Frost Control Application/Operation - Greenheck APEX-200 Installation, Operation & Maintenance Manual

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 supply air inlet compartment (see
FIGURE 15) 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
Table 2 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. 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 FIGURE 14)
• 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. When electric preheat frost
control is initiated, the electric pre-heater 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 set point, 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 set point, the wheel will run at minimum speed until the pressure differential falls
below the set point. The temperature and pressure differential set points are set at the factory, but are field-
adjustable. The variable frequency drive will be fully programmed at the factory.
Installation
Indoor RH @ 70°F
TABLE 2: Frost threshold temperatures
10
Frost Threshold Temp
20% -10°F
30% -5°F
40% 0°F
T2
Timer
Scale
T1
FIGURE 14
®
A1 B1 15
0.8 0.6
Dip
1.0 0.4
Switch
0 0.2
1 MIN T2
1 MIN T1
0.8 0.6
1.0 0.4
0 0.2
16 18 A2