Optional Accessories
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 intake compartment and a pressure sensor to monitor pressure drop across the wheel.
Frost Threshold Temperatures
Indoor RH @ 70°F | Frost Threshold Temp |
|
|
20% | |
|
|
30% | |
|
|
40% | 0º F |
|
|
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 wheel 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 Timer Faceplate drawing below)
•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 (at outdoor air intake) and an airflow pressure switch (located at the preheater) 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 outdoor air side doors on or temporarily jumper the airflow pressure switch in the preheater control center to avoid nuisance tripping of the pressure switch. Also check the airflow switch pressure tap located at the supply discharge blower to ensure the tubing is connected and the tap is not blocked. 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 reduced speed until the pressure differential falls below the set point. The temperature and pressure differential set points are set at the factory,
•Jumper the wheel pressure switch in the unit control center. 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. (normal position)
Dip
Switch
T2
Timer
Scale
T1
but are
• Turn the temperature | 16 | 18 | A2 |
sensor up as high as
possible. The supply blower should cycle on for one minute, then turn off for one minute.
14Model ERV Energy Recovery Unit
