Frost Control Strategies
Engineering Bulletin
Order No.
VCES-DEF-EB-1
Date
July 2015
Rev. No.
1 (July 2015)
Frost control must be considered when specifying an
energy recovery unit for climates with severe winter
conditions. Frost buildup on the energy recovery device
results in reduced airflow through the heat exchanger,
reduced energy savings and potential damage to the
device. This bulletin describes the need for frost control
and provides both simple and more complex frost control
strategies that will allow continuous ventilation and
prevent frost formation.
At a Glance
• Why do we need frost control?
–– Flat plate heat exchangers
–– Heat pipe heat exchangers
–– Energy recovery wheels
• Frost control strategies
–– Exhaust only
–– Recirculation
–– Face and bypass
–– Traversing
–– Variable frequency drive
–– Preheat
©2015 Nortek Air Solutions, LLC
Why Do We Need Frost Control?
Supply air inlet
(Entering)
X4
Air-to-air heat exchanger
X1
Supply air outlet
(Leaving)
Exhaust air outlet
(Leaving)
X2
X3
Exhaust air inlet
(Entering)
Figure 1: Airflow diagram
Frost control must be considered when specifying an energy recovery unit for
climates with severe winter conditions. This is true whether you are using a flat
late or heat pipe heat exchanger or you are using an energy recovery wheel. Frost
buildup on the energy recovery device results in reduced airflow through the
exchanger, reduced energy savings and potential damage to the device.
Determining or measuring frost threshold is not an easy task and different
approaches can be used. These include measuring temperature, humidity, or
pressure drop variation. In general, frost buildup becomes a problem when
outdoor air temperatures reach −5°F [−15°C].
Figure 1 illustrates the flow of air through an air-to-air heat exchanger. Frost
formation can occur at Station X4 as the exhaust air temperature drops below the
frost threshold while at saturation. This is due to the energy transfer to the supply
side.
Venmar CES typically monitors air temperature and/or relative humidity at Station
X4 to ensure that the air temperature remains above frost threshold values and/
or the humidity remains below them. In addition, to maximize effectiveness, the
outdoor air temperature at Station X1 may also be monitored to start or modify
the frost control sequence.
Figure 2: Flat plate heat exchanger
Figure 3: Heat pipe heat exchanger
Requirement for Frost Control on Flat Plate Heat Exchangers
In flat plate heat exchangers, warm return air cools as it passes through the heat
exchanger. See Figure 2. Moisture from the warm air condenses and creates water
droplets on the media. As cool outdoor air enters the heat exchanger, the water
droplets start to freeze, creating ice on the cold corner of the core. If the ice keeps
building, it will eventually block the flow of air through the heat exchanger. A
defrost cycle is required to warm up the cold side of the core, to melt the ice and
to maintain airflow and heat transfer through the heat exchanger.
Requirement for Frost Control on Heat Pipe Heat Exchangers
In heat pipe heat exchangers, warm return air cools as it passes through the
heat exchanger. See Figure 3. Moisture from the warm air condenses and creates
water droplets on the heat pipe. As cool outdoor air enters the heat exchanger,
the water droplets start to freeze, creating ice on the cold side of the heat pipe.
If the ice keeps building, it will eventually block the flow of air through the heat
exchanger. A defrost cycle is required to warm up the cold side of the heat pipe, to
melt the ice and to maintain airflow and heat transfer through the heat exchanger.
Requirement for Frost Control on Energy Recovery Wheels
In energy recovery wheels, warm return air cools as it passes through the rotating
wheel. See Figure 4. Moisture from the warm air condenses and creates water
droplets on the sensible wheel. The water molecules are then absorbed by the
desiccant on the enthalpy wheel. As the wheel rotates through the cold supply air,
the water can start to freeze and form ice before the water droplets can be cleared
on the sensible wheel and before the water molecules can be desorbed on the
enthalpy wheel. The ice typically forms on the internal surface of the wheel, near
the outdoor air entrance.
Figure 4: Energy recovery wheel
Frost Control Strategies
VCES-DEF-EB-1
July 2015
Page 2 of 6
Frost Control Strategies
Simple strategies can be used to defrost ice formation in both flat plate and
heat pipe heat exchangers. These include exhaust-only and recirculation frost
control. There are also more complex frost control strategies that allow continuous
ventilation and prevent frost formation on flat plate and heat pipe heat exchangers
and on energy recovery wheel units. Examples include face and bypass, traversing,
variable frequency drive (VFD) and pre-heat frost control.
If supply air temperature is critical, post-heating must be included or applied to
trim the supply air temperature to the desired setpoint, as some frost control
strategies can drop the supply air temperature below freezing.
These strategies are discussed in more detail on the following pages, along with
some guidelines to keep in mind when evaluating frost control options. The
selection of frost control should take into consideration energy savings and the
operating sequence for cooling, heating, occupied, unoccupied and economizer
operation.
Exhaust Only Frost Control
Flat plate or heat pipe heat exchangers only
Exhaust Only Defrost
1
OA
EA
3
4
RA
2
SA
When the unit goes into a defrost cycle, the exhaust fan continues to operate, the
supply fan is de-energized and the outdoor air damper closes. This method can
be used with flat plate or heat pipe heat exchangers only and is ideal for source
control applications where continuous exhaust is required.
Figure 5: Exhaust only frost control –
flat plate heat exchanger
Exhaust Only Defrost
EA
OA
4
3
1
RA
2
Figure 6: Exhaust only frost control –
heat pipe heat exchanger
Exhaust only frost control is a simple, cost-effective method, which periodically
defrosts ice formation on the heat exchanger by shutting down the supply fan to
remove the source of cold air. See Figures 5 and 6.
SA
Since the building is under a negative pressure during the defrost cycle, there
is the potential for combustion appliances or equipment to backdraft into the
mechanical room where the unit is installed. Pressure relief should be provided
through the use of fresh air dampers as part of the building ventilation system.
A drawback of this method is that ventilation is interrupted when the supply fan
shuts down during the defrost cycle. This may not be acceptable in all applications
and may not meet indoor air quality (IAQ) requirements as set out in ASHRAE
Standard 62.
Shutting down the supply fan may also be an issue if the unit has high thermal
inertia heating devices, such as electric heating coils or an indirect gas-fired
heater.CFM spikes may cause overheating, potential operation faults and supply
temperature variations.
Exhaust only frost control has two temperature-initiated, time-based cycles of
defrost to ventilation:
1. At an initiating outdoor air temperature, duration is a linear, time-versustemperature variant in minutes of defrost to minutes of ventilation until a
lower outdoor air temperature is reached.
2. Below the lower outdoor air temperature, duration and cycle time is a
constant in minutes of defrost to minutes of ventilation.
Frost Control Strategies
VCES-DEF-EB-1
July 2015
Page 3 of 6
Recirculation Frost Control
Standard: Flat plate or heat pipe heat exchanger units only
Recirculation Defrost
3
OA
EA
RA
2
SA
Figure 7: Recirculation frost control –
flat plate heat exchanger
Recirculation Defrost
EA
4
OA
3
1
RA
2
SA
Figure 8: Recirculation frost control –
heat pipe heat exchanger
Recirculation frost control is another low cost method of frost control. Ice
formation on the heat exchanger is periodically defrosted by shutting down the
exhaust fan, by closing the outdoor and exhaust air dampers and by opening a
recirculation air damper to remove the source of cold air. When the unit goes into
a defrost cycle, the supply fan remains on to recirculate building exhaust air back
into the occupied space. The exhaust air goes through the heat exchanger and
provides defrosting in the absence of cold outdoor air. See Figures 7 and 8.
A drawback of this method is that ventilation is interrupted when the exhaust fan
shuts down during the defrost cycle. This may not be acceptable in all applications
and may not meet indoor air quality (IAQ) requirements as set out in ASHRAE
Standard 62.
Standard recirculation frost control has two temperature-initiated, time-based
cycles of defrost to ventilation:
1. At an initiating outdoor temperature, duration is a linear, time-versustemperature variant in minutes of defrost to minutes of ventilation until a
lower outdoor temperature is reached.
2. Below the lower outdoor temperature, duration and cycle time is constant in
minutes of defrost to minutes of ventilation.
Modulating: Energy recovery wheel units only
Recirculation Defrost
EA
OA
4
3
1
RA
2
Figure 9: Recirculation frost control –
energy recovery wheel
SA
In energy recovery wheel units, recirculation frost control prevents frost buildup
inside the wheel by mixing building exhaust air with outdoor air through
modulating recirculation. See Figure 9. The outdoor and exhaust air dampers
reduce the cold air entering the wheel, thereby increasing the exhaust air
temperature above the frost threshold. A recirculation air damper, located
downstream of the exhaust fan, is used to remove the source of cold air. Both
supply and exhaust fans continue operating but at a reduced ventilation rate. A
plenum or FANWALL® exhaust fan is required to keep the increase in unit length to
a minimum and for best performance.
Recirculation frost control for energy recovery wheel units with modulating
dampers has the following advantages over 100% fixed ventilation.
•
Frost prevention is used instead of defrost, eliminating possible heat wheel
damage.
•
Exhaust air temperature and humidity sensors are used instead of an outdoor
air temperature sensor. The result is a lower leaving exhaust air temperature
and, hence, higher efficiencies and lower reheat requirements.
•
Exhaust and ventilation operation continues during defrost, though at a
reduced rate, versus shutting down.
A drawback of this method is that a percentage of exhaust air is recirculated to
the supply air and the actual ventilation rate is reduced.
Modulating recirculation frost control starts when the exhaust air temperature
drops below the frost threshold value and will modulate the mixing dampers to
maintain the exhaust air humidity below the frost threshold. Extensive testing has
shown that the exhaust temperature can drop well below the starting exhaust
temperature before frost crystals appear on the wheel.
Frost Control Strategies
VCES-DEF-EB-1
July 2015
Page 4 of 6
Face and Bypass Frost Control
1
OA
EA
Flat plate or heat pipe heat exchanger units only
3
4
RA
2
SA
Exchanger Side View
Face and bypass is a preventative strategy for flat plate or heat pipe heat
exchangers, with the objective of preventing frost formation while maintaining
100% ventilation. See Figures 10 and 11. As the outdoor air becomes colder,
face and bypass dampers upstream of the heat exchanger modulate to reduce
the amount of outdoor air flowing through the heat exchanger. This reduces the
amount of energy recovered and keeps the exhaust temperature above the frost
threshold.
The supply and exhaust fans, as well as the outdoor air and exhaust air dampers,
continue to operate during the frost control cycle. Thus there is no interruption to
ventilation, making this strategy ideal for harsher environments or source control
applications like laboratories.
Figure 10: Face and bypass frost
control – flat plate heat exchanger
As both fans remain running, there is no depressurization of the building,
eliminating the potential for combustion appliance backdraft into the occupied
space.
The face and bypass dampers can also be used for free cooling (economizer) and/
or supply air temperature control in different modes of operation, if required.
Face and bypass frost control starts when the exhaust air temperature drops
below the frost threshold value. The face and bypass dampers will modulate to
maintain the exhaust air temperature above the frost threshold. This method
is proportionally controlled, meaning the face and bypass dampers will be
modulated linearly to maintain the exhaust air temperature setpoint.
Figure 11: Face and bypass frost
control – heat pipe heat exchanger
Traversing Frost Control
1
OA
EA
4
Flat plate heat exchanger units only
3
RA
2
Exchanger Side View
SA
Traversing frost control is similar to face and bypass control, but uses several
motorized face dampers in series upstream of the heat exchanger. The dampers
periodically block a portion of the outdoor air from flowing through the heat
exchanger to defrost ice formation on the exhaust portion of the heat exchanger
while maintaining 100% ventilation. There are typically five motorized dampers so
that, at any given time, four out of five dampers are open and one is closed. See
Figure 12.
The advantages of this strategy are similar to face and bypass except for free
cooling, where the addition of an outdoor air bypassing section and motorized
damper is required.
Figure 12: Traversing frost control –
flat plate heat exchanger
Traversing frost control has two temperature-initiated, time-based cycles of frost
control:
1. At an initiating outdoor air temperature, the first damper of the set closes
to defrost while all of others are open and exchanging heat. Then the first
damper of the set re-opens and the second closes, and so on. A cycle is
finished when all of the dampers in the set have closed once. If, after the
cycle is complete, the outdoor air temperature is still lower than the set point,
a new cycle starts. The duration of closure of each damper in the set is a
linear, time-versus-temperature variant in minutes of defrost to minutes of
ventilation until a lower outdoor temperature is reached.
2. Below the lower outdoor temperature, duration and closing time for each
damper of the set remains constant in minutes of defrost to minutes of
ventilation until a cycle is completed.
Frost Control Strategies
VCES-DEF-EB-1
July 2015
Page 5 of 6
Variable Frequency Drive (VFD) Frost Control
Energy recovery wheel units only
The objective of VFD frost control is to prevent frost formation within the energy
recovery wheel and maintain 100% ventilation. At the normal running rpm of the
wheel, as the outdoor air temperature becomes colder, the exhaust temperature
also becomes colder and the exhaust relative humidity rises. With this frost control
strategy, the VFD slows down the rpm of the energy recovery wheel drive motor.
This reduces the sensible and latent effectiveness of the wheel, thereby reducing
the risk of frost crystals forming on the internal media.
VFD frost control starts when the exhaust air temperature drops below the frost
threshold value. Then the VFD will vary the speed of the wheel drive motor to
maintain the exhaust air humidity below the frost threshold. Extensive testing has
shown that the exhaust temperature can drop well below the starting exhaust
temperature before frost crystals appear on the enthalpy wheel, making it the
most efficient frost control strategy and requiring the lowest reheat.
Preheat Frost Control
Flat plate or heat pipe heat exchangers and energy recovery wheel units
OA
−22ºF
[−30ºC]
EA
23ºF
[−5ºC]
Pre-heat
Figure 13: Preheat frost control – energy
wheel or heat pipe heat exchanger
4
1
3
RA
SA
2
Figure 14: Preheat frost control – flat
plate heat exchanger
Pre-heat frost control is a preventative strategy for flat plate or heat pipe heat
exchanger and for energy recovery wheel units. See Figures 13 and 14. The
objective is preventing frost from occurring within the heat exchanger while
maintaining 100% or continuous ventilation. The benefit is efficiency as energy
recovery operates at full capacity.
Heating coils (electric, steam or hot water) are duct-mounted or integrated into
the unit in the outdoor airstream so that the entering outdoor air temperature is
pre-conditioned to a temperature above the frost threshold for the technology
being used.
Because there is no interruption to fan operation, this strategy provides continuous
ventilation, making it ideal for harsh environments or source control applications.
While pre-heat typically has higher up-front costs than other strategies, it can
result in significant operating savings in climates where frost control is required for
a long period of time (more than a few hours).
The pre-heater should be sized for the coldest outdoor air design temperature and
to the appropriate outdoor air temperature where the exhaust air frost threshold
is reached based on the design conditions and airflows for the type of heat
exchanger in the selection report.
Nortek Air Solutions has a policy of continuous product improvement and reserves
the right to change design and specifications without notice.
Nortek Air Solutions is a leader in innovative custom and engineered HVAC solutions for commercial, industrial and critical environments through our brands Governair, Huntair,
Mammoth, Temtrol, Venmar CES, Ventrol and Webco. Nortek Air Solutions, LLC is a subsidiary of Nortek, Inc., a global, diversified company whose many market leading brands deliver
broad capabilities and a wide array of innovative, technology-driven products and solutions for lifestyle improvement at home and at work.
nortekair.com
R
©2015 Nortek Air Solutions, LLC
VCES-DEF-EB-1
July 2015