Variable Refrigerant Flow
ASHRAE Handbook Chapter XX
VARIABLE REFRIGERANT FLOW (VRF) DEFINED…………………………2
GENERAL DESIGN CONSIDERATIONS………………………………...……16
User Requirements……………………………………………………………17
Diversity and Zoning………………………………………………………….18
Installation……………………………………………………………………..20
Refrigerant Pipe Design……………………………………………………….20
Maintenance Concerns………………………………...………………………20
Sustainability…………………………………………………………………23
TYPES OF VRF SYSTEMS………………………………………………...……16
EQUIPMENT AND SYSTEM STANDARDS…………………………...………23
AHRI Certification Programs…………………………...…………………….23
Ventilation Standards…………………………...……………………………..23
Refrigerant Management………………………………………………………30
Green Buildings………………………………………………………...……..34
COMPONENTS AND SYSTEM LAYOUT………………………………………34
Software for Designing Systems.………………………...…………………….23
Indoor Unit Styles……………….………………………...……………………23
Controls………………………….………………………...……………………23
Refrigerant Circuit and Components……………………………………………23
Typical System Layout…………………………………………………………23
SYSTEM OPERATION………………………………………………………..…36
Explanations of P-H Diagram (Refrigerant Characteristics Table) ............... 36
Concept of Basic Refrigeration Cycle ........................................................... 37
Points of Refrigerant Control of VRF System ............................................... 38
Cooling Operation ........................................................................................... 38
Heating Operation ........................................................................................... 39
Control of Electronic Expansion Valve .......................................................... 41
Heating and Defrost Operations……………………………..…………………43
Heat-recovery Operations………………………………………………………43
APPLICATIONS —BUILDING TYPES (NEW CONSTRUCTION AND
RETROFIT)…...……………………………………………………………………43
Offices…………………………………………….……………………………43
Schools and Universities………………………………….……………………43
Limited Care Facilities; Nursing Homes………………………….……………43
Multi-tenant Dwellings, Apartments………………………………………...…43
Hotel and Motel……………………………………………………...…………43
Churches……………………………………………………………..…………43
Residential…………………………………………………………...…………43
Hospitals…………………………………………………………..……………43
REFERENCES………………………………………………...………………… 44
1
VARIABLE REFRIGERANT FLOW DEFINED
Many HVAC professionals are familiar with mini-split systems: an air conditioner or heat pump
with more than one factory-made assembly (e.g., one indoor and one outdoor unit). A variation
of this product, often referred to as a multi-split or a variable-refrigerant flow (VRF) system,
typically consists of:
1. A condensing section housing compressor(s) and condenser heat exchanger,
2. Multiple indoor direct-expansion (DX) evaporator fan-coil indoor units with electronic
expansion devices, temperature sensing capabilities, and a dedicated microprocessor for
individual control,
3. A single set of refrigerant piping that interconnects the condensing unit and the evaporator
units,
4. A zone temperature control device that may or may not be interlocked with a system controller.
VRF multi-split products are fundamentally different from unitary or other types of traditional
HVAC systems in that heat is transferred to or from the space directly by circulating refrigerant
to evaporators located near or within one conditioned space. In contrast, conventional systems
transfer heat from the space to the refrigerant by circulating air (in ducted unitary systems) or
water (in chillers) throughout the building. The main advantage of a VRF system is its ability to
respond to fluctuations in space load conditions by allowing each individual thermostat to
modulate its corresponding electronic expansion valve to maintain its space temperature set point
(see Tables 1 and 2 for a comparison of VRF to other systems).
Table 1 Comparison of VRF and Unitary HVAC Systems
Item
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
1.10
1.11
1.12
2.0
2.1
2.2
3.0
3.1
3.2
3.3
3.4
Description
VRF System
Condensing units components
Single or multiple compressor
Yes
Oil separator for each compressor or for all
Yes
compressors
Oil level control
Yes
Active oil return
Yes
Option for heating and cooling
Yes
Simultaneous heating / cooling
Yes
Air cooled or water cooled condenser
Yes
Liquid receiver
Yes
Control of the refrigerant level in the liquid receiver
Yes
Condensing temperature control
Yes
Capacity control by the suction pressure
Yes
Compressor cooling capacity control by speed
Yes
(RPM) or steps
Suction accumulator
Depending on the System
Refrigerant lines
Long liquid lines to many evaporators
Yes
Refrigerant pipes special design procedure due to
Yes
pressure drop and oil return
Internal units
Several units any size
Yes
Independent control for each evaporator by an
Yes
electronic expansion valve
Mechanical sub-cooling
Provided for pressure drop
(if necessary) and to
improve performance
Expansion valve able to handle different cooling Electronic expansion valve
2
Unitary System
Yes
Yes
Yes
In some units
Yes for hot gas defrost
No
Yes
Yes
Yes
It is an option
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Provided to improve
performance
Thermostatic or electronic
3.5
3.6
3.7
4.0
4.1
4.2
4.3
4.4
4.5
capacities and pressure differential
Coil and drain defrost
Only necessary for the
external unit heating
Yes
Depends
Air filter
Drainage pump
Controls
Microprocessor control condensing unit
Microprocessor in the evaporator
BMS available
Inverters for power
Alarm codes
Yes
Yes
Yes
Yes
Yes
expansion valve
Operational and protection
Not necessary
Depends
Yes
Yes
Yes
Yes
Yes
Table 2 Comparison of VRF and Chiller Systems
Item Description
1.0
Sensible cooling
capacity
1.1
Latent cooling
capacity
1.2
Total cooling
capacity
1.3
Capacity Increase
or adjustment for
Sensible Heat
Factor
Air Cooled
Condenser
Capacity Increase
or adjustment for
Sensible Heat
Factor
Water Cooled
Condenser
Air flow in m3/h
1.4
2.0
Chilled Water System
OK – selection will
always meet the thermal
load and air flow
OK – selection will
always meet the thermal
load. It will be necessary
to add a heating device to
control humidity
OK – selection will
always meet the thermal
load
VRF System
There is no option to
select equal to the
thermal load and air flow
There is no option to
select equal to the
thermal load
Possible – new coil and
control valve selection or
change in chilled water
temperature
It should be provide
room for expansion or
capacity increase easy to
be done.
Difficult to change the
sensible heating factor
There is a band for
adjustment between a
maximum and minimum
value
There is a tap in the
motor for adjustment for
a higher value
There is no option to
select equal to the
thermal load and the air
flow
Possible – new coil and
There is no option, it
control valve selection or should be another
change in chilled water
equipment or another
temperature
refrigerant lines
Adjustable – it may need
a motor change
2.1
Air flow pressure
drop
Adjustable – it may need
a motor change
2.2
Air filter efficiency
Compatible with almost
air filter efficiency
2.3
Electrical motor
efficiency internal
unit
Condensate water
3.0
It uses a standard a low
efficiency filter 50%
efficiency gravimetric
test not better than
MERV 4
Higher efficiency, could Lower efficiency
be better than 90%
Depends on the model
minimum 60%
Inside the machine room, It may need pump and
3
Comments
Usually sensible cooling load for
VRF is lower than the air flow,
you may have to oversize the unit
Usually latent cooling load for
VRF is a consequence from the
sensible load it will be necessary
to add electrical heating for
humidity control
Usually total cooling load for
VRF is lower than the thermal
load or you may have to oversize
the unit
Chilled water is more flexible.
VRF was design to be
compatible with usual Offices
and comfort jobs SHF from 0.70
to 0.80
VRF was design to be
compatible with usual Offices
and comfort jobs SHF from 0.70
to 0.80
Usually you should oversize the
cooling capacity to match air
flow
Very narrow band to adjust for
VRF. If there is duct, the duct
should be calculated according to
the external pressure of the
internal unit
VRF there are options up to 85%
efficiency dust spot test MERV
11, but will reduce the external
pressure and will have a higher
cost
There is no option to change the
motor for VRF. Not good for
ASHRAE Standard 90.1-2004
Very unreliable for VRF
drainage
no problem
4.0
Long pipes - Air
Cooled Condenser
Increase chilled water
pump power but doesn’t
change capacity
4.1
Long condensing
water lines
4.2
Refrigerant lines
safety and leakage
Air cooled units
4.3
Refrigerant lines
safety and leakage
Water cooled units
5.0
Coefficient of
performance
5.1
Capacity control
5.2
Water cooled
Condenser
6.0
6.1
needs proper insulation
It reduces the capacity
for long lines up to 75%.
It reduces the latent
cooling capacity
Increase condensing
Increase condensing
water pump power but
water pump power but
doesn’t change capacity doesn’t change capacity
Only in the machine
It is all over the building
room or in the outside air difficult to control and to
locate the leakage. High
risk for the occupants
Only in the machine
It is all over the same
room or in the outside air floor not so difficult to
control and to locate the
leakage. High risk for the
occupants
Easy to calculate it
Condensing unit is
depends on the %
almost constant
cooling capacity and the regarding the cooling
outside air
capacity, but depends of
the outside air
Leaving water
Suction pressure of the
temperature keep
compressor keep
constant, by the capacity constant by the capacity
control on the
control on the
compressor
compressor speed or
stages
Shell and tube
condensers, standard
procedures and easy to
clean
Plate heat exchanger or
tube in tube, it needs a
closed circuit with the
use of an intermediate
heat exchanger
Heating and cooling Needs four pipes to heat Almost standard easy to
and cool at the same time do and low cost
with heat recovery
Cooling and
Very sophisticate not so Easy to use is the same
Heating Control
easy to use for the
as the mini-split
costumer
Equipment may be over electrical
devices
Very important to verify the real
capacity, including the suction
pressure drop for VRF. Great
issue
Both are very similar
Very difficult to certified the
VRF system according to
ASHRAE 15/1999
VRF system may be possible to
certified according to ASHRAE
15-2007.
High efficiency Chilled Plant
could be 0.8 kW/Ton and VRF
condensing units could be 0.95
kW/Ton all year around average
for Sao Paulo, Brazil
Constant pressure control in the
suction line near the compressor
keeps the COP constant, but it
doesn’t gives the same value for
the evaporator due to the
pressure drop. Reduces the latent
cooling capacity for VRF
Higher initial cost but very low
maintenance for VRF
Advantage for the VRF
Advantage for VRF, it doesn’t
need trained personal to operate
There are two basic types of VRF multi-split systems: heat pump and heat recovery (see Figure
1). Heat pumps can operate in heating or cooling mode. A heat-recovery system, by managing
the refrigerant through a gas flow device, can simultaneously heat and cool—some indoor fan
coil units in heating and some in cooling, depending on the requirements of each building zone.
The majority of VRF systems are equipped with variable-speed compressors. Often called
variable-frequency drives (VFD) or inverter compressors (Figure 2), this component responds to
indoor temperature changes, varying the speed to operate only at the levels necessary to maintain
a constant and comfortable indoor environment. Due to this flexibility, VRF systems that include
inverter compressors are inherently energy efficient. Heat-recovery systems increase VRF
efficiency because, when operating in simultaneous heating and cooling, energy from one zone
can be transferred to meet the needs of another.
Figure 1: Heat-recovery and Heat-pump Systems
4
Figure 2: Compressor Frequency
VRF outdoor units can have cooling and heating capacities from 12,000 Btu/h (3,508 W) to
300,000 Btu/h (87,692 W); VRF indoor units can have cooling and heating capacities from 5,000
Btu/h (1,462 W) to 120,000 Btu/h (17,538 W). The outdoor unit may support up to 50 indoor
evaporator units with capacities that collectively add up to 150% capacity of the condensing unit.
VRF equipment is divided into three general categories: residential, light commercial, and
applied. Residential equipment is single-phase with a cooling capacity of 65,000 Btu/h or less.
Light commercial equipment is generally three-phase, with cooling capacity greater than 65,000
Btu/h and is designed for small businesses and commercial properties. Applied equipment has
cooling capacities higher than 135,000 Btu/h and is designed for large commercial buildings.
Definitions
Heat pump multi-split. An encased, factory-made assembly or assemblies designed to be used as permanently
installed equipment to take heat from a heat source and deliver it to the conditioned space when heating is desired.
It may be constructed to remove heat from the conditioned space and discharge it to a heat sink if cooling and
5
dehumidification are desired from the same equipment. It normally includes multiple indoor conditioning coils,
compressor(s), and outdoor coil(s). Such equipment may be provided in more than one assembly, the separated
assemblies of which are intended to be used together. The equipment may also provide the functions of cleaning,
circulating and humidifying the air.
Variable Refrigerant Flow (VRF) System. An engineered direct exchange (DX) multi-split system incorporating at
least one variable capacity compressor distributing refrigerant through a piping network to fan coil units each
capable of individual zone temperature control, through proprietary multiple indoor zone temperature control
devices and common communications network.
VRF heat-recovery multi-split system. A split system air-conditioner or heat pump incorporating a single refrigerant
circuit, with one or more outdoor units at least one variable-speed compressor or an alternate compressor
combination for varying the capacity of the system by three or more steps, multiple indoor fan coil units, each of
which is individually metered and individually controlled by a proprietary control device and common
communications network. This system is capable of operating as an air-conditioner or as a heat pump. The system is
also capable of providing simultaneous heating and cooling operation, where recovered energy from the indoor units
operating in one mode can be transferred to one or more other indoor units operating in the other mode. Variable
refrigerant flow implies 3 or more steps of control on common, interconnecting piping.
VRF multi-split system. A split system air-conditioner or heat pump incorporating a single refrigerant circuit, with
one or more outdoor units, at least one variable speed compressor or an alternative compressor combination for
varying the capacity of the system by three or more steps, multiple indoor fan coil units, each of which is
individually metered and individually controlled by a proprietary control device and common communications
network. The system shall be capable of operating either as an air conditioner or a heat pump.
GENERAL DESIGN CONSIDERATIONS
User Requirements
The user primarily needs space conditioning for occupant comfort. Cooling, dehumidification,
and air circulation often meet those needs, although heating, humidification, and ventilation are
also required in many applications. Components other than the base outdoor and indoor units
may need to be installed for VRF systems to satisfy all requirements.
Applications
VRF systems have many advantages over more traditional HVAC units. The advantages and
disadvantages for a VRF system, when compared to a chilled system, are presented in Table 1.
Table 1: VRF System Advantages and Disadvantages
Item Description
1
Human Comfort
2
3
Process cooling, heating,
humidification and
dehumidification
Internal Air Quality
4
5
Initial Cost
Operational Cost
6
Cooling capacity
Variable Refrigerant Flow AC
System
Partial – no humidity control,
not so good air distribution
Not applicable - no humidity
control, not so good air
distribution
Partial – needs a auxiliary air
make-up system and special
filters
No duct work is good
Similar
Little higher at full load 1.25
kW/ton
Good performance until 100 m
equivalent length
Poor performance above 100 m
6
Chilled Water AC System
Good – true air conditioning
Good - May by designed for any
condition
Good – may be designed for any
condition.
Ducts need to be cleanable
Similar
At full load 1.18 kW/ton
Distance is only a matter of
pumps’ selection and operational
power consumption
7
Increasing cooling capacity
8
9
11
Operation at partial load
Customer or tenant control
on the operational cost
Compatibility with
standards, guides and
regulations
Long distance pipes
12
Refrigerant management
13
Customer operation
14
Malfunction Possibility
15
16
Operational life expectation
Maintenance
17
Sales strategy
10
equivalent length
Not so easy, it may be necessary It could be done by changing the
to change the refrigerant lines
control valves and or the coils.
and the condensing unit
Chiller plant doesn’t change or
chilled water pipes
Good performance and control Good performance and control
Good - full control
No control on the operational and
Very important
maintenance cost
Partial. It is necessary to solve
Fully compatible
the compatibilities issue during
the design
Up to 100 m is OK, more there No problem.
is a cooling capacity reduction
up to 75%
Difficult it depends on the
Concentrate in a single
design of the system for
equipment easy and simple –
monitoring, identification and
Good
repair
Easy and simple – Good
Not so clear to customer Very important
Acceptable
To many parts and components More reliable, just a few parts
and long refrigerant lines –
and equipment – Good
Acceptable
Up to 15 years - Acceptable
Up to 25 years – Good
Depends on the design, access
No problem - Good
may be a problem
It is necessary to verify, the say To much engineering stuff,
what the customer would like to difficult for the costumer to
listen, but not all is true
understand
VRF systems are not suitable for all applications. Some limitations include:

There is a limitation on the indoor coil maximum and minimum entering dry- and wetbulb temperatures, which makes the units unsuitable for 100% outside air applications
especially in hot and humid climates.

The cooling capacity available to an indoor section is reduced at lower outdoor
temperatures. This limits the use of the system in cold climates to serve rooms that
require year-round cooling, such as telecom rooms.

The external static pressure available for ducted indoor sections is limited. For ducted
indoor sections, the permissible ductwork lengths and fittings must be kept to a minimum.
Ducted indoor sections should be placed near the zones they serve.
Diversity and Zoning
The complete specification of a VRF system requires careful planning. Each indoor section is
selected based on the greater of the heating or cooling loads in the area it serves. In cold climates
where the VRF system is used as the primary source for heating, some of the indoor sections will
need to be sized based on heating requirements.
Once all indoor sections are sized, the outdoor unit is selected based on the load profile of the
facility (Example 1). The combined cooling capacity of the indoor sections can match, exceed, or
be lower than the capacity of the outdoor section connected to them. An engineer can specify an
7
outdoor unit with a capacity that constitutes anywhere between 70% and 130% of the combined
indoor units capacities. The design engineer must review the load profile for the building so that
each outdoor section is sized based on the peak load of all the indoor sections at any given time.
Adding up the peak load for each indoor unit and using that total number to size the outdoor unit
likely will result in an unnecessarily oversized outdoor section. Although an oversized outdoor
unit in a VRF system is capable of operating at lower capacity, avoid oversizing unless it is
required for a particular project due to an anticipated future expansion or other criteria. Also,
when indoor sections are greatly oversized, the modulation function of the expansion valve is
reduced or entirely lost. Most manufacturers offer selection software to help simplify the
optimization process for the system’s components.
Sizing Example 1
Peak cooling load for Zone 1
3 ton
Peak cooling load for Zone 2
2.5 ton
Peak cooling load for Zone 3
4 ton
Zones peak load = 3 + 2.5 + 4
9.5 ton
Building peak load
7.0 ton
Available sizes for outdoor unit
7.5 ton and 10 ton
Selection: Unless additional indoor units are planned for the future, select a 7.5 ton
outdoor section.
Installation
In deciding if a VRF system is feasible for a particular project, the designer should consider
building characteristics; cooling and heating load requirements; peak occurrence; simultaneous
8
heating and cooling requirements; fresh air needs; electrical and accessibility requirements for all
system components; minimum and maximum outdoor temperatures; sustainability; and acoustic
characteristics. The physical size of the outdoor section of a typical VRF is somewhat larger than
that of a conventional DX condensing unit, with a height up to 6 ft. (1.8 m) excluding supports.
The chosen location should have enough space to accommodate the condensing unit(s) and any
clearance requirements necessary for proper operation.
Refrigerant Piping Design
Building geometry must be studied carefully so that refrigerant piping lines are properly
designed. The system should not be considered if the expected pipe lengths or height difference
exceed those listed in the manufacturer’s catalog. In buildings where several outdoor locations
are available for the installation of the outdoor units, such as roof, setback, and ground floor,
each condensing section should be placed as close as possible to the indoor units it serves.
Although manufacturers routinely increase the maximum allowable refrigerant pipe run, the
longer the lengths of refrigerant pipes, the more expensive the initial and operating costs. For
most VRF units, the maximum allowable vertical distance between an outdoor unit and its
farthest indoor unit is approximately 164 ft ( m); the maximum permissible vertical distance
between two individual indoor units is approximately 49 ft ( m); and the maximum refrigerant
piping lengths allowable between outdoor and farthest indoor units is up to 541 ft. ( m) (see
Figure 2 and Table 2).
Figure 2: Maximum Allowable Distances and Piping Lengths
9
Table 2: Maximum Allowable Distances and Piping Length Ranges
Maintenance Considerations
Ductless VRF indoor units have some considerations in reference to maintenance:
 Draining condensate water from the indoor and outdoor units
 Changing air filters
 Repairs
 Cleaning
Ease of maintenance depends on the relative position of the indoor and outdoor units and the
room to ensure access for changing filters, repairing, and cleaning. The installer must make sure
there is enough slope to drain condensate water generated by both the indoor and outdoor units.
Depending on the location where the indoor unit is installed, it may be necessary to install a
pump so that water drains properly.
Sustainability
VRF systems feature higher efficiencies in comparison to conventional heat pump units. Less
power is consumed by heat-recovery VRF systems at part load, which is due to the variable
speed driven compressors and fans at outdoor sections. The designer should consider other
factors to increase the system efficiency and sustainability. Again, sizing should be carefully
evaluated.
Environmentally friendly refrigerants such as R-410A should be specified. Relying on the heat
pump cycle for heating, in lieu of electric resistance heat, should be considered, depending on
outdoor air conditions and building heating loads. This is because significant heating capacities
are available at low ambient temperatures (e.g., the heating capacity available at 5°F (– 15°C)
can be up to 70% of the heating capacity available at 60°F (16°C), depending on the particular
design of the VRF system).
TYPES OF VRF SYSTEMS
Both heat-pump and heat-recovery VRF systems are available in air-to-air and water-source
(water-to-refrigerant) configurations (see Table 1). Air-cooled condensing units contain a
propeller fan to transfer heat from the refrigerant to the air; water-cooled condensing units,
10
which are usually installed indoors, uses a closed or open water loop to transfer heat from the
refrigerant.
Closed Loop Configurations
Water-loop heat pump application: Water-to-air heat pump using liquid circulating in a common
piping loop functioning as a heat source/heat sink.
NOTE: The temperature of the liquid loop is usually mechanically controlled within a
temperature range of 59°F [15°C] to 104°F [40°C].
Ground-loop heat pump application: Brine-to-air heat pump using a brine solution circulating
through a subsurface piping loop functioning as a heat source/heat sink.
NOTES
1. The heat exchange loop may be placed in horizontal trenches or vertical bores, or be
submerged in a body of surface water. ANSI/ARI/ASHRAE ISO Standard 132561:1998
2. The temperature of the brine is related to the climatic conditions and may vary from 23º
to 104ºF [–5° to 40°C].
Water-to-air heat pump and/or brine-to-air heat pump: Heat pump which consists of one or more
factory-made assemblies which normally include an indoor conditioning coil with air-moving
means, compressor(s), and refrigerant-to-water or refrigerant-to-brine heat exchanger(s),
including means to provide both cooling and heating, cooling-only, or heating-only functions.
NOTES
1. When such equipment is provided in more than one assembly, the separated assemblies
should be designed to be used together.
2. Such equipment may also provide functions of sanitary water heating, air cleaning,
dehumidifying, and humidifying.
Open Loop Configuration
Groundwater heat pump: Water-to-air heat pump using water pumped from a well, lake, or
stream functioning as a heat source/heat sink.
11
NOTE: The temperature of the water is related to the climatic conditions and may vary from 41º
to 77ºF (5° to 25°C) for deep wells.
Table 1. Classification of VRF Multi-Split Systems
System Identification
VRF Heat-pump Multi-split
VRF Heat-recovery
Multi-split
1 Shared to all indoor units
1 or More Variable Speed or
alternative method resulting in
3 or more steps of capacity
Greater than one indoor unit
Individual
Zones/Temp
1 Shared to all indoor units
1 or More Variable Speed or
alternative method resulting in
3 or more steps of capacity
1 or More
Steps of Control
1 or multiple-manifolded
outdoor units with a specific
model number.
3 or More
Mode of Operation
A/C, H/P
A/C, H/P, H/R
Heat Exchanger
One or more circuits of shared
refrigerant flow
MSV-A-CB
One or more circuits of shared
refrigerant flow
Attribute
Refrigerant Circuits
Compressors
Indoor
Units
Qty.
Operation
Outdoor
Unit(s)
Qty.
Classific
ation
Air-Conditioner
(air-to-air)
Air-Conditioner
(water-to-air)
Heat Pump (air-toair)
Heat Pump (waterto-air)
Individual
Zones/Temp
3 or More
MSV-W-CB
HMSV-A-CB
HMSR-A-CB
HMSV-W-CB
HMSR-W-CB
NOTES:
1 A suffix of “-O” following any of the above classifications indicates equipment not intended for
use with field-installed duct systems (6.1.5.1.2).
2 A suffix of “-A” indicates air-cooled condenser and “-W” indicates water-cooled condenser.
3 For the purposes of the tested combination definition, when two or more outdoor units are
connected, they will be considered as one outdoor unit.
Heat Rejection. VRF condensers may be air-cooled or water-cooled; the letters A or W follow the Air-Conditioning
Heating and Refrigeration Institute (AHRI) designation.
Heat Source/Sink. Unitary heat pump outdoor coils are designated as air-source or water-source by an A or W,
following AHRI practice. The same coils that act as a heat sink in the cooling mode act as the heat source in the
heating mode.
Unit Exterior. The unit exterior should be decorative for in-space application, functional for equipment room and
ducts, and weatherproofed for outdoors.
EQUIPMENT AND SYSTEM STANDARDS
AHRI Certification Programs
AHRI is developing a certification program for VRF multi-split air-conditioning and heat-pump
equipment up to 300,000 Btu/h that will be based on AHRI Draft Standard 1230 and ASHRAE
Standard 37. The certification program includes all VRF multi-split air-conditioning, air- and
12
water-source heat-pump equipment rated up to 300,000 Btu/h (88,000 W) at AHRI Standard
Rating Conditions.
The following Certification Program ratings are verified by test:
VRF Multi-Split Air-Conditioning and Heat Pump Equipment
a. For VRF Multi-Split Air-Conditioners < 65,000 Btu/h (19,000 W)
1. ARI Standard Rating Cooling Capacity, Btu/h (W)
2. Seasonal Energy Efficiency Ratio, SEER, Btu/(Wh)
b. For VRF Multi-Split Air-Conditioners ≥ 65,000 Btu/h (19,000 W)
1. ARI Standard Rating Cooling Capacity, Btu/h (W)
2. Energy Efficiency Ratio, EER, Btu/(Wh)
3. Integrated Part-Load Value, IPLV/IEER
c. For all VRF Multi-Split Heat Pumps < 65,000 Btu/h (19,000 W)
1. ARI Standard Rating Cooling Capacity, Btu/h (W)
2. Seasonal Energy Efficiency Ratio, SEER
3. High Temperature Heating Standard Rating Capacity, Btu/h (W)
4. Region IV Heating Seasonal Performance Factor, HSPF, Minimum Design
Heating Requirement, Btu/(Wh)
d. For VRF Multi-Split Heat Pumps ≥ 65,000 Btu/h (19,000 W)
1. ARI Standard Rating Cooling Capacity, Btu/h (W)
2. Energy Efficiency Ratio, EER, Btu/(Wh)
3. Integrated Part-Load Value, IPLV/IEER
4. High Temperature Heating Standard Rating Capacity, Btu/h (W)
5. High Temperature Coefficient of Performance, COP
6. Low Temperature Heating Standard Rating Capacity, Btu/h (W)
7. Low Temperature Coefficient of Performance, COP
e. For VRF Multi-Split Heat Recovery Heat Pumps
1. Ratings Appropriate in (c) (d) above
2. Simultaneous Cooling and Heating Efficiency (SCHE) (50% heating/50%
cooling)
f. For VRF Multi-Split Heat Pumps Systems that Use a Water Source for Heat Rejection
1. ARI Standard Rating Cooling Capacity, Btu/h (W)
2. Energy Efficiency Ratio, EER, Btu/(Wh)
3. Integrated Part-Load Value, IPLV/IEER
4. Heating Standard Rating Capacity, Btu/h (W)
5. Heating Coefficient of Performance, COP
6. Simultaneous Cooling and Heating Efficiency (SCHE) (50% heating/50% cooling)
(Heat Recovery models only)
Energy Efficiency Ratings—Definitions
Coefficient of performance (COP). A ratio of the heating capacity in watts [W] to the power input values in watts
[W] at any given set of rating conditions expressed in watts/watts [W/W]. For heating COP, supplementary
resistance heat shall be excluded.
13
Energy efficiency ratio (EER). A ratio of the Cooling Capacity in Btu/h to the power input values in watts at any
given set of rating conditions expressed in Btu/W·h.
Heating Seasonal Performance Factor (HSPF). The total heating output of a heat pump, including supplementary
electric heat necessary to achieve building heating requirements during its normal annual usage period for heating
divided by the total electric power during the same period, as determined in Appendix C expressed in Btu/[Wh].
Integrated Energy Efficiency Ratio (IEER). A single number that is a cooling part-load efficiency figure of merit
calculated per the method described in paragraph 6.5.
Integrated Part-Load Value (IPLV). A single number that is a cooling part-load efficiency figure of merit calculated
per the method described in Appendix H.
Seasonal Energy Efficiency Ratio (SEER). The total cooling of a systems covered by this standard with a capacity
<65,000 Btu/h [19,000 W]during its normal usage period for cooling (not to exceed 12 months) divided by the total
electric energy input during the same period as determined in Appendix C, expressed in Btu/[Wh].
Simultaneous cooling and heating efficiency (SCHE means the ratio of the total capacity of the system (heating and
cooling capacity) to the effective power when operating in the heat recovery mode. (Where SCHE is stated without
an indication of units, it shall be understood that it is expressed in Btu/[Wh].)
Tested Combination. The term “tested combination” means a sample basic model comprised of units that are
production units, or are representative of production units, of the basic model being tested. The tested combination
shall have the following features:
The basic model of a variable refrigerant flow system (“VRF system”) used as a tested combination shall
consist of an outdoor unit (an outdoor unit can include multiple outdoor units that have been manifolded into
a single refrigeration system, with a specific model number) that is matched with between 2 and 5 indoor
units (for systems with nominal cooling capacities greater than 150,000 Btu/h (43,846 W), the number of
indoor units may be as high as 8 to be able to test non-ducted indoor unit combinations).
The indoor units shall: Represent the highest sales model family as determined by type of indoor unit e.g.
ceiling cassette, wall-mounted, ceiling concealed, etc. If 5 are insufficient to reach capacity another model
family can be used for testing.
Together, have a nominal cooling capacity between 95% and 105% of the nominal cooling capacity of the
outdoor unit.
Not, individually, have a nominal cooling capacity greater than 50% of the nominal cooling capacity of the
outdoor unit, unless the nominal cooling capacity of the outdoor unit is 24,000 Btu/h (7,016 W) or less.
Have a fan speed that is consistent with the manufacturer's specifications.
All have the same external static pressure.
Ventilation Standards
One of the most challenging aspects of designing VRF systems is the need to provide a separate
outside air supply to each unit to comply with ANSI/ASHRAE Standard 62.1-2004, Ventilation
for Acceptable Indoor Air Quality (ANSI approved), and building codes. Most manufacturers
offer an outside air kit, for connecting to outside air ductwork. A separate outside air fan and
control system is generally required for larger buildings. In humid climates, providing
preconditioned outside air to each indoor unit ensures good indoor air quality.
Item 5.9 from ASHRAE Standard 62.1-2004 specifically discusses particulate matter removal
and how VRF indoor units can or cannot uphold the requirements.
 Item 5.9 – Particulate Matter Removal: Particulate matter filters or air cleaners having a
minimum efficiency reporting value (MERV) of not less than 6 when rated in
accordance with ANSI/ASHRAE 52.2-1999, shall be provided upstream of all cooling
coils or others devices with wetted surfaces trough which the air is supplied to an
occupied space.
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o The standard filter with 50% efficiency gravimetric test which is MERV 1 or 2 –
not acceptable;
o They may use an option with 65% efficiency dust spot test which is MERV 11
available for ducted units– Approved OK
o There is another option with 85% efficiency dust spot test which is MERV 13
available for dusted units – Approved OK
o Those filters have a higher cost and reduce the static external pressure available
and can only be used in ducted units and select ductless units.
o Higher efficiency filters are available for:
 Ceiling Mounted Cassette Type – double flow
 Ceiling Mounted Cassette Type – multi-flow
 Ceiling Mounted Built-in Type
 Ceiling Mounted Duct Type
 Slim Ceiling Mounted Duct Type, can be installed with field supplied
return air grill + filter
 Console – Ceiling Suspended Type
o Higher filter aren’t available for:
 Ceiling Mounted Cassette Corner Type
 Console – Ceiling Suspended Type
 Wall Mounted Type
 Floor Standing Type/Concealed Floor Standing Type
 Ceiling Suspended Cassette Type
Refrigerant Management Standards
HVAC systems must comply with ASHRAE Standard 15-2007—Safety Standard for
Refrigeration Systems (ANSI approved).
 Refrigerant leak detector to activate alarms and mechanical ventilation system
o Difficult to provide, because you don’t know where the leaks may occur
o If machine room is for an air cooled VRF systems, it is external – OK
o If machine room is for an water cooled VRF systems, it is internal and needs the
leak detector and ventilation – OK
o Refrigerant lines between the floors are external, usually in the corner of the
building – OK
o Refrigerant lines inside the roof and the ceiling may need a refrigerant leak
detector – OK
o Mechanical ventilation system shall be provided by the installation, could be
provided – OK
o All the items above are possible, but they mean more cost
 Machinery room shall be vented to the outdoors, utilizing mechanical ventilation.
o Machine room are for air cooled VRF systems external – OK
o Machine room for water cooled VRF systems needs the leak detector and
ventilation – OK
o The air supply and exhaust ducts for the machinery room shall serve no other area
– OK
o All the items above are possible, but they means more cost
 Refrigerant Quantity Limits. The quantity of refrigerant in each independent circuit of
high probability systems shall not exceed the amounts shown in Table 1, except as
provided in 7.2.1 and 7.2.2, based on volumes determined in accordance with 7.3. For
15

refrigerant blends not listed in Table 1, the amount of each component shall be limited in
the same manner and the total of all components in each circuit shall not exceed the
quantity that would equal 69,100 ppm by volume upon release to the volume determined
by 7.3.
o It is possible to accomplish – seems to be OK
Declaration. A dated declaration of test shall be provided for all systems containing 55 lb
(25 kg) or more of refrigerant. The declaration shall give the name of the refrigerant and
the field test pressure applied to the high-side and the low-side of the system. The test
declaration shall be signed by the installer and, if an inspector is present at the tests, the
inspector shall also sign the declaration. When requested, copies of this declaration shall
be furnished to the authority having jurisdiction.
o It is possible to accomplish - OK
Since introduction, interest has been generated in regards to designing R410A VRF systems to
meet ASHRAE Standard 15 (Safety Standards for Refrigerant Systems) requirements. Specific
designs must focus on the refrigerant flow attributes of these systems, and ASHRAE 15 instructs
designers in many aspects of refrigerant safety.
ASHRAE Standard 15
ASHRAE 15 is a “National Voluntary Consensus Standard”; but, equipment listed by a
Nationally Recognized Testing Laboratory (NRTL) and identified as being in compliance with
Standard 15 meets the applicable provisions of the Standard (ASHRAE Standard 15-2007,
Section 13). Also, regulatory language was incorporated in the 2001 revision and, by adoption,
can be made part of local code requirements. This is specific to each jurisdiction, so it is
important for the designer to be familiar with local codes and regulations.
Applying ASHRAE Standards 15 and 34 to R410A
R410A is the refrigerant used in newer and more energy-efficient systems. Though ASHRAE 15
was last revised in 2007, it does not directly reference R-410A refrigerant except by footnote “a”
under “Table 1 Refrigerant and Amounts” that states:
aThe refrigerant safety groups in Table 1 are not part of ASHRAE Standard 15. The
classifications shown are from ASHRAE Standard 34, which governs in the event of a
difference.” Therefore, system designers must refer to Standard 34 when applying Standard 15
safety principles to R410A refrigerant.
The overall purpose of ASHRAE Standard 34 is “to establish a simple means of referring to
common refrigerants… It also establishes a uniform system for assigning reference numbers and
safety classifications to refrigerants. The standard identifies requirements to apply for
designations and safety classifications for refrigerants, including blends, in addenda or revisions
to this standard.” (“Designation and Safety Classification of Refrigerants” ASHRAE Standard
34-2007, Section 1).
A main point of discussion under ASHRAE Standard 34 is Refrigerant Concentration Limit
(RCL) (ASHRAE Standard 34-2007, Section 7), which is defined as “the refrigerant
concentration limit, in air, determined in accordance with this standard and intended to reduce
the risks of acute toxicity asphyxiation and flammability hazards in normally occupied, enclosed
spaces”
RCL can be expressed in:
 ppm v/v
 g/m3
16

lb./Mcf (or lb./1,000 ft3)
Limits have been developed as indicated (ASHRAE Standard 34, Section 7.4.1):
Mass per Unit Volume. The following equation shall be used to convert the RCL from a
volumetric ratio, ppm by volume, to mass per unit volume, g/m3 (lb./Mcf):
RCLM = RCL · a · M
where
RCLM = The RCL expressed as g/m3 (lb./Mcf)
RCL = the RCL expressed as ppm v/v
a = 4.096 · 10-5 for g/m3 (1.160 x 10-3 for lb./Mcf)
M = The molecular mass of the refrigerant in g/mol (lb./mol)
RCL values are the lowest of the following three factors:
Acute Toxicity Exposure Limit (ATEL): “The refrigerant concentration limit determined in
accordance with this standard (34-2007) and intended to reduce the risks of acute toxicity
hazards in normally occupied, enclosed spaces” (ASHRAE Standard 34-2007, Section 7.1.1).
ATEL includes consideration of mortality, cardiac sensitization, anesthetic or central nervous
system effects and other escape impairing effects and permanent injury.
Oxygen Deprivation Limit (ODL): “The concentration of a refrigerant or other gas that results
in insufficient oxygen for normal breathing” (ASHRAE Standard 34-2007, Section 7.1.2).
Flammable Concentration Limit (FCL): “The refrigerant concentration limit, in air,
determined in accordance with this standard and intended to reduce the risk of fire or explosion
in normally occupied spaces” which is 25% of the Lower Flammability Limit (LFL) (LFL is the
minimum concentration of refrigerant that is capable of propagating a flame…) (ASHRAE
Standard 34-2007, Section 7.1.3).
RCL for R-410A is based on the ATEL (Acute Toxicity Exposure Limit) because it is
lower than the ODL (Oxygen Deprivation Limit). Toxicologists considered the elderly and
children when determining the RCL values for refrigerants. (No discussion on this in Standard.
The toxicology subcommittee of SSPC 34 includes toxicologists from Honeywell, DuPont, and
Arkema, PhD Consulting and representatives from Trane and IIAR.)
ASHRAE Standard 34-2007
Table 10 – Data & Safety: Classifications for Refrigerant Blends
Refrigerant Safety Data from Table 1 of ASHRAE Standard 34-2007
Refrigerant
Safety Group
RCL lb./Mcf
Highly Toxic or Toxic Under
Code Classification
R-22 (CHCIF2)
A1
13
Neither
R-134A (CH2FCF3)
A1
13
Neither
R-407C (Blend)
A1
17
Neither
R-410A (blend)
A1
25
Neither
R410A Qty per Occupied Space = RCL =130,000 ppm v/v or = 390 g/m3 = 25 lb./MCF.
Designing VRF Systems with ASHRAE 15 and 34
Occupied Spaces
17
Standard 15 guides designers on how to apply a refrigeration system in a safe manner, and
details information on the type and amount of refrigerant allowed in an occupied space, defined
as “that portion of the premises accessible to or occupied by people, excluding machinery
rooms” (ASHRAE Standard 15-2004, Section 3).
Standard 15 also lists occupancy classifications (Section 4) with recommendations of allowable
conditions for each class:
Institutional Occupancy
Public Assembly
Residential Occupancy
Commercial Occupancy
Large Mercantile Occupancy
Industrial Occupancy
Mixed Occupancy
Standard 15 (Section 5) also defines Refrigerating System Classifications with guidance for
applications for:
 Direct Systems, which are systems having evaporator, condenser, or refrigerant lines in
direct contact with the material (air) to be cooled or heated
 Indirect Open Spray Systems
 Double Indirect Open Spray Systems
 Indirect Closed Systems
 Indirect Vented Closed Systems
In reviewing specific applications, the designer must look at the space any HVAC system serves,
as well as the refrigerant line paths. If system components are located in normally occupied
spaces, then they must be evaluated for safety and suitability. Corridors and lobbies – especially
points of egress - should be evaluated as well since their volume is, by definition, part of the
connected spaces volume and the restrictions in the Standard limit refrigeration concentrations in
these areas to specified amounts. In most cases such system components – including refrigerant
piping – do not pose a safety or suitability issue. ASHRAE 15 requires factory testing on all
refrigerant containing components; as a result, the likelihood of subsequent failure is remote.
Field fabricated connections also require inspection and evaluation. VRF systems require
evacuation of the complete system and all piping, including field fabricated connections, and
vacuum must be held with no leaks as a part of the commissioning process for every system
installed.
Refrigerant Leaks in Occupied Spaces
Leaks are not defined in Standard 15, but it generally addresses a catastrophic event where full
circuit refrigerant volume is to be considered as available for discharge into the occupied space.
Standard 15 also does not address any time period over which a leak might occur. Even in the
unlikely event of a line rupture, the amount of refrigerant in a circuit would require a significant
period to escape from the system.
The design professional should keep in mind that ASHRAE 15 was primarily developed and
written for the catastrophic release of the entire contents of a pressure vessel thru a safety valve
of large diameter in a short time.
There is a clearly defined relationship between the amount of refrigerant in a system and the
volume of the occupied space into which the refrigerant could flow. According to Standard 15,
“the volume used to determine the refrigerant quantity limits for refrigerants in 7.2 shall be
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based on the volume of space to which the refrigerant disperses in the event of a refrigerant
leak” (ASHRAE Standard 15-2007, Section 7). Occupied space is not necessarily a single room
or area. If a group of rooms or spaces (offices, corridors, other spaces off the corridor, etc.) are
connected by ductwork or other means, then all of their connected volumes are counted in
calculating the affected volume. These “connected spaces” could also include louvers or
“permanent” openings to adjacent spaces or to the outside, as in a ventilation source or exhaust,
and even undercuts on connecting doors, provided there is forced movement of air (Note that R410A is heavier than air and would spread along floor surfaces as a free gas). Standard 15
specifically lists ventilation as a remedy in establishing occupied space, but does not quantify the
amount or type of ventilation required only, the “smallest volume in which the leaked refrigerant
disperses... (ASHRAE Standard 15-2007, Section 7.3.2).
Fig. 4
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Table 11 VRF System Volume Example (Using a CITY MULTI System)
Room Name
Lobby/Waiting
Room
Conference Room
TENANT UPFIT SPACES
Room Area (sq
Ceiling Height (ft)
ft)
450
10
Room Volume (cu ft)
4500
235
12
2820
Office #1
Office #2
Open Work Room
115
70
944
10
10
12
1150
700
11328
Break Room
Men's Bathroom
127
42
10
9
1270
378
Women's Bathroom
42
9
378
Electrical Room
39
14
546
Janitor Closet
36
14
504
To further illustrate the design options, a system with a P72 outdoor unit is shown in Table 11.
The space is a single bay tenant upfit within a strip mall located in a mild climate. The space is
30 feet by 70 feet with 14 feet from the floor to the underside of the roof deck. The interior walls
shown extend 18 inches above the dropped ceiling.
The 72,000 Btu/h capacity unit contains 23 lbs 3 oz of R410A refrigerant. ASHRAE Standard 34
lists the refrigerant concentration limit (RCL) as 25 lb./Mcf for R410A. As shown in Table 11,
the minimum room volume needed to handle the full refrigerant charge of the system can be
easily calculated.
RCL (R410A) = 25 lb./Mcf
Refrigerant Charge (Rc) of a P72 = 23 lbs 3 oz = 23.1875 lbs
MRV = Minimum Room Volume (cubic feet)
MRV = Rc/RCL
MRV = (23.1875 lbs)/(25 lbs/1000 cu ft)
MRV = 927.5 cu ft
Table 12
To summarize the above equation, the smallest space which any of the indoor units could be
located in would have to be capable of dispersing the refrigerant charge into 927.5 cu ft.
As per Table 11, the only spaces of concern would be
-Men’s Bathroom
-Women’s Bathroom
-Electrical Room
-Janitor Closet
-Office 2
There are several options available to deal with the smaller spaces. In cases such as the
bathrooms, the code required ventilation will likely be all that is required to maintain conditions
in the space. Should extra cooling be required, a ducted unit located in the workroom corridor
would solve the problem. The electrical room/janitor closet area provides another opportunity for
the architect to help the mechanical design team. An opening located low along the common wall
between the two spaces would increase the available volume from 504 cu ft minimum to over
20
1000 cu ft. Should the electrical closet require a rated enclosure as required by NFPA-70 a fire
damper could be installed. Office 2 was provided with a ducted unit located in the corridor to
meet the requirements of the Standard. Another option for office 2 would be to omit the ceiling
entirely or install it at a level which provides the needed volume. It shall be noted that ASHRAE
15 Section 7.3.2 clearly states “the space above a suspended ceiling shall not be included in
calculating the refrigerant quantity limit in the system…”
As illustrated by the example, the only requirement to meet the standard was an understanding of
the language and not major accommodations or changes. With the application of sound
engineering practice, any design professional can easily integrate VRF technology into his or her
design.
Conclusion
Engineers and designers have great flexibility in applying VRF systems to ensure the design is
“ASHRAE 15 compliant.” Examining the project spaces and determining the occupied and
connected spaces needs to be a primary consideration, and care must be taken in the location and
layout of refrigerant lines and indoor units.
Green Buildings
The U.S. Green Building Council (USGBC) is the nation’s foremost coalition of leaders from
across the building industry working to promote buildings that are environmentally responsible,
profitable, and healthy places to live and work. The core purpose of USGBC is:
To transform the way buildings are designed, built, and operated enabling an
environmentally and socially responsible, healthy, and prosperous built
environment that improves the quality of life in communities.
In order to further that purpose, USGBC developed the LEED® (Leadership in Energy and
Environmental Design) Green Building Rating System™. The LEED Green Building Rating
System™ is a voluntary, consensus-based national standard for developing high-performance,
sustainable buildings.
VRF systems can be used to help buildings to achieve LEED certification in many ways; the
credits discussed in the paragraphs below are based off of the LEED New Construction (NC)
v2.2 rating system.
Energy and Atmosphere
Prerequisite 1: Fundamental Commissioning of the Building Energy Systems Required
A VRF controls system assists building commissioning by allowing easy testing, setting, and
adjusting of the entire HVAC system.
Prerequisite 2: Minimum Energy Performance Required
All buildings must be designed at a minimum to meet both the mandatory and prescriptive or
performance requirements of ASHRAE 90.1-2004. VRF equipment has many energy saving
features, further described under EAc1, which helps with meeting this prerequisite.
Prerequisite 3: Fundamental Refrigerant Management Required
Newer VRF systems use R410A, which is a HFC based refrigerant, CFC free and has no ozone
depletion potential.
Credit 1: Optimize Energy Performance 1-10 points (2 Points Required)
Some VRF systems, in addition to the variable refrigerant flow through the indoor units, use an
inverter drive on the compressor and the outdoor fan motor, feature simultaneous heating and
cooling operation, and include an integrated control system allowing for scheduling of
equipment in each room to maximize energy performance. VRF systems can be coupled with an
energy recovery ventilator (ERV) to further reduce energy usage. Building energy savings can be
demonstrated by performing a building energy model using the EnergyPro software available
21
from EnergySoft, LLC. and comparing the building design with a baseline building as defined by
ASHRAE 90.1 2004. Energy Pro has been approved by the USGBC as acceptable for EAc1
calculations.
Credit 5: Measurement and Verification (1 point)
Some VRF system manufacturers offer software to provide for the ongoing accountability and
optimization of building energy consumption over time, monitoring and logging energy
consumption, heat-recovery cycles, static pressure and ventilation air volumes, and other
building specific systems and equipment. Energy usage data obtained from such software can be
compared with a building energy model prepared in Energy Pro in order to verify the energy
savings shown by the model.
Indoor Environmental Quality
Prerequisite 1: Minimum IAQ Performance Required
VRF systems can often meet minimum outside air requirements through the ventilation
connections of the indoor units. In applications where more outside air is required and the indoor
units capacity is exceeded, an ERV can bring in outside air by using the exhaust air from the
building and transferring energy and moisture to or from the outside air before delivering it to
occupied zones.
Credit 1: Outdoor Air Delivery Monitoring (1 point)
The ERV can be fully integrated within a VRF controls systems, which allows the unit to be
programmed based on occupancy. An ERV can also be integrated with a C02 sensor to energize
the unit and or vary the airflow based on C02 levels within the space.
Credit 2: Increased Ventilation (1 point)
An ERV can be used to exchange a high percentage of air, which when used with adequate air
distribution from ducted units, can increase the ventilation rates above the requirements of
ASHRAE 62.1-2004.
Credit 3.2: Construction IAQ Management Plan: Before Occupancy (1 point)
An ERV can be used to flush the building prior to occupancy.
Credit 5: Indoor Chemical and Pollutant Source Control (1 point)
Many VRF system indoor units can be installed with a filter. A design professional should be
consulted to ensure that adequate static pressure is available to provide desired airflow
performance.
Credit 6.2: Controllability of Systems: Thermal Comfort (1 point)
VRF systems can be controlled by the occupant via the wall-mounted remote controller that can
be provided in every room, or centrally via web-based control. The occupant has the ability to
control airflow direction, fan speed and temperature set points.
Credit 7.1: Thermal Comfort: (1 point)
When VRF systems are properly designed into a building, temperature and humidity control can
be provided in accordance with the ASHRAE 55-2004 guidelines.
Credit 7.2: Thermal Comfort: Verification (1 point)
The trending software that many VRF system manufacturers can install provide verification of
the space temperature, set temperature and mode of operation. The data obtained can be used in
congruence with the thermal comfort surveys, required by this credit, to develop a plan to correct
zones that present thermal comfort issues.
Note: The LEED rating system is a measure of whole building sustainability and to effectively
pursue a LEED certification for any building, the entire team (owner, architect, engineer,
contractor, etc.) must work together to maximize potential. No single equipment selection can
assure any level of certification.
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COMPONENTS AND SYSTEM LAYOUT
Software for Designing Systems
Most VRF manufacturers provide software that makes designing for VRF systems quick and easy. In
some systems, the designer just needs to drag and drop components to complete the design. The program
has built in safeguards against exceeding limitations and shows if there is an error. Assuring line lengths,
maximum connected capacities, component selection, control scheme, etc. are within the system
requirements.
23
Indoor Unit Types
Condensing units (or outdoor unit) of an air conditioning system contains the compressor, circuit
board, and heat exchanger coil; pumps refrigerant to the evaporator coil (or indoor unit).
These indoor units are available in multiple configurations such as wall-mounted, ceilingmounted cassette suspended, and concealed ducted types. Multiple types of indoor units can be
combined with a single outdoor unit.
Controls
Each individual indoor unit can be controlled by a programmable thermostat or a multiple indoor
units serving the same zone can be controlled by the same thermostat. Most VRF manufacturers
offer a centralized control option, which enables the user to monitor and control the entire system
from a single location or via the Internet.
System Controls
An integral network operations and communications system with sensors to monitor and forecast
the status of items such as temperature, pressure, oil, refrigerant levels and fan speed.
A micro-processor, algorithm-based control scheme to: (1) communicate with an optimally
managed variable capacity compressor, fan speed of indoor units, fan speed of the outdoor unit,
solenoids, various accessories; (2) manage metering devices; and (3) concurrently operate
various parts of the system.
These controls optimize system efficiency and refrigerant flow through an engineered distributed
refrigerant system to conduct zoning operations, matching capacity to the load in each of the
zones.
24
Refrigerant Circuit and Components
VRF systems use a sophisticated refrigerant circuit that monitors mass flow, oil flow, and
balance to ensure optimum performance. This is accomplished in unison with variable-speed
compressors and condenser fan motors. Both of these components adjust their frequency in
reaction to changing mass flow conditions and refrigerant operating pressures and temperatures.
A dedicated microprocessor continuously monitors and controls these key components to ensure
proper refrigerant is delivered to each indoor unit in cooling or heating.
25
VRF heat-pump systems include either a two-pipe (liquid and suction gas) or three-pipe (liquid,
suction gas, and discharge gas) configuration. Heat-recovery systems use similar pipe
configurations, but add a gas flow device that determines the proper routing of refrigerant gas to
a particular indoor unit.
Typical System Layout
Figure 1 illustrates a standard VRF configuration, while Figure 2 shows a heat recovery unit
providing simultaneous heating and cooling.
Fig. 1: Typical VRF Configuration in an Office Building.
Fig. 2 Typical VRF Water Source Heat Pump Application
26
Fig. 3: Heat Recovery VRF System.
Designing Refrigeration Lines—A New Approach.
Calculating the refrigerant lines for VRF systems introduces new issues in reference to the liquid
and suction lines:
 Diameter – It’s always the same; it doesn’t matter the distance
 Liquid line –Pressure drop isn’t important, the expansion valve will handle it
 Suction line –Have the same diameter to ensure oil return during the active oil return
cycle and inform the cooling capacity reduction.
Liquid Line
The liquid line should be selected for the minimum refrigerant charge with a smaller diameter
(less refrigerant charge), and minimum pressure drop, larger diameter (bigger refrigerant charge).
The option was to choose the smaller diameter for less refrigerant charge and as a penalty the
higher pressure drop in the liquid line.
Manage the higher pressure drop plus vertical line risers in the refrigerant lines following these
suggestions:
 Mechanical liquid sub-cooling to avoid flash gas, with less refrigerant in circulation and
pumped by the compressor;
o Increase the sub-cooling by increasing the condensing pressure:
 Refrigerant flash gas reduction is OK
 Higher condensing pressure means increase in the power consumption for
the same capacity not acceptable
o Heat exchange between the liquid line and the suction line:
 Refrigerant flash gas is OK
 Using almost all the exchange in the internal unit for evaporation is OK
 No increase in cooling capacity or in power consumption acceptable
o Liquid evaporation from the liquid line to cool down the liquid line:
 Refrigerant flash gas is OK
 Cooling capacity is almost the same by increasing the refrigerant specific
enthalpy difference but reducing the mass flow – acceptable
27

Expansion valve operates together with the liquid line pressure drop to keep the
condensing pressure in reasonable level. Flash gas refrigerant gas is acceptable, but the
design of the liquid line is more difficult (refinet);
o The total pressure drop between the high pressure and low pressure is part
pressure drop and part expansion valve;
o It keeps the condensing pressure always in its minimum value - OK
o Refrigerant flash gas is OK;
o It is necessary to use electronic expansion valve – acceptable but with higher cost;
o The design of the liquid line needs closer attention to keep the same ration of
vapor mass and total mass – needs attention;
o Best control to be used
o
Liquid Line Pressure Drop and Expansion Valve
Pressure
MPa
Total pressure drop
Liquid line pressure drop
Condenser
Compressor
Evaporator
Enthalpy kJ/kg
Expansion valve pressure drop
Note:





Refrigeration
effect
No problems
Usually maximum capacity loss 2%
Total pressure drop is the sum of liquid line and expansion valve
Enthalpy is the same, it doesn’t matter the pressure drop
It’s necessary an electronic expansion valve
Fig. 3 Refrigerant P&h diagram
28
The refrigerant lines in current VRF systems can have up to 50 m rise and 190 m equivalent
length considering that the equipment is operating with full capacity and we have the following
data:
Table 4 Differential Pressure for Vertical Rise
Outside air Saturated
dry bulb pressure
ºC
kPA
34.1
3000
36.9
3200
39.6
3400
Bubble
temp.
ºC
48.99
51.81
54.49
Dew temp.
ºC
49.1
51.91
54.59
Liquid line
Differential Pressure in kPa
temp.
Density
Vertical Rise
ºC
kg/m3 10 m 20 m 30 m 40 m 50 m
38.99
914.5
90
179 269 358 448
41.81
892.6
87
175 262 350 437
44.49
870.0
85
171 256 341 426
Note: The difference between the saturated temperature and the dry bulb outside air temperature
is 15.0ºC.

50 m Vertical Rise:
o Pressure differential 448 kPa for dry bulb outside air 34.1ºC;
o Liquid line sub-cooling 10ºC;
o Equivalent saturated temperature difference 7.8 ºC
o Liquid sub-cooling due to the 50 m vertical rise 2.2 ºC
Table 5 Equivalent Temperature Difference Due to the Pressure Loss Equivalent Length
Cooling
capacity
kW
70.32
87.90
105.48
123.06
140.64
Ton
20
25
30
35
40
Liquid
line Dia.
inches
3/4
3/4
3/4
3/4
3/4
mm
19,1
19,1
19,1
19,1
19,1
ASHRAE
Table
0.,02ºC/m
kW
47.3
47.3
47.3
47.3
47.3
20
0.82
1.22
1.69
2.24
2.84
Equivalent length liquid line
60
100
140
2.45
4.08
5.72
3.66
6.10
8.54
5.08
8.47
11.86
6.71
11.18
15.65
8.53
14.22
19.91
180
7.35
10.98
15.25
20.13
25.59
Note: Calculated according to the ASHRAE table 8 ASHRAE 2006 Refrigeration Handbook
chapter 2 System Practices for Halocarbon Refrigerants.
Equivalent length 180 m plus 50 m vertical rise for 40 tons and 19.1 mm tube diameter:
 Pressure drop
o Vertical rise: 448 kPa or 7.8ºC
o Equivalent length 180 m: 1525 kPa or 25.6ºC
o Total pressure drop: 1973 kPa or 33.4 ºC
o Minimum sub-cooling to assure only liquid 5ºC
o Natural sub-cooling 10ºC
o Mechanical sub-cooling necessary 28.4ºC
o Total pressure differential 2005 kPa
o Pressure differential available for the expansion valve 32 kPa for condensing
pressure 3000 kPa and evaporating pressure 950 kPa
 Mechanical sub-cooling should be at least 28.4ºC and natural sub-cooling 10ºC, with a
final sub-cooling 5ºC
 If flash refrigerant gas is used, there may be natural sub-cooling 10ºC and the refrigerant
will have 25% mass of refrigerant vapor
29
 Mass flow will be the same, because enthalpy is the same.
It’s important to remember that these numbers are for full cooling capacity, at partial load this
won’t be an issue. Most of the time, the system is operating at partial cooling capacity.
The following tables show the pressure drop.
Table 6 Pressure Drop for a Vertical Rise Liquid Line – Refrigerant HFC 410A
Liquid
DB Out- Saturated Bubble Dew Line
Liquid
Vertical liquid line rise – pressure increase for side Air Pressure Temp. Temp Temp. Saturated Density
HFC 410A
ºC
kPA
ºC
ºC
ºC
kPa
kg/m3 10 20 30
40
50
60
70
80
28.0
2600
42.9
43
34.9
2100
957 94 188 281 375 469 563 657
750
31.1
2800
46.02 46.14 38.0
2300
935.8 92 183 275 367 459 550 642
734
34.1
3000
48.99 49.1 41.0
2500
914.5 90 179 269 358 448 538 627
717
36.9
3200
51.81 51.91 43.8
2700
892.6 87 175 262 350 437 525 612
700
39.6
3400
54.49 54.59 46.5
2800
870.0 85 171 256 341 426 512 597
682
42.2
3600
57.05 57.15 49.1
3000
846.3 83 166 249 332 415 498 581
663
44.6
3800
59.5 59.59 51.5
3200
821.0 80 161 241 322 402 483 563
644
46.9
4000
61.85 61.93 53.9
3400
793.5 78 156 233 311 389 467 544
622
49.2
3500
762.6 75 149 224 299 374 448 523
598
4200
64.1 64.17 56.1
Note:
A - Liquid sub-cooling 10ºC or 650 kPa, minimum sub-cooling 5ºC or 325 kPa
B – Blue columns liquid sub-cooling is OK
C – Yellow columns liquid sub-cooling near zero
D – Gray columns no more liquid sub-cooling, flash gas
E – The maximum vertical rise liquid line without mechanical sub-cooling is 40 m
F – It doesn’t consider the pressure drop of the equivalent length
At least 10ºC sub-cooling is necessary to ensure only liquid in all branches for vertical rise.
Table 7 Pressure Drop in ºC for the Equivalent Length of Liquid Line at Same Level
Cooling Capacity Diameter
kW
Tr
pol
Units
70
20
3/4
ºC
87
25
3/4
ºC
105
30
3/4
ºC
123
35
3/4
ºC
140
40
3/4
ºC
140
40
3/4
kPa
20
0.82
1.22
1.69
2.24
2.84
169
40
1.63
2.44
3.39
4.47
5.69
339
Equivalent length liquid line in meters
60
80
100
120
140
2.45
3.27
4.08
4.90
5.72
3.66
4.88
6.10
7.32
8.54
5.08
6.78
8.47 10.17 11.86
6.71
8.95
11.18 13.42 15.65
8.53
11.38 14.22 17.06 19.91
508
678
847
1017 1187
160
6.53
9.76
13.56
17.89
22.75
1356
Note:
A - Liquid sub-cooling 10ºC
B – Blue columns liquid sub-cooling is OK
C – Yellow columns liquid sub-cooling near zero
D – Gray columns indicate the lack of liquid sub-cooling, flash gas
E – Liquid line diameter ¾’ (19.05 mm)
F – The maximum equivalent length for the liquid line without mechanical sub-cooling for 100% cooling capacity is
40 m
G – The maximum equivalent length for the liquid line without mechanical sub-cooling for 50% cooling capacity is
140 m
30
As indicated, there is a possibility of using flash gas or increasing the mechanical cooling to
ensure unit performance.
Pressure drop in the liquid line doesn’t reduce the cooling capacity or increase the power
consumption. If the expansion valve is properly sized, the cooling capacity is reduced only 2% to
3% due to design of the liquid line.
Suction Line
Suction line is different. The pressure drop in the suction could be responsible for up to 20%
reduction of cooling capacity. VRF system manufacturers decided to maintain the COP as high
as possible by keeping the suction pressure near the compressor constant.
To keep the performance and compensate for the pressure drop, it is usually necessary to reduce
the pressure at the compressor’s suction. This will keep the evaporator cooling capacity, but
reduces the compressor cooling capacity and with the same power consumption, will lower the
COP.
VRF system manufacturers decided to maintain the compressor suction saturated temperature
always near 5.5ºC. (42°F) It doesn’t matter how the evaporators are operating—the saturated
temperature at the evaporators will be always the compressor suction saturated temperature plus
the pressure drop.
For the compressor, maintaining the temperature near 5.5ºC (42°F) is perfect—the COP is
almost constant. But for the evaporator that means higher saturated temperature, which results in:
 Small sensible cooling capacity variation 10% – so dry bulb will be OK
 100% latent cooling capacity variation – problems with humidity
 Large variation in total cooling capacity – due to the latent cooling capacity variation –
problems with humidity
 Evaporators aren’t operational for evaporating temperature above 10ºC
Table 8 Coil Cooling Capacity – Different Evaporating Temperature
Coil Cooling
Condition
1
2
3
4
Performance
Saturated
ºC
4
6
8
10
Sensible
kW
15.4
14
12.8
12.7
Percentage
%
110%
100%
91%
91%
Cooling Capacity
Latent
Percentage
kW
%
6.8
139%
4.9
100%
3
61%
0
0%
Coil Specifications:
 Application – Cooling
 Tube Diameter – ½” Copper
 Fin Material – Aluminum 0.006” thick
 Fin length 30”
 Fin Height 20”
 Rows 4, 12 FPI 8 circuits
 Air Flow – 3400 m3/h or 2000 CFM
 EAT-DB – 26.7ºC and EAT-WB – 19.4ºC
 Refrigerant HCFC-22
 Suction Temperature: 4ºC, 6ºC, 8ºC, and 10ºC
31
Total
kW
22.2
18.9
15.8
12.7
Percentage
%
117%
100%
84%
67%
Table 9 Suction Line Pressure Drop in ºC – HFC-410A
kW
Refrigerant
Charge
Cooling Capacity
70.32
87.90
105.48
123.06
140.64
Diameter
pol
mm
kg
Tr
20
25
30
35
40
1-5/8
42
1-5/8
1-5/8
1-5/8
1-5/8
1-5/8
42
42
42
42
42
20
40
Suction Line - Equivalent length
60
80
100 120 140
160
180
1.01
2.02 3.04 4.05 5.06 6.07 7.08 8.10 9.11
Pressure Drop in ºC – saturated temperature equivalent
0.28 0.55 0.83 1.10 1.38 1.66 1.93 2.21 2.48
0.41 0.82 1.24 1.65 2.06 2.47 2.89 3.30 3.71
0.57 1.14 1.72 2.29 2.86 3.43 4.01 4.58 5.15
0.76 1.51 2.27 3.02 3.78 4.53 5.29 6.04 6.80
0.96 1.92 2.88 3.84 4.80 5.76 6.72 7.68 8.65
Note:
A – Pressure drop suction line in equivalent temperature ºC
B – Blue columns pressure drop is up to 2ºC, good performance OK
C – Yellow columns pressure drop is from 2ºC up to 3ºC, acceptable
D – Pink columns pressure drop from 3ºC up to 5ºC not acceptable latent cooling is zero
E – Red columns pressure drop above 5ºC equipment won’t cool, totally wrong
E – Suction line diameter 1-5/8” (42 mm)
F – The maximum equivalent length 180 m, cooling capacity shouldn’t be greater than 75% of
the nominal cooling capacity
G – For full cooling capacity maximum equivalent length should be not greater than 100 m
H – DOAS – Dedicated outside air is mandatory for VRF with pressure drop higher than 3ºC
If the hypothesis is that the compressor cooling capacity is controlled by the suction pressure at
the compressor, for lines greater than 100 m, the unit will never be at full capacity and probably
the highest cooling capacity will be 75% of the nominal.
Active Oil Return
Active oil return is well known. The equipment opens all the expansion valves, the compressors
operate at the manufacturer’s predefined speed, and the refrigerant’s high velocity through the
suction line will return the oil.
Ducts No Longer Necessary
In small jobs or jobs with small rooms, it’s easy to use a ductless system. In large systems
usually with chilled water (high temperature differential; variable flow) and the air side (variable
flow with VAV; diffusers for variable air flow), the cost of the air side is so high that is an issue
for a complete system:
 Equipment: 18% Chiller Plant +1% Splits +9% F&C
=
28%;
 Air Distribution: 5% VAV +6% Sound attenuation +24% Ducts =
35%;
 Electrical Installation
=
10%;
 Hydraulic Installation
=
10%;
 Controls:
=
9%
 Exhaust and Ventilation
=
3%
 Fire protection system
=
3%
 Engineering
=
1%
32
Air distribution system that includes ducts, VAV box, controls air side, grilles, diffusers and
labor represents 35% of the total system price, which is why using ductless VRF systems is
advantageous. VRF system equipment will cost more than air distribution system equipment,
but when the total cost is compared, it could be less expensive. Another advantage for ductless
systems is the reduced electrical power consumption. VRF will push harder for ductless system.
SYSTEM OPERATION
Explanation of P-H Diagram (Refrigerant Characteristics Table)
The following P-H (pressure, enthalpy) diagram shows characteristics of various refrigerants
with pressure on the vertical axis and enthalpy on the horizontal axis.

The change of state from gas to liquid is called condensing and that from liquid to
gas is called evaporating. The boundary state of each change is called saturation, and the
temperature generating saturation is called the saturation temperature.

Saturation temperature depends on the kind of refrigerant and pressure. The
characteristics of saturation temperature are shown on P-H diagrams of various
refrigerants, and are called the saturation curve.

The characteristics of temperature gradients for pressure and enthalpy are shown
on P-H diagrams, called isothermal lines. By knowing the zone divided with saturation
curve in which the intersection point of pressure and isothermal line is included, the
information on the state of refrigerant can be provided. The intersection above can be
obtained by measuring pressure and temperature of refrigerant at a certain point.

For single refrigerants such as R22 and R134A, the isothermal line has no
gradient in the saturated area, that is, the saturation temperature under certain pressure is
the same at both the liquid side and the gas side. For mixed or blended refrigerants such
as R407C and R410A, in which multiple refrigerants with different boiling points are
mixed, their isothermal lines have gradients in the saturated area, so the saturation
temperatures under certain pressure are different at the liquid side and the gas side. They
are called zeotropic refrigerants, with the exception that R410A is called an quasi
azeotropic refrigerant.
States of refrigerants are classified in the following three categories:
 Superheated vapor: state that refrigerant exists as gas
 Saturated vapor: state that is a mixture of liquid and gas (this is also called wet vapor)
33

Subcooled liquid: state that refrigerant exists as liquid.
Concept of Basic Refrigeration Cycle
The following P-H diagram shows characteristics of various refrigerants with pressure on
the vertical axis and enthalpy on the horizontal axis. Theoretical refrigeration cycle
neglecting pressure loss is shown.
The difference between temperature and pressure equivalent saturation temperature is
called the Superheated Degree.



The difference between discharge pipe temperature and condensing temperature is
called the Discharging Superheated Degree (DSH).
The difference between suction pipe temperature and evaporating temperature is
called Suction Superheated Degree (SH). Generally, superheated degree means
suction-superheated degree.
The difference between temperature and pressure equivalent saturation temperature
in subcooled liquid is called Subcooled Degree (SC).
34
In order to prevent wet operation (*), the superheated degree is calculated at the evaporator
outlet, and the refrigerant flow rate into the evaporator is regulated with the expansion
valve, so that the superheated vapor only is returned to the compressor.
* Wet operation is a state of operation where wet vapor not completely vaporized in the
evaporator is sucked by the compressor, causing liquid return or liquid hammering.
Points of Refrigerant Control of VRF System
Cooling Operation
Influenced by the number of operating (thermostat-on) units, capacity, airflow rate, returnair temperature, and humidity of indoor units:
Load on total system changes.
Loads on every indoor unit are different.
Compressor Capacity Control
In order to maintain the cooling capacity corresponding to the capacity of evaporator and load
fluctuation, based on the pressure detected by low pressure sensor of the outdoor unit (Pe), the
compressor capacity is controlled so as to put the low pressure equivalent saturation
temperatures (evaporation temperature = Te) close to target value.
Superheated Degree Control of Indoor Electronic Expansion Valve
To maintain the superheated degree in the evaporator and to distribute proper refrigerant flow
rate regardless of different loads on every indoor unit, based on the temperature detected by
thermistors on the liquid pipes and gas pipes, the indoor electronic expansion valve is regulated
so as to put superheated degree at the evaporator outlet close to target value.
* Superheated degree SH = (indoor gas pipe temperature - indoor liquid pipe temperature)
*1. When sizing indoor units, caution should be taken to ensure that the unit is not oversized for
the calculated load; otherwise, large temperature swings, poor comfort levels, and overall
system inefficiencies may occur.
Heating Operation
35
Influenced by change the number of operating (thermostat-on) units, capacity, airflow rate, and
return-air temperature of indoor units:
Load on total system changes.
Loads on every indoor unit are different.
Compressor Capacity Control
To maintain the heating capacity against condenser capacity and load fluctuation based on the
pressure detected by high-pressure sensor control (Pc), compressor capacity is controlled so as to
put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to
target value.
Superheated Degree Control of Indoor Electronic Expansion Valve
To maintain the superheated degree in the evaporator, based on the pressure detected and
calculated low pressure sensor equivalent saturation temperature (Te) & the temperature detected
by the suction pipe thermistor, the outdoor unit electronic expansion valve is controlled to
maintain the superheat value of the evaporator outlet close to the target value.
* Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating
temperature)
Subcooled Degree Control of Indoor Electronic Expansion Valve
To distribute proper refrigerant flow rate regardless of different loads on every indoor unit, based
on the pressure detected, and calculated high pressure equivalent saturation temperature of the
outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe, the indoor
electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close
to target value.
* Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)
*1. When sizing indoor units, caution should be taken to ensure that the unit is not oversized for
the calculated load; otherwise the phenomenon of the EEV not fully closing can cause the zone
to heat up even during thermostat-OFF, causing user discomfort and an ineffective system.
36
Compressor Capacity Control
Using the compressor capacity controller of the VRF system, the pressure detected (Pe or
Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent
saturation temperature, and the evaporating temperature (Te) while cooling, or the
condensing temperature (Tc) while heating, are controlled with PI control so as to put them
close to the target value. This maintains stable capacity regardless of incessantly varying
loads. Refer to the following target value table. All target temperatures represent mean
saturation temperatures on the gas side
The pressure loss in piping increases depending on connected pipe length and operation
capacity of the compressor. In order to compensate the reduction of capacity caused by the
pressure loss in piping the following correction is made:

The target value can be adjusted with a field setting.
37



Long connection piping at the installation site may increase pressure loss in piping and
an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid
pipe inside resistance. In this event, a “lower” setting of target evaporation temperature
by using field setting helps to give stable operation.
For short connection piping, a higher setting enables stable operation.
In addition, samplings of evaporating temperature and condensing temperature are made
so that the pressure detected by pressure sensors of high/low pressure are read every 20
seconds and calculated. With each reading, the compressor capacity (INV frequency or
STD ON/OFF) is controlled to eliminate deviation from target value.
Control of Electronic Expansion Valves
Electronic Expansion Valve of Outdoor Unit:
In Cooling Operation
In cooling operation, the outdoor electronic expansion valve is basically in the fully open
position.
Note: The valve can be fully closed when a bridge circuit is included.
In Heating Operation = Superheated Degree Control
Superheated degree [SH] is calculated from the low-pressure equivalent saturation temperature
(Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe)
and temperature detected by the suction pipe thermistor (Te). The electronic expansion valve
opening degree is regulated so that the superheated degree [SH] becomes close to target
superheated degree [SHS].
 When SH > SHS, adjust to make opening degree of the electronic expansion valve
larger than the present one.
 When SH< SHS, adjust to make opening degree of the electronic expansion valve
smaller than the present one.
SH : Superheated degree (Ts
SHS : Target superheated degree (Normally 9° F / 5°C)
REFERENCE: Control range of outdoor electronic expansion valve:
R410A unit ... 0 to 1400 pulses
Electronic Expansion Valve of Indoor Unit
In Cooling Operation = Superheated Degree Control
Superheated degree [SH] is calculated from temperature detected by the gas pipe
thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor
(Tl). The electronic expansion valve opening degree is controlled so that the superheated
degree [SH] is close to the targeted superheated degree [SHS].
The compensation is made based on the temperature difference between set-point
temperature and the return-air thermistor temperature (ΔT).
 When SH > SHS, adjust to make opening degree of the electronic expansion valve
larger than the present one.
 When SH< SHS, adjust to make opening degree of the electronic expansion valve
smaller than the present one.
o
38
o SHS : Target superheated degree
 Normally 9° F (5°C), but when the temperature difference (ΔT)
decreases, SHS increases. Even when SH is large, the opening
degree of the electronic expansion valve becomes small.
o (ΔT): Remote controller set-air thermistor
detection value
In Heating Operation = Subcooled Degree Control
Subcooled degree [SC] is calculated from the high pressure equivalent saturation
temperature (Tc) converted from the pressure detected by high pressure sensor of the
outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit
(Tl). Electronic expansion valve opening degree is regulated so that the subcooled degree
[SC] is close to target subcooled degree [SCS].
The compensation is made based on the temperature difference between set-point
temperature and the return-air thermistor temperature (ΔT).
 When SC > SCS, adjust to make opening degree of the electronic expansion valve
larger than the present one.
 When SC < SCS, adjust to make opening degree of the electronic expansion valve
smaller than the present one.
o SC : Subcooled degree (Tc - Tl)
o SCS : Target Subcooled degree
o Normally 9° F (5°C), but when the temperature difference (ΔT) decreases,
SCS increases. Even when SC is large, the opening degree of the
electronic expansion valve becomes small.
o (ΔT): Remote controller set-point temperature - return-air thermistor
detection.
Heating Operation
Using VRF heat pump units for heating and cooling can increase building energy efficiency,
especially when the heating obtained from the heat pump mode replaces an electric resistance
heating coil. Most VRF units provide higher heating capacities than conventional DX heat
pumps at low ambient temperatures. The designer must evaluate the heat output for the units at
the outdoor design temperature. Manufacturers indicate the heating capacities at catalog
minimum outside temperature, after which point, a low ambient kit is sometimes offered as an
option. When the outdoor temperature drops below the temperature indicated in the catalog, the
heating output from the heat pump cycle decreases. Supplemental heating should be considered
when the heating capacity of the VRF units is below the heating capacity required by the
application. Sequence of operation and commissioning must specify and prevent premature
activation of supplemental heating.
Simultaneous Heating and Cooling Operation
In heat-recovery VRF systems, although several indoor sections are connected to one outdoor
section, some indoor sections can provide heating, while others provide cooling. The prices for
those units and their installation are higher than that of cooling- or heating-only units. More
economical design can sometimes be achieved by combining zones with similar heating or
cooling requirements together. When zones with different cooling/heating requirements are
connected to the same outdoor section, consider units that are capable of providing simultaneous
heating and cooling. Examples of zones that may require simultaneous heating and cooling when
39
combined are interior and exterior zones; exterior zones with different exposures; and zones
requiring comfort cooling with rooms requiring close environmental control. Units capable of
providing simultaneous heating and cooling are not available in smaller sizes (e.g., capacities
below 6 tons [21 kW]).
Heating and Defrost Operation
In heating mode, VRF systems typically must defrost like any mechanical heat pump, using
reverse cycle valves to temporarily operate the outdoor coil in cooling mode. Oil return and
balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil
entrained in the low side of the system is brought back to the high side by increasing the
refrigerant velocity using a high-frequency operation performed automatically based on hours of
operation.
The DX fan coils are constant air volume, but use variable refrigerant flow through an electronic
expansion valve. The electronic expansion valve reacts to several temperature-sensing devices
such as return air, inlet and outlet refrigerant temperatures, or suction pressure. The electronic
expansion valve modulates to maintain the desired set point.
APPLICATIONS—BUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)
Offices
Schools and universities
Limited care facilities; nursing homes
Multi-tenant dwellings; apartments
Hotel and motel
Churches
Residential
Hospitals
REFERENCES
Refrigerants (from ASHRAE Standard 34)
R-22 — Single Compound - HCFC – Methane-based, Contains Chlorine - Safety Group A1
(S34-Table 1)
R-32 — Single Compound – HFC – No Chlorine – Safety Group A2 (S34-Table 1)
R-125 — Single Compound – HFC – No Chlorine – Safety Group A1 (S34-Table 1)
40
R-134A— Single Compound - HFC – Ethane-based, No Chlorine - Safety Group A1 (S34-Table
1)
R-407C — Zeotropic Blend (23.0% R-32, 25.0% R-125, 52.0% R-134A) — HFC – No chlorine
(S34-Table 2)
R-410A — Zeotropic Blend (50% R-32, 50% R-125) — HFC – No chlorine (S34-Table 2)
Safety Group Classifications (from ASHRAE Standard 34)
Classification consists of two alphanumeric characters. The capital letter indicates toxicity and
the numeral indicates flammability (S34-6.1.1).
Class A signifies refrigerants for which toxicity has not been identified at concentrations less
than 400 ppm (S34-6.1.2).
Class 1 indicates refrigerants that do not show flame propagation, Class 2 indicates refrigerants
that have a low flammability limit (S34-6.1.3)
Some Related Standards: UBC - Chapter 11 and ISO 5149
Code Documents: OSHA 29 CFR 1910.119 and EPA 40 CFR 68
1-“Green Buildings show higher rents, occupancy” Building Operating Management
July 2008. http://www.facilitiesnet.com/bom/article.asp?id=9225
41