Greenheck Fan HEAT RECOVERY UNIT WITH EVAPORATIVE COOLING HRE-20 - Manual (Page 18)

18
Frost Control Application/Operation
Extremely cold outdoor air temperatures can cause moisture condensation and frosting on the energy recovery
wheel. Frost control is an optional feature that will prevent/control wheel frosting. Three options are available:
1) Timed Exhaust frost control
2) Electric preheat frost control
3) Modulating wheel frost control
All of these options are provided with a thermostat (with
probe) mounted in the outdoor air inlet compartment and
a pressure sensor to monitor pressure drop across the
wheel . The typical temperature setting corresponds to the
indoor air relative humidity as shown in the Frost Threshold Temperatures Table and represents when frost can
occur . An increase in pressure drop would indicate that frost is occurring . Both the pressure sensor AND the
outdoor air temperature sensor must trigger in order to initiate frost control . The two sensors together insure
that frost control is only initiated during a real frost condition . Field wiring of a light (or other alarm) between
6 & C in the control center will notify personnel when unit is in frost control mode (refer to Remote Panel Wiring
schematics section for wiring details) . The following explains the three options in more detail .
Timed exhaust frost control includes a timer in addition to the thermostat
and pressure sensor . When timed exhaust frost control is initiated, the timer
will turn the supply blower on and off to allow the warm exhaust air to defrost
the energy recovery wheel . Default factory settings are 5 minutes off and 30
minutes on . Use the following test procedure for troubleshooting .
Testing (refer to diagram at right)
• Jumper the pressure switch. Set the Timer Scale for T1 and T2 to
1 minute . Set the Timer Settings for T1 and T2 to 1 .0 . Set the dip
switch to the down position .
• Turn the temperature sensor up as high as possible. The supply blower
should cycle on for one minute, then turn off for one minute .
• After testing, set the Timer Scale as follows: T1 = 10 minutes, T2 =
1 hour
• Set the Timer Settings as follows: T1 = 0.5, T2 = 0.5. The timer is now
set for 5 minutes off and 30 minutes on . Remember to remove the jumper.
Electric preheat frost control includes an electric heater (outdoor air inlet) and an air pressure switch
(outdoor air outlet) in addition to the thermostat and pressure sensor on wheel . (Refer to Electric Heater
Application/Operation for electric preheater location) . When electric preheat frost control is initiated, the electric
preheater will turn on and warm the air entering the energy wheel to avoid frosting . Use the following test
procedure for troubleshooting .
Testing
• Turn the thermostat as high as it will go and jumper the wheel pressure sensor. The heater
should turn on .
• If it doesn’t, either put the supply side doors on or temporarily jumper the air pressure switch to
avoid nuisance tripping of the pressure switch . Remember to remove the jumpers.
Modulating wheel frost control includes a variable frequency drive in addition to the thermostat and pressure
sensor . When modulating wheel frost control is initiated, the variable frequency drive will reduce the speed
of the wheel reducing the speed of the energy wheel reduces its effectiveness, which keeps the exhaust air
condition from reaching saturation, thus, eliminating condensation and frosting . If the outdoor air temperature is
greater than the frost threshold temperature OR the pressure differential is less than the setpoint, the wheel will
run at full speed. If the outdoor air temperature is less than the frost threshold temperature AND the pressure
differential is greater than the setpoint, the wheel will run at reduced speed until the pressure differential falls
below the setpoint . The temperature and pressure differential set points are set at the factory, but are field-
adjustable (refer to VFD section for more information) . The variable frequency drive will be fully programmed at
the factory .
A1 B1 15
16 18 A2
0.2 0
0.4 1.0
0.6 0.8
0.2 0
0.4 1.0
0.6 0.8
T1
T2
T2 1 MIN
T1 1 MIN
Timer
Scale
Dip
Switch
Frost Threshold Temperatures
Indoor RH @ 70º F Frost Threshold Temp
20% 2º F
25% 7º F
30% 14º F
Timer
OPTIONAL ACCESSORIES