
damper)
■ Open the heat-exchanger bypass damper
6
DBToa  DBTcritical
DPToa  DPTca target
Full recovery for
preconditioning,
supplemental heating
■
7
DBToa  DBTfrost threshold
Full recovery for
preconditioning,
supplemental heating,
frost prevention
■
Wheel operates at full capacity for maximum
recovery
Cooling coil maintains the target CA dew point
Heat exchanger satisfies the CA dry-bulb target
with help from the reheat coil, if necessary
Dehumidification, SA tempering are
unnecessary
■ Wheel recovers cooling energy while
maintaining the CA dew point at/below the
target
■ Cooling coil satisfies the CA dry-bulb target
■ Bypassing the heat exchanger avoids unwanted
heat transfer and saves fan energy
■
Wheel satisfies the CA dry-bulb target without
overheating
■ Bypassing the heat exchanger saves fan energy
Rotate the wheel (close the bypass damper) ■ Wheel operates a full capacity in cold weather
Modulate the heating coil
■ Heating coil satisfies the CA dry-bulb target
■ Open the heat-exchanger bypass damper
■ Bypassing the heat exchanger saves fan energy
■
Modulate the preheat coil
■
Rotate the wheel (close the bypass damper)
■
■ Open the heat-exchanger bypass damper
■
■ Modulate the heating coil
■
Preheat coil protects the wheel from frost and
maximizes energy recovery
Bypassing the heat exchanger saves fan energy
Heating coil satisfies the CA dry-bulb target
1 ca = conditioned air, DBT = dry-bulb temperature, DPT = dew-point temperature, h = enthalpy, oa = outdoor air, ra = return air
2 Region 3 operation described above is based on the assumption that DPTca target is a maximum limit. In applications that require mechanical humidification, modulate the wheel
capacity to maintain the desired conditioned-air dew point and modulate the cooling coil to maintain the target dry-bulb temperature.
3 To respond to the pressure change caused by opening the bypass damper, provide for active control of building pressurization (by modulating the exhaust fan, for example).
4 Do not continuously operate the wheel at full capacity when Region 5 conditions exist. To avoid overhumidification when the return-air dew point is unusually high, modulate the
wheel capacity to maintain the desired conditioned-air dew point; use partial recovery for tempering to achieve the target dry-bulb temperature.
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Alternatives for Tempering 
with Sensible-Energy Recovery
The “dual-function,” dedicated OA system introduced on p. 84 tempers supply
air with sensible energy recovered by a fixed-plate heat exchanger. Substituting
a coil loop, a heat pipe, or a sensible-energy wheel as in Figure 77 will not
significantly alter the control sequence in Figure 76 (p. 87), provided that the
design includes a means for controlling the capacity of the tempering device.
Figure 77. “Dual-function” dedicated OA unit with sensible-energy wheel for tempering
In contrast to a total-energy wheel,
which is a passive desiccant device, an
active desiccant device adds heat to
“regenerate” the desiccant.
The Trane applications engineering
manual, Dehumidification in HVAC
Systems (order number SYS-APM004EN), further discusses “active”
desiccant devices. It is available from
the Trane bookstore at www.trane.com/
bookstore. ■
Tempering (Recooling) 
in an Active-Desiccant System
Figure 78 illustrates a dedicated OA system with active-desiccant
dehumidification. The system includes two energy-recovery devices and uses
a different psychrometric process to perform the same role as the “dualfunction,” dedicated OA system:
■
An active-desiccant wheel, which conditions the outdoor air by heating and
dehumidifying the air stream rather than cooling and dehumidifying it
■
A sensible wheel, which tempers the conditioned air by recooling the air
stream rather than reheating it
The active-desiccant wheel removes moisture from the outdoor air stream,
dehumidifying it to a low dew point; it also transfers sensible heat to the
conditioned air. The dry conditioned air then passes through a sensible-energy
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Figure 78. “Dual-function,” dedicated OA system with active-desiccant wheel
wheel, which removes sensible heat and rejects it into the exhaust air stream
(as shown in Figure 78) or into a separate outdoor air stream. A cooling coil
further reduces the temperature of the conditioned air to a nearly neutral
condition.
On the regeneration side of the system, the sensible-energy wheel preheats the
exhaust air stream with sensible heat recovered from the conditioned air. A gasfired heat exchanger further heats the “regeneration” air stream. Depending on
the desired conditioned-air dew point, the temperature of the “regeneration”
air entering the active-desiccant wheel typically ranges from 130°F to 250°F
(54°C to 121°C).
Increasing the regeneration temperature of the active-desiccant wheel allows it
to remove more moisture from the conditioned air stream; however, it further
increases the dry-bulb temperature of the conditioned air leaving this device
and, therefore, requires more recooling. ■
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Suggestions for Effective
Airside Energy Recovery
The preceding chapters discussed how air-to-air energy recovery can reduce
the costs of operating an HVAC system … and may even permit downsizing of
mechanical equipment used for heating, cooling, humidification, and
dehumidification. This section summarizes the key points from those chapters;
it also offers general recommendations to promote the effective use of air-to-air
energy-recovery devices in HVAC applications. Economic considerations, local
energy codes, and other issues such as spatial constraints, maintenance
requirements, and air-stream locations will ultimately determine the feasibility
of air-to-air energy recovery for a particular project.
OA Preconditioning
When an application that preconditions
outdoor air requires recovered energy to
comply with ASHRAE Standard
90.1 – 2007 (see pp. 2 – 4), the total
effectiveness of the air-to-air energyrecovery device must be at least 
50 percent. ■
Some local codes require the use of recovered energy for outdoor-air (OA)
preconditioning. Although relatively few installations are currently affected by
these mandates, many more could economically benefit from this use of airside
energy recovery. The benefits of preconditioning outdoor air with recovered
energy accrue during both cooling and heating operation — and they are not
confined to hot, humid climates. In fact, the largest economic benefits typically
result in climates where both cooling and heating demands exist.
Sensible- or Total-Energy Recovery?
Budgetary constraints generally determine whether a sensible- or total-energy
recovery device best suits a particular system design. In most climates, a totalenergy recovery device improves the opportunity for downsizing the cooling
and heating equipment (and usually provides the best payback) because it
recovers both sensible and latent energy. The most notable exceptions are dry
climates where it is unnecessary to mechanically dehumidify the outdoor air.
Certified performance permits
meaningful comparisons of air-to-air
energy-recovery devices. Look for
equipment that bears the ARI 1060 seal
(page 61), and include ARI-certified
performance when specifying energyrecovery devices for OA preconditioning
applications. ■
90
Which technologies offer the best value? Trane engineers studied five airto-air energy-recovery technologies — heat pipes, sensible-energy wheels, totalenergy wheels, coil loops, and fixed-plate heat exchangers — to determine their
value in OA preconditioning applications. The analysis compared the first costs
(normalized) of providing a defined rate of heat transfer at various volumes of
airflow. Figure 79 summarizes the results of that study. At every point of
comparison, the total-energy wheels offer the best value — that is, the lowest
first cost for a given quantity of transferred heat.
The best value among the sensible-energy-recovery technologies depends on
airflow capacity. At less than 15,000 cfm (7 m³/s), fixed-plate heat exchangers
provide the best value; at higher airflows, coil loops prevail. Given the higher
first costs of recovering the same amount of energy, heat pipes and sensibleenergy wheels provide the least value; however, they also deliver the required
performance across a broader range of airflows.
SYS-APM003-EN
Suggestions for Effective
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Figure 79. Comparison of airside energy-recovery technologies for OA preconditioning
Of course, intangible issues such as the condition of the exhaust air stream
must be considered when selecting an air-to-air energy recovery device. Coil
loops, for example, become the best energy-recovery option when crossleakage between air streams is unacceptable.
Total-energy recovery in predominantly heating climates. “Only hot,
humid climates justify total energy recovery because the device can function as
a dehumidifier during many hours of the year.” … “Buildings in cold climates
require many hours of heating, so why pay a higher price to recover total
energy when sensible-energy recovery will suffice?”
Too often, these misperceptions influence the design of HVAC systems that
precondition outdoor air with recovered energy. When compared with sensibleenergy recovery, however, total-energy-recovery devices can provide significant
operating advantages in climates where heating operation prevails:
SYS-APM003-EN
■
Frost forms at a much colder outdoor temperature, which allows the
total-energy-recovery device to recover more heat during cold weather and
lessens the need for preheat (see p. 55).
■
The generally higher effectiveness of total-energy recovery devices saves
more heating energy and permits further downsizing of the heating plant.
■
Moisture recovered by a total-energy-recovery device humidifies (as well as
warms) the entering outdoor air during the heating season, which helps
keep the space from becoming too dry. “Free” humidification also reduces
the energy used by the mechanical humidification system and permits
equipment downsizing.
■
Most heating climates include a cooling season. Applying a total-energyrecovery device enables a larger reduction in cooling-plant capacity, which
can substantially reduce the first-cost premium for energy recovery.
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Suggestions for Effective
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Table 17 compares the expected number of annual operating hours for one
system when it is applied in two distinctly different climates: Jacksonville,
Florida, where cooling operation prevails; and Minneapolis, Minnesota, where
heating operation prevails. The mixed-air VAV system includes a comparative
enthalpy-economizer and uses a total-energy-recovery device to precondition
the outdoor air stream. (For a detailed description of the control sequence, see
Figure 67, p. 78.) The analysis reveals two interesting points:
■
Despite the difference in climate, the total number of operating hours for the
energy-recovery device —represented by Regions 1, 4, 5, and 6 — is roughly
the same in both locations … 1,647 hours in Jacksonville and 1,489 hours in
Minneapolis.
■
Centralizing the rest-room-exhaust airflow for the Minneapolis system
significantly reduced the amount of “new” energy needed for supplemental
heating. The reduction results from more hours of “free” heating provided
by the energy-recovery device (Region 4) coupled with fewer hours that
require supplemental heat (Region 5).
Preconditioning outdoor air with recovered energy may yield even greater
economic benefits in other climates where heating predominates.
Table 17. Suggested control modes to precondition outdoor air in a VAV system
Annual Hours of Operation 2
Psychrometric Conditions 1
Control Mode
(see Figure 67, p. 78)
1
h oa  h ra
Full recovery,
partial cooling
1,585 hr
(61 %)
468 hr
( 18 %)
2
DBToa  DBT sa
H oa  H ra
No recovery,
economizer,
partial cooling
672 hr
(26 %)
701 hr
(27 %)
3
DBToa  DBT sa
Vot  minimum setpoint
No recovery,
economizer only
281 hr
( 11 %)
410 hr
( 16 %)
4
Vot  minimum setpoint
DBT oa  DBT critical
Partial recovery only
62 hr
[62 hr
( 2 %)
( 2 %)]
496 hr
[853 hr
( 19 %)
(32 %)]
5
DBT oa  DBT critical
Full recovery,
supplemental heating
0 hr
( 0 %)
481 hr
[124 hr
( 18 %)
( 5 %)]
6
DBT oa  DBT frost threshold
Full recovery,
supplemental heating,
frost prevention
0 hr
( 0 %)
44 hr
(2 %)
Jacksonville, Fla.
Minneapolis, Minn.
1 DBT = dry-bulb temperature, h = enthalpy, oa = outdoor air, ra = return air, sa = supply air, Vot = total (system) outdoor
airflow
2 Annual operation is based on 10,000 cfm (4.7 m³/s) of outdoor air and 7,000 cfm (3.3 m³/s) of system-level exhaust air.
Values enclosed in brackets [ ] represent annual operation that is based on system-level exhaust air plus local exhaust
air from rest rooms, for a total exhaust airflow of 9,500 cfm (4.5 m³/s).
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Central or Local OA Preconditioning?
When the design of an HVAC system includes multiple air handlers, energy
recovery for OA preconditioning can be implemented in one of two ways:
■
A single, large energy-recovery device can precondition all of the outdoor
air before delivering it to the individual air handlers. Known as central OA
preconditioning, this approach may reduce the first cost of the system.
■
By contrast, local OA preconditioning devotes an individual energy-recovery
device to each air handler.
If the energy-recovery device is packaged with supply- and exhaust-side fans, it
is called an energy-recovery ventilator or ERV.
Central OA preconditioning with multiple, dedicated OA systems.
Figure 80 illustrates a central ERV, which preconditions the outdoor air stream
using a total-energy wheel and then delivers that air to two or more dedicated
OA units. Although this configuration reduces the number of wheels (not the
energy-recovery capacity) for the installation, ducting the exhaust air from all of
the air handlers back to a single ERV typically requires more ductwork.
Control of a central OA-preconditioning ERV can be straightforward, but only if
the dedicated OA units share the same operating schedule and deliver air at
identical conditions (dew point, dry-bulb temperature). A more complex, less
energy-efficient control sequence is needed to maintain proper building
pressurization if the individual systems deliver air at different conditions:
■
The relief damper associated with each dedicated OA unit must modulate to
appropriately regulate building pressure in the areas served by that unit.
■
The exhaust-side fan in the ERV must modulate to assure that none of the
relief dampers reaches the wide-open position.
Figure 80. Central OA preconditioning for two or more dedicated OA units
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Suggestions for Effective
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Central OA preconditioning with multiple mixed-air systems. Figure 81
shows a central total-energy wheel that preconditions the outdoor air before
delivering it to the OA inlet of each air handler. Unlike the example in Figure 80,
however, each air handler now includes a path for recirculated return air (RRA),
making it a mixed-air system. Additional ductwork is needed to carry the
conditioned outdoor air from the energy-recovery ventilator to the air handlers.
Of course, carrying the exhaust air from each of the air handlers back to the ERV
typically requires more ductwork, too.
Adding an airside economizer to each air handler further complicates the
design and increases the first cost. As shown in Figure 82, each air handler
requires an additional damper to modulate the economizer airflow. Installing
the air handlers indoors will require another path and more ductwork to deliver
the outdoor air needed for economizer cooling.
The additional air paths, dampers, and complexity of maintaining proper
building pressurization can pose a formidable challenge for the engineer
charged with designing an HVAC system that combines central OA
preconditioning and multiple mixed-air systems … especially when the air
handlers include airside economizers. The system is easiest to control during
extremely hot or cold weather: the wheel simply operates at full capacity. At
other operating conditions, however, the air handlers may send conflicting
control signals to the energy-recovery ventilator. For example, psychrometric
conditions at one air handler may require partial recovery to heat the outdoor
air while conditions at another air handler dictate economizer operation without
recovery. Satisfying both demands will unavoidably sacrifice part of the
potential energy savings.
Figure 81. Central OA preconditioning for multiple mixed-air systems
(without economizers)
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Figure 82. Central OA preconditioning for multiple mixed-air systems (with economizers)
To help address the control challenges of central OA preconditioning for
multiple mixed-air systems:
■
Use pressure-independent OA dampers at the air handlers to assure
delivery of the proper quantity of outdoor air for ventilation.
■
Provide a means to modulate the ERV supply-side fan so that it only delivers
the quantity of air required by the operating air handlers.
■
Control building pressurization directly. The relief damper at each
air handler must modulate to control building pressurization in the areas
served, and the ERV exhaust-side fan must modulate to assure that none of
the relief dampers reaches the wide-open position.
Note: Centralizing the rest-room exhaust at the ERV further complicates
control of the exhaust-side fan.
Minimizing Life-Cycle Cost
Analyze the options. Economically, the main reason for preconditioning
outdoor air with recovered energy is the opportunity to downsize cooling and
heating equipment. If the first-cost savings do not entirely offset the initial
investment in the energy-recovery device, operating cost savings may quickly
make up the difference.
Energy analysis software can make it easier to estimate the performance of
various air-to-air energy-recovery technologies. Performing a whole-building
analysis helps you predict the length of the payback period based on building
SYS-APM003-EN
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Airside Energy Recovery
location, type and operating schedule, as well as HVAC system type, system
control, type of economizer, and local utility rates. Make sure that the analysis
accounts not only for economizer operation, but also for the effects of the
energy-recovery device —that is, the cooling and heating energy savings, as
well as the extra fan energy needed to overcome the additional pressure drop.
Only size for ventilation airflow. For mixed-air systems, size the energyrecovery device to condition only the minimum outdoor airflow required for
ventilation, not full economizer airflow. This design strategy permits the
selection of a smaller-capacity device, which lowers the first cost. To save fan
energy, include bypass dampers or a separate path to direct airflow around the
energy-recovery device during economizer operation.
Strive for balanced airflows. Local exhausts, infiltration, and (potentially)
cross-leakage create an imbalance between the incoming outdoor air stream
and the system-level exhaust air stream. To maximize both energy recovery
(which reduces operating costs) and potential downsizing of the heating and
cooling plants, duct as much of the local exhaust airflow to the energy-recovery
device as possible. Remember: The less disparity between the outdoor and
exhaust airflows, the more energy that can be recovered.
Figure 83 shows an example of a mixed-air VAV system that channels the
rest-room exhaust back to a central point and mixes it with the system exhaust
immediately upstream of the energy-recovery device. Because the entering
air pressure on the exhaust side of the wheel changes, the system must
somehow maintain constant exhaust airflow from the rest rooms to comply
with building codes. A pressure-independent, flow-measuring damper — which
operates in concert with the relief damper in the return air path — is the most
effective way to monitor and control the total quantity of rest-room exhaust air.
Figure 83. Mixed-air VAV system with centralized rest-room exhaust
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If the flow-measuring damper is wide open and the exhaust-side pressure is
high enough that the required rest-room-exhaust airflow cannot be maintained,
the relief damper modulates closed to lower the exhaust-side pressure. The
exhaust fan then modulates (using a variable-speed drive, for example) to
directly control the building pressure relative to outdoors.
Note: Consider specifying a “minimum flow” alarm to notify the operator
whenever the building pressure prevents the rest-room exhaust fans from
providing the required airflow.
Implement a carefully considered control strategy to lower operating
costs and shorten the payback period. It is important to integrate control of the
energy-recovery device with the control schemes for the unit and the system.
Factory-installed controls with preprogrammed operating strategies can
simplify engineering in the field and help to assure that the system delivers
optimum performance.
The previous section (“System-Level Control of Recovered Energy”)
demonstrated the importance of devising a control strategy that accounts for
the type of system, the type of recovered energy (total or sensible), and — in the
case of mixed-air systems — the control and type of airside economizer. To
avoid overheating the air, most systems will require a means to modulate
energy-recovery capacity when the device operates in the heating mode. (See
“Energy-Recovery Technologies,” pp. 38 – 64.) Unnecessarily operating the
device at full capacity requires recooling and wastes energy.
Common modulation methods include:
■
Mixing valves (coil loops)
■
Variable-speed drives (coil loops, rotary heat exchangers)
■
Bypass or face-and-bypass dampers (rotary heat exchangers, fixed-plate
heat exchangers, heat pipes)
■
Tilt control (heat pipes)
Capacity modulation of the energy-recovery device may also serve another
function: frost prevention for OA preconditioning applications in cold climates.
Note: For cold-climate applications, consider adding a preheat coil to ensure
that the temperature of the entering outdoor (or exhaust) air remains above the
frost threshold. Preheat permits the energy-recovery device to operate at full
capacity during cold weather, maximizing the amount of recovered energy.
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SA Tempering
To comply with ASHRAE Standard
90.1 –2007 (see pp. 2 – 4), an air-to-air
energy-recovery device that is used for
tempering supply air must be capable of
providing at least 75 percent of the
energy used to reheat the air. ■
“Energy” codes may actually require the use of energy recovery for supply-air
(SA) tempering in specific applications — most notably in constant-volume,
mixed-air systems that directly control space humidity. (See “Compliance with
Codes,” p. 2.) Even where it is not mandated, however, tempering supply air
with recovered energy often makes economic sense, especially for dedicated
outdoor-air systems that deliver neutral-temperature air.
Sensible- or Total-Energy Recovery?
Supply-air tempering adds sensible heat to the air downstream of the cooling
coil. Because the cooling coil also dehumidifies the passing air stream, an
energy-recovery device that transfers moisture as well as sensible heat is
undesirable. For this reason, sensible-energy recovery is usually the best choice
for SA tempering applications.
Series or Parallel Arrangement?
As discussed on pp. 22 – 26 and pp. 28 – 32, sensible-energy recovery for supplyair tempering is accomplished using one of two arrangements:
Figure 84. Energy-recovery arrangements
for SA tempering in mixed-air systems
■
A series configuration, which relies on the air entering the cooling coil as a
source for varying amounts of transferable heat
■
A parallel configuration, which can recover a relatively constant amount of
heat from the return air stream.
The best choice for a particular application depends, in part, on whether the
HVAC system uses a mixed-air or a dedicated-outdoor-air design. Other
important factors include first cost, savings in cooling and heating energy, and
the additional fan energy needed to overcome the static pressure loss imposed
by the energy-recovery device.
Mixed-air systems. Both series and parallel configurations (Figure 84) reduce
the heating energy required for tempering. Although the series configuration
saves more cooling energy than the parallel configuration, neither arrangement
permits downsizing of the cooling and heating plants.
Outdoor conditions ultimately determine which arrangement provides the
highest potential for energy savings. The series arrangement can transfer more
heat if the outdoor air is warmer than the return air; but when the outdoor air is
cooler than the return air, the parallel arrangement will recover more sensible
energy. In many applications, cooler outdoor conditions require more
tempering.
98
SYS-APM003-EN
Suggestions for Effective
Airside Energy Recovery
Figure 85. Energy-recovery arrangements
for tempering in dedicated OA systems
Dedicated OA systems. Budgetary constraints often simplify the choice
between series and parallel energy-recovery arrangements in dedicated OA
systems that deliver neutral-temperature air. Most of these applications use the
series arrangement because it avoids the expense of routing the building
exhaust to a central location (Figure 85). Beyond that consideration, however,
the series arrangement:
■
Is only available during cooling modes of operation
■
Transfers more heat (and saves more new “reheat” energy) than the parallel
arrangement whenever the outdoor air is warmer than the exhaust air.
■
Saves cooling energy and may even reduce the required capacity of the
cooling plant
By comparison, the parallel arrangement:
■
Is available during cooling and heating modes of operation
■
Transfers more heat (and saves more new “reheat” energy) than the series
arrangement whenever the outdoor air is cooler than the exhaust air
■
May recover enough heat from the exhaust air stream to permit downsizing
of the heating plant
Note: Adding a second energy-recovery device to precondition the outdoor air
lessens the first-cost penalty of the parallel arrangement. (See pp. 84 – 88 for an
example of a “dual-function” energy-recovery system.)
Which technologies offer the best value? Trane engineers analyzed the
performance of three types of sensible-energy-recovery devices — heat pipes,
fixed-plate heat exchangers, and coil loops — to determine their practicality for
tempering supply air. (Sensible-energy wheels were omitted from the study
because the high first cost often precludes their use.) The analysis compared
the first costs (normalized) of providing a defined rate of heat transfer at various
airflows. Figure 86 (p. 100) summarizes the results of that study.
Of the devices studied, fixed-plate heat exchangers and coil loops offer the best
value for supply-air tempering. At less than 12,000 cfm (5.6 m³/s), fixed-plate
heat exchangers provide the specified performance at the lowest first cost; at
higher airflows, coil loops prevail. Although heat pipes perform across a
broader range of airflows, their first cost is considerably higher than the other
devices in the study.
Initial investment, though important, is only one consideration when choosing
between sensible-energy-recovery technologies. Other factors include space
limitations, maintenance requirements, and (for parallel arrangements) the
location and condition of the exhaust air stream. For example, a coil loop
becomes the best energy-recovery option when cross-leakage is unacceptable,
despite its higher first cost at low volumes of airflow.
SYS-APM003-EN
99
Suggestions for Effective
Airside Energy Recovery
Figure 86. Comparison of airside energy-recovery technologies for SA tempering
Minimizing Life-Cycle Cost
Analyze the options. The economic justification for tempering supply air
with recovered energy typically lies in saving as much reheat energy as
possible and minimizing the negative effect on fan energy consumption. When
applied in a dedicated OA system and depending on the arrangement (series or
parallel), sensible-energy recovery may also reduce the required capacity of the
cooling or heating plant. If so, the first cost savings from equipment downsizing
can help offset the initial investment in energy recovery.
Energy analysis software makes it easier to estimate and compare the cooling
and heating energy savings provided by various air-to-air energy-recovery
technologies. Performing a whole-building analysis will help you predict the
length of the payback period based on building location, building type and
operating schedule, HVAC system type, system control, and local utility rates.
Make sure that the analysis accounts not only for the humidity levels inside the
building during various operating modes, but also for the extra fan energy
needed to overcome the pressure drop of the energy-recovery device.
Implement a carefully considered control strategy. Proper control of
the sensible-energy-recovery device is essential to realize the anticipated
energy savings. Optimum performance depends on integrating the control
schemes of the energy-recovery device and air handlers with the rest of the
HVAC system. Factory-installed controls with preprogrammed operating
strategies can simplify engineering in the field and help to assure that the
system delivers optimum performance.
“Dedicated OA System for Neutral-Temperature Air” (pp. 80 – 83) demonstrated
the importance of devising an operating strategy that accounts for the type of
100
SYS-APM003-EN
Suggestions for Effective
Airside Energy Recovery
ventilation system (mixed-air or dedicated outdoor air) and the arrangement of
the energy-recovery device (series or parallel). For most systems, the control
scheme must also modulate energy-recovery capacity in some way to avoid
overheating the supply air. Common methods of modulation include:
■
Mixing valves (coil loops)
■
Variable-speed drives (coil loops, rotary heat exchangers)
■
Bypass or face-and-bypass dampers (rotary heat exchangers, fixed-plate
heat exchangers, heat pipes)
■
Tilt control (heat pipes)
Review “Energy-Recovery Technologies,” pp. 38 –64, for more information. ■
SYS-APM003-EN
101

Glossary
active-desiccant wheel. Rotary device that removes moisture from the
passing air stream with a desiccant-coated media and releases it to an adjacent
air stream. It is typically regenerated at high temperatures from an external
heat source. Also called dehumidification wheel.
air handler (AHU). A piece of equipment that moves and/or conditions air for
maintaining space comfort.
air-to-air energy recovery. The transfer of sensible heat, or sensible plus
latent heat, between two or more air streams, or between two locations within
the same air stream.
ANSI. American National Standards Institute (www.ansi.org)
ANSI/ASHRAE Standard 62.1. Titled Ventilation for Acceptable Indoor Air
Quality, this standard specifies minimum ventilation rates to minimize the
potential for adverse health effects of buildings.
ANSI/ASHRAE/IESNA Standard 90.1. Titled Energy Standard for Buildings
Except Low-Rise Residential Buildings, this standard sets minimum design
requirements that encourage energy efficiency throughout the building — that
is, for the envelope, lighting, motors, HVAC, and service-water heating systems.
ARI Standard 1060. Titled Performance Rating of Air-to-Air Heat Exchangers
for Energy Recovery Ventilation Equipment, this standard defines the
requirements for testing, rating, and marking factory-made energy-recovery
devices, including rotary heat exchangers, heat pipes, and fixed-plate heat
exchangers.
ASHRAE. American Society of Heating, Refrigerating, and Air-Conditioning
Engineers (www.ashrae.org)
coil loop. An energy-recovery device consisting of two or more finned-tube
coils that are piped together in a closed loop to transfer sensible heat. A small
pump circulates the working fluid through the two coils. Also called coil
runaround loop or runaround loop.
conditioned outdoor air (CA). Outdoor air that has been heated, cooled,
humidified, or dehumidified by a dedicated outdoor-air unit before being
delivered directly to the conditioned space or to the mixing boxes of other local
units. Also called conditioned air.
constant-volume system. Type of air-conditioning system that varies the
temperature of a constant volume of air supplied to meet the changing load
conditions of the space.
critical temperature. Psychrometric condition at which an energy-recovery
device, operating at full heating capacity, can no longer maintain the target
supply-air dry-bulb temperature. It is defined by the dry-bulb temperatures of
102
SYS-APM003-EN

Glossary
the supply air and return air, and the effectiveness of the energy-recovery
device at full capacity:
·
m s   T critical – T 2 
 sensible = -------------------------------------------------·
m e   T critical – T 3 
where,
 sensible = sensible effectiveness
m· s
= mass flow rate, in lb /hr (kg/s), through the supply side of the
heat-exchange device
m· e
= mass flow rate, in lb/hr (kg/s), through the exhaust side of the
Tcritical
= critical entering supply-air temperature, in °F (°C)
T2
= target leaving supply-air dry bulb, in °F (°C)
T3
= entering exhaust-air dry bulb, in °F (°C)
heat-exchange device
dedicated outdoor-air unit. An air handler used to cool, heat, dehumidify, or
humidify all of the outdoor air brought into the building for ventilation. This
conditioned outdoor air may be delivered directly to the conditioned spaces or
to other air handlers or terminal equipment. Also called makeup-air unit or
100% outdoor-air unit.
desiccant. An absorbent or adsorbent substance, liquid or solid, that removes
water or water vapor from a material. Activated alumina and silica gel are two
desiccants that are commonly used as drying agents in refrigerating systems.
economizer air. Outdoor air brought into the building to offset the cooling
load in the space.
effectiveness. A measure used to quantify the performance of an air-to-air
heat exchanger. Defined as the ratio of the amount of energy (sensible, latent,
or total) that a device actually transfers divided by the maximum energy
transfer possible between the two air streams. Also called effectiveness ratio.
energy-recovery unit. An air handler or rooftop air conditioner that includes
an integral (built-in) energy-recovery device for outdoor-air preconditioning. It
also includes a cooling coil within the same unit casing.
energy-recovery ventilator. An air handler that consists of an energyrecovery device for outdoor-air preconditioning, a supply fan, and an exhaust
fan; it does not include a cooling coil. These units are typically used to
precondition outdoor air before delivering it to other air handlers or rooftop air
conditioners.
exhaust air (EA). Air removed from the conditioned space and expelled from
the building by mechanical ventilation.
SYS-APM003-EN
103

Glossary
exhaust-air transfer ratio (EATR). A measurement, defined by ARI
Standard 1060, used to quantify cross-leakage of air from the exhaust air
stream to the supply air stream. Expressed as a percentage of supply airflow.
fixed-membrane heat exchanger. An energy-recovery device that consists
of multiple, paper-like membranes arranged in a cross-flow configuration to
transfer both sensible (temperature) and latent (moisture) heat.
fixed-plate heat exchanger. An energy-recovery device that consists of
multiple, thermally conductive plates arranged in a cross-flow configuration to
transfer sensible heat.
frost threshold. Temperature at which frost begins to form on the exhaust
side of an air-to-air energy-recovery device.
heat pipe. An energy-recovery device consisting of refrigerant-filled, finned
tubes that transfer sensible heat between the two ends of the tubes.
latent-design condition. Design dew-point and mean coincident dry-bulb
temperatures, as tabulated in the ASHRAE Handbook–Fundamentals.
neutral air. Air that is delivered to the conditioned space at a dry-bulb
temperature and humidity ratio that does not impose a heating or cooling load
in the space.
outdoor air (OA). Air brought into the building — either by a ventilation
system or through openings provided for natural ventilation — from outside the
building. (Also called outside air or fresh air, but outdoor air is preferred.
outdoor air correction factor (OACF). A measurement, defined by ARI
Standard 1060, used to quantify cross-leakage of air from the supply air stream
to the exhaust air stream. Expressed as the supply-side airflow upstream of the
device divided by the supply-side airflow downstream of the device.
outdoor-air preconditioning. The process of using an air-to-air energyrecovery device to cool, dehumidify, heat, or humidify outdoor air by
transferring energy (either sensible or sensible plus latent) to and from another
air stream.
parallel energy recovery. A configuration that transfers heat between one air
stream and a second, separate air stream.
recirculated return air (RRA). Air removed from the conditioned space(s)
and reused as supply air — usually after passing through an air-cleaning and
-conditioning system — for delivery to the conditioned space. See also return air
(RA).
104
SYS-APM003-EN

Glossary
return air (RA). Air removed from the conditioned space(s) and either
recirculated or exhausted. See also recirculated return air (RRA) and exhaust air
(EA).
rotary heat exchanger. An energy-recovery device that consists of a
revolving cylinder that rotates between two air streams. Air streams pass
through a matrix of channels that direct air through the device in a counterflow
arrangement. Rotary heat exchangers can transfer either sensible energy or
sensible plus latent energy. Also called wheel.
sensible-design condition. Design dry-bulb and mean coincident wet-bulb
temperatures, as tabulated in the ASHRAE Handbook – Fundamentals.
sensible-energy recovery. The transfer of sensible heat between two or
more air streams or between two locations within the same air stream.
series energy transfer. A configuration that transfers heat from one
location in an air stream to a second location within the same air stream. This
configuration, used only in supply-air tempering applications, reduces the need
for tempering (reheating) with new energy.
sensible-energy wheel. Rotary heat exchanger that transfers sensible heat
from one air stream and another. Also called heat wheel.
supply air (SA). Air delivered to the conditioned space by mechanical means
for ventilation, heating, cooling, humidification, or dehumidification.
supply-air tempering. The process of adding sensible heat to the air
downstream of the cooling coil to allow independent control of latent and
sensible loads in the space. Only systems that directly control supply-air dew
point or space humidity are candidates for supply-air tempering.
total-energy recovery. The transfer of sensible and latent (moisture) heat
between two or more air streams or between two locations within the same air
stream.
total-energy wheel. A desiccant-coated, rotary heat exchanger that transfers
sensible (temperature) and latent (moisture) heat between two air streams. The
desiccant typically regenerates at room temperatures. Also called enthalpy
wheel or passive desiccant wheel.
variable-air-volume (VAV) system. Type of air-conditioning system that
varies the volume of constant-temperature air, which is supplied to meet the
changing load conditions in the space. ■
SYS-APM003-EN
105

References
Air-Conditioning and Refrigeration Institute (ARI). 2005. Performance Rating of
Air-to-Air Heat Exchangers for Energy Recovery Ventilation Equipment, ARI
Standard 1060 – 2005. Arlington, VA: ARI.
American Society of Heating, Refrigeration and Air-Conditioning Engineers, Inc.
(ASHRAE). 1991. Method of Testing Air-to-Air Heat Exchangers, ASHRAE
Standard 84–1991. Atlanta, GA: ASHRAE.
. 2004. Air-to-Air Energy Recovery. In Chapter 44, Systems and Equipment.
ASHRAE Handbook series.
. 2007. Ventilation for Acceptable Indoor Air Quality, ASHRAE
Standard 62.1–2007.
ASHRAE and Illuminating Engineering Society of North America (IESNA). 2007.
Energy Standard for Buildings Except Low-Rise Residential Buildings, BSR/
ASHRAE/IESNA Standard 90.1–2007. Atlanta, GA: ASHRAE.
Murphy, J. 2006. "Smart Dedicated Outdoor Air Systems," ASHRAE Journal
(July): pp. 30-37.
To obtain copies of Trane publications,
including videocassettes, contact your
local Trane office. Some publications are
are also available from the Trane’s
online bookstore at www.trane.com/
bookstore. ■
Trane. 2003. Designing Dedicated Outdoor-Air Systems, SYS-APG001-EN. 
La Crosse, WI: Inland Printing Company.
. 1999. Air-to-Air (Fixed-Plate Type) Heat Exchanger Module, CLCH-EB-43
(March).
. 1999. Psychrometry, TRG-TRC001-EN. Air Conditioning Clinic series.
. 2000. “Air-to-Air Energy Recovery,” Engineers Newsletter Live satellite
broadcast, APP-APV006-EN (October 27, videocassette). La Crosse, WI: AVS
Group.
. 2001. Energy Wheel Overview for Modular and T-Series Climate
Changer ™ Air Handlers, CLCH-PRB0012-EN.
. 2000. Recirculation Energy Recovery Unit Controls, CLCH-PRB009-EN.
. 2001. “Dedicated Outdoor-Air Ventilation Systems,” Engineers Newsletter
Trane publishes the Engineers
Newsletter several times annually.
Back issues, including the newsletters
listed here, are available from the
Trane Web site at www.trane.com/
engineersnewsletter. ■
106
Live satellite broadcast, APP-APV008-EN (September 19, videocassette). La
Crosse, WI: AVS Group.
 : Stanke, D. and Bradley, B. 2000. “Dehumidify with Constant-Volume
Systems.” Trane Engineers Newsletter, volume 29, number 4.
SYS-APM003-EN

References
 : Stanke, D. and Bradley, B. 2001. “Design Tips for Effective, Efficient
Dedicated Ventilation Systems.” Trane Engineers Newsletter, volume 30,
number 3.
 : Stanke, D., Murphy, J., and Bradley, B. 2000. “Air-to-Air Energy
Recovery.” Trane Engineers Newsletter, volume 29, number 5.
United States Environmental Protection Agency (EPA). Indoor Environments
Division, Office of Air and Radiation. 2000. Energy Cost and IAQ Performance of
Ventilation Systems and Controls, EPA-402-S-01-001 (January). www.epa.gov/
iaq/largebldgs
Wright, W. 1984. “Control of Coil Loop Heat Recovery Systems”, HPAC
Engineering (August): pp. 69–71. ■
SYS-APM003-EN
107

Index
a
active-desiccant wheels 88 – 89
adjacent air streams 13 – 14, 32, 33
air blenders 57
Air-Conditioning, Heating, and Refrigeration Institute (AHRI) 60
air-handler casing
potential path for cross-leakage 50
airside pressure drop
bypass dampers to reduce 35
coil loops 38 – 40
effect of outdoor-air preconditioning with recovered energy 16
effect of supply-air tempering with recovered energy 35
fixed-plate heat exchangers 44, 45
heat pipes 47 – 48
rotary heat exchangers 52 – 53
summary of typical performance for energy-recovery devices 64
ANSI/ASHRAE Standard 62.1 2
acceptable cross-leakage 20
requirements for outdoor-air intakes 19 – 20
ANSI/ASHRAE Standard 84 7
ANSI/ASHRAE/IESNA Standard 90.1
discussion about 2 – 3
economizer operation 17
excerpts from 3, 4
implications for reheat 68
reheat limitations for supply-air tempering 21, 67, 80
sizing components for heating and tempering 80, 82
ARI (see Air-Conditioning, Heating, and Refrigeration Institute)
ARI Standard 1060 6, 60 – 63, 64
ARI Standard 410 6, 60
ASHRAE Standard 62.1 (see ANSI/ASHRAE Standard 62.1)
ASHRAE Standard 90.1 (see ANSI/ASHRAE/IESNA Standard 90.1)
auxiliary heat (see supplemental heat and reheat)
b
balanced airflows 7 – 8
benefits 96 – 97
building exhaust
proximity to outdoor-air intake 19 – 20
building pressure 97
direct control 95
bypass dampers
capacity control 53 – 55
economizer operation 3, 16 – 17, 41, 96
exhaust-air bypass for linear unloading 54
fan energy consumption 35, 41
outdoor-air bypass for frost prevention 56 – 57
c
capacity control 64, 97
coil loops 40 – 41
fixed-plate heat exchangers
108
44 – 45
SYS-APM003-EN

Index
for frost prevention 19, 50, 56 – 57, 97
heat pipes 49 – 50
outdoor-air preconditioning with recovered energy 17 – 18
rotary heat exchangers 53 – 55
supply-air tempering with recovered energy 35 – 36
central outdoor-air preconditioning
for multiple dedicated outdoor-air systems 93
for multiple mixed-air systems 94 – 95
certified performance 60 – 63
coil loops 38 – 43, 64
three-coil 32, 42 – 43
coincident heating and cooling 3
cold conditioned air
defined 65
sensible-energy recovery for outdoor-air preconditioning 73
total-energy recovery for outdoor-air preconditioning 72
compressor short-cycling 14
constant-volume mixed-air systems 66
outdoor-air preconditioning 74 – 76
supply-air tempering 82 – 83
control sequences
dual-function energy recovery 86 – 87
outdoor-air preconditioning with constant-volume mixed-air systems 75, 76
outdoor-air preconditioning with dedicated outdoor-air systems 69, 70, 72, 73
outdoor-air preconditioning with variable-air-volume systems 78, 79
supply-air tempering 81, 83
control strategies
integration 67, 97, 100
cooling coil
humidity control 21, 26
cooling-plant capacity
effect of balanced airflows 8
effect of outdoor-air preconditioning 14 – 15
effect of supply-air tempering with parallel energy recovery 25, 30
effect of supply-air tempering with series energy recovery 24, 29
supply-air tempering with dedicated outdoor-air systems 32 – 33
critical temperature
defined 102 – 103
cross-leakage 20, 37
air-handler casing 42, 46, 50
coil loops 42
effect of fan configuration 58
exhaust air transfer ratio 62
fixed-plate heat exchangers 46
heat pipes 50 – 51
likelihood of (table) 64
purge 59
rest-room exhaust 57
rotary heat exchangers 57 – 59
using EATR to calculate 62 – 63
see also exhaust air transfer ratio
SYS-APM003-EN
109

Index
d
dampers
summary of uses for energy recovery 64
see also face-and-bypass, flow-measuring, and frost-avoidance dampers
dedicated outdoor-air systems
capacity control for supply-air tempering 35 – 36
defined 65 – 66
dual-function energy recovery 84 – 88
outdoor-air preconditioning for cold air 71 – 73
outdoor-air preconditioning for neutral air 68 – 70
series versus parallel arrangements for supply-air tempering 28 – 32, 99
supply-air tempering for neutral-temperature air 26 – 32, 80 – 81, 99
tempering supply air with new energy 68
dehumidification
ANSI/ASHRAE/IESNA Standard 90.1 4
in constant-volume mixed-air systems 21 – 22
in dedicated outdoor-air systems 26
with active-desiccant wheels 88 – 89
desiccant 52
see also rotary heat exchangers and active-desiccant wheels
direct-expansion refrigeration systems 3, 14
DOE-2 energy analysis software 16
see also energy simulation software
dual-function energy recovery
benefits 85 – 86
control sequence 86 – 87
e
EATR (see exhaust air transfer ratio)
economic justification for energy recovery
outdoor-air preconditioning 13 – 18, 95 – 97
supply-air tempering 32 – 36, 100 – 101
economizers
and coil loops 41
ANSI/ASHRAE/IESNA Standard 90.1 3
coordination with energy recovery 16 – 17, 75 – 76, 78 – 79
effect on sizing energy-recovery devices 16, 41, 54, 77
implications for central outdoor-air preconditioning 94 – 95
effectiveness
ASHRAE 90.1 minimum requirement for exhaust-air energy recovery
coil loops 38 – 40
defined 7
equation 6
fixed-plate heat exchangers 43 – 44
heat pipes 47 – 49
implications of unbalanced airflows 7 – 8
rotary heat exchangers 52
summary of typical performance for energy-recovery devices 64
see also sensible effectiveness and total effectiveness
110
3, 4
SYS-APM003-EN

Index
energy-recovery arrangements 63
series versus parallel 98 – 99
energy-recovery technologies
performance comparison 90 – 91, 99 – 100
summary of characteristics (table) 64
summary of common (table) 38
energy-recovery ventilator 93
energy simulation software 16, 33
exfiltration 8
exhaust air transfer ratio 61 – 63
exhaust airflow
adjacent ductwork for energy recovery 13 – 14, 32, 33
local 8
exhaust-air energy recovery
ANSI/ASHRAE/IESNA Standard 90.1 requirements 4
f
face velocity
versus sensible effectiveness 39, 44, 48
versus total effectiveness 52
face-and-bypass dampers 44, 45, 50, 64
fan motors
effect of outdoor-air preconditioning on sizing 14
effect of supply-air tempering on sizing 33
fans
arrangements 58, 63
energy consumption 16, 35, 62
sizing calculations 63
first cost (see initial cost of owning)
fixed-membrane heat exchangers 43
fixed-plate heat exchangers 43 – 47, 64
flow-measuring dampers 62, 77, 95, 96 – 97
frost prevention 97
coil loops 41 – 42
fixed-plate heat exchangers 45 – 46
for parallel energy recovery in cold climates 31
heat pipes 50
outdoor-air preconditioning applications 18 – 19
rotary heat exchangers 55 – 57
supply-air tempering applications 37
techniques 64
wheel speed control 56
frost threshold 18 – 19, 46, 55
frost-avoidance dampers 46
SYS-APM003-EN
111

Index
h
heat pipes 47 – 51, 64
heat wheels (see rotary heat exchangers, sensible-energy wheels, and total-energy wheels
heating-plant capacity
effect of balanced airflows 8
effect of supply-air tempering with parallel energy recovery 30 – 31
supply-air tempering with dedicated outdoor-air systems 32 – 33
humidification
with total-energy recovery 13, 91
i
IAQ (see indoor air quality)
indoor air quality 2
initial cost of owning
effect of outdoor-air preconditioning with recovered energy
supply-air tempering with recovered energy 32 – 33
13 – 15
l
latent heat of condensation 5, 11, 19, 28
local exhaust 8, 96 – 97
local outdoor-air preconditioning 93
m
microbial growth 14
mixed-air ventilation systems 66
series versus parallel arrangements for supply-air tempering
supply-air tempering using recovered heat 21 – 26
mixing valves 40 – 41, 41 – 42, 64
25 – 26, 98
n
neutral conditioned air 65
dedicated outdoor-air systems 32, 65 – 66, 68, 80, 84 – 88
sensible-energy recovery for outdoor-air preconditioning 70
sensible-energy recovery for supply-air tempering 81, 87
total-energy recovery for outdoor-air preconditioning 69, 87
o
OACF (see outdoor air correction factor)
operating costs
outdoor-air preconditioning with recovered energy 16 – 19
supply-air tempering with recovered energy 33 – 37
see also economic justification
outdoor air correction factor 61 – 63
112
SYS-APM003-EN

Index
outdoor-air intake
cross-leakage from building exhaust 42, 46, 51, 59
proximity to building exhaust 19 – 20
outdoor-air preconditioning
fixed-plate heat-exchanger applications 44
outdoor-air preconditioning with recovered energy
ARI Standard 1060 60
ASHRAE Standard 90.1 requirements 2 – 4
capacity control to avoid overheating supply air 17 – 18
coil-loop applications 40
constant-volume mixed-air systems 74 – 76
dedicated outdoor-air systems 68 – 70, 71 – 73
defined 9
economic justification for energy recovery 13 – 18, 95 – 96
effect on initial cost of owning 13 – 15
effect on operating costs 16 – 19
energy-recovery arrangements 9
fan sizing 14, 63
fixed-plate heat-exchanger applications 44
heat-pipe applications 48
local versus centralized 93 – 95
rotary heat-exchanger (wheel) applications 52
sizing energy-recovery devices 96
variable-air-volume systems 77 – 79
p
parallel arrangement
benefits for supply-air tempering 24 – 26, 29 – 32, 98 – 99
effect on cooling-plant capacity 25, 30
effect on heating-plant capacity 30 – 31
implications for dual-function energy recovery 86
performance example for supply-air tempering 24 – 25, 29 – 31
payback
effect of balanced airflows 8
predicting 95 – 96
performance comparison
balanced versus unbalanced airflows 7 – 8
coil loops 38 – 40
energy-recovery technologies 90 – 91, 99 – 100
fixed-plate heat exchangers 43 – 44
heat pipes 47 – 49
rotary heat exchangers 52 – 53
performance examples
outdoor-air preconditioning with sensible-energy recovery 10 – 11
outdoor-air preconditioning with total-energy recovery 12 – 13
supply-air tempering with parallel energy recovery 24 – 25, 29 – 31
supply-air tempering with series energy recovery 22 – 24, 28 – 29
performance ratings 60 – 63
plate heat exchangers (see fixed-plate heat exchangers)
SYS-APM003-EN
113

Index
preheat 19, 56, 64
see also frost prevention and auxiliary heat
purge section in rotary heat exchangers 59, 62
r
regeneration air 89
see also active-desiccant wheels
reheat 5
implications of ANSI/ASHRAE/IESNA Standard 90.1 for supply-air tempering
outdoor-air preconditioning with dedicated outdoor-air systems 68
supply-air tempering with constant-volume mixed-air systems 82
supply-air tempering with dedicated outdoor-air systems 80
rest-room exhaust
centralized 14 – 15, 92, 95, 96 – 97
cross leakage 57
recirculation 20
rotary heat exchangers 51 – 59, 64
see also sensible-energy wheels and total-energy wheels
21, 80
s
saturation curve 19, 55
sensible effectiveness 6
coil loops 38 – 40
equation 6
fixed-plate heat exchangers 44
heat pipes 47 – 48
sensible-energy wheels 52
sensible-energy recovery 5
constant-volume mixed-air systems 76
dedicated outdoor-air systems 70, 73
outdoor-air preconditioning 10 – 11, 90 – 91
supply-air tempering 67, 88
variable-air-volume systems 79
sensible-energy wheels 51, 52, 64
recooling in an active-desiccant system 88 – 89
see also rotary heat exchangers
sensible-energy-recovery devices
performance comparison 90 – 91, 99 – 100
sensible-heat transfer
calculating 6
series arrangement
benefits for supply-air tempering 25 – 26, 31 – 32, 99
effect on cooling-plant capacity 24, 29
performance example for supply-air tempering 22 – 24, 28 – 29
supply-air tempering 40, 49
simultaneous heating and cooling 3
114
SYS-APM003-EN

Index
sizing energy-recovery devices 41, 54, 77, 96
speed control 53 – 55, 64
and frost prevention 56
Standard 62.1 (see ANSI/ASHRAE Standard 62.1)
Standard 90.1 (see ANSI/ASHRAE/IESNA Standard 90.1)
supplemental heat 34 – 35
supply-air tempering with recovered energy
capacity-control considerations 35 – 36
coil-loop applications 40
common applications 20
constant-volume mixed-air systems 21 – 26, 82 – 83
dedicated outdoor-air systems 26 – 32, 80 – 81
defined 5
economic justification for energy recovery 100 – 101
effect on initial cost of owning 32 – 33
effect on operating costs 33 – 37
frost prevention 37
heat-pipe applications 49
sensible- versus total-energy recovery 98
series versus parallel energy recovery 25 – 26, 31 – 32, 98 – 100
System Analyzer energy analysis software 16
see also energy simulation software
t
tilt control 49, 50, 64
total effectiveness 6
equation 6
total-energy recovery 5
constant-volume mixed-air systems 75
dedicated outdoor-air systems 69, 72
example performance for outdoor-air preconditioning
humidification 13
variable-air-volume systems 78
see also total-energy-recovery devices
total-energy wheels 51 – 52, 64
see also rotary heat exchangers
total-energy-recovery devices
in dry climates 90
in predominantly heating climates 91 – 92
outdoor-air preconditioning 9
performance comparison 90 – 91
see also total-energy recovery
total-heat transfer
calculating 6
TRACE energy analysis software 16
see also energy simulation software
turbulators 40
12 – 13
u
U.S. Environmental Protection Agency
unbalanced airflows 7 – 8, 96 – 97
SYS-APM003-EN
2
115

Index
v
variable-air-volume systems 66
outdoor-air preconditioning 77 – 79
variable-speed drives
capacity control 40 – 41, 53 – 55
frost prevention 41 – 42, 56, 64
ventilation systems 65 – 67
116
SYS-APM003-EN
Trane - by Trane Technologies (NYSE: TT), a global climate innovator - creates comfortable, energy efficient indoor environments for commercial
and residential applications. For more information, please visit trane.com or tranetechnologies.com.
Trane has a policy of continuous product and product data improvement and reserves the right to change design and specifications without notice. We are committed to using
environmentally conscious print practices.
Trane, the Circle Logo, Let’s Go Beyond, TRACE, System Analyzer, and Traq are trademarks of Trane in the United States and other countries. ASHRAE is a trademark of
the American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc. ANSI is a trademark of American National Standards Institute, Inc. All trademarks
referenced are the trademarks of their respective owners.
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SYS-APM003D-EN 09 Sept 2020
Supersedes SYS-APM003C-EN 11 March 2011
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