Demand Defrost Control - Rheem RQMP 14 Series Installation Instructions Manual

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2. In the heating mode, the first heat stage of the thermostat will energize one or
more supplementary resistance heaters. If required or considered desirable, the
resistance heat may also be controlled by outdoor thermostats. In the heating
mode, the thermostat will, on a call for heating, energize the indoor blower relay.
XVII. DEMAND DEFROST CONTROL
The demand defrost control is a printed circuit board assembly consisting of solid state
control devices with electro-mechanical outputs. The demand defrost control monitors
the outdoor ambient temperature, outdoor coil temperature, and the compressor run-
time to determine when a defrost cycle is required.
DEFROST INITIATION
A
defrost will be initiated when the three conditions below are satisfied:
1) The outdoor coil temperature is below 35°F.
2) The compressor has operated for at least 34 minutes with the outdoor coil tempera-
ture below 35°F.
3) The measured difference between the ambient temperature and the outdoor coil
temperature is greater than the calculated delta T.
Additionally, a defrost will be initiated if six hours of accumulated compressor run-time
has elapsed without a defrost with the outdoor coil temperature below 35°F.
DEFROST TERMINATION
Once a defrost is initiated, the defrost will continue until fourteen minutes has elapsed or
the coil temperature has reached the terminate temperature. The terminate temperature
is factory set at 70°F, although the temperature can be changed to 50°F, 60°F, 70°F or
80°F by relocating a jumper on the board.
TEMPERATURE SENSORS
The coil sensor is clipped to the top tube on the outdoor coil at the point fed by the distri-
bution tubes from the expansion device (short 3/8" dia. tube). The air sensor is located
behind the compressor access panel.
If the ambient sensor fails the defrost control will initiate a defrost every 34 minutes with
the coil temperature below 35°F.
If the coil sensor fails the defrost control will not initiate a defrost.
TEST MODE
The test mode is initiated by shorting the TEST pins. In this mode of operation, the
enable temperature is ignored and all timers are sped up by a factor of 240. To initiate a
manual defrost, short the TEST pins. Remove the short when the system switches to
defrost mode. The defrost will terminate on time (14 minutes) or when the termination
temperature has been achieved. Short TEST pins again to terminate the defrost immedi-
ately.
TROUBLE SHOOTING DEMAND DEFROST
Set the indoor thermostat select switch to heat and thermostat lever to a call for heat.
Jumper the "test pins" to put the unit into defrost. If the unit goes into defrost and comes
back out of defrost, the indication is that the control is working properly.
If the unit did not go into defrost using the test pins, check to ensure that 24V is being
supplied to the control board. If 24V is present then replace the control.