Frost Control Application/Operation; Timed Exhaust Frost Control; Modulating Wheel Frost Control - Bosch FHP RT-20 Installation, Operation And Maintenance Manual

Frost Control Application/Operation

Extremely cold outdoor air temperatures can cause
moisture condensation and frosting on the enthalpy
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 enthalpy wheel. The typical
temperature setting corresponds to the indoor air
relative humidity as shown in the Frost Threshold
Temperatures
Frost Threshold Temperatures
Table and
Indoor RH
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
ensure 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 enthalpy
wheel. Default factory settings are 5 minutes off and
30 minutes on. Use the following test procedure for
troubleshooting.
Testing (refer to Timer
diagram)
• 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)
24 Model RT Energy Recovery Unit
Frost Threshold
at 70°F Temperature
20% -10º F
30% -5º F
40% 0º F
A1 B1 15
0.8 0.6
1.0 0.4
0 0.2
T2
1 MIN T2
Timer
Scale
1 MIN T1
0.8 0.6
1.0 0.4
0 0.2
T1
16 18 A2
Timer
• 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 (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 airside 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
Dip
the frost threshold temperature AND the pressure
Switch
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,
but are field-adjustable (refer to VFD section for more
information). The variable frequency drive will be fully
programmed at the factory.