Upon cooling demand, the thermostat closes circuit R to O and Y.
Closing R to O and Y energizes the reversing valve for cooling
operation and closes the heat pump contactor, starting the
compressor and outdoor fan. The thermostat automatically
closes R to G circuit, which also brings on the indoor fan at the
same time. Upon satisfying cooling demand, the thermostat will
open the above circuits and open the main contactor, stopping
the compressor and outdoor fan. If the indoor unit is equipped
with a delay timer, the blower will continue to operate for 60 to
90 seconds, which improves system efficiency.
Upon heating demand, the thermostat closes circuit R to Y, which
closes the heat pump contactor, starting the compressor and
outdoor fan. The reversing valve is not energized in the heating
mode. The thermostat again automatically brings on the indoor
fan at the same time. Upon satisfying heating demand, the
thermostat opens above circuits and stops heat pump operation.
If the outdoor ambient conditions are such that frost forms on the
outdoor coil, the defrost control monitors the need for and
initiates and terminates defrost cycles as necessary to maintain
The defrost control is time/temperature initiated and temperature
terminated with a maximum defrost time (time-out) of 10 minutes.
The time between defrost cycles is preset at 60-minute intervals
at the factory, but can be field adjusted between 30, 60, or
90 minutes. To adjust the time period between defrost cycles,
see "Adjust Time Between Defrost Cycles" in "Adjusting Defrost
The defrost control will initiate a defrost cycle when the selected
time period has elapsed and the defrost sensor sees a
temperature below freezing. At the start of a defrost cycle, the
defrost control will energize the reversing valve solenoid, shifting
the reversing valve and de-energizing the outdoor fan. The
defrost relay will also close, energizing temporary heat for
increased comfort during defrost (if the indoor unit is so
equipped). The heat pump will remain in defrost until the defrost
sensor has determined that the frost has been removed from the
coil or a 10-minute period has elapsed, whichever comes first.
SEQUENCE OF OPERATION
The defrost thermostat is located on the liquid line between the
check/expansion valve and the distributor. When defrost
thermostat senses 42ºF (5.6ºC) or cooler, the thermostat contacts
close and send a signal to the defrost control board to start the
defrost timing. It also terminates defrost when the liquid line
warms up to 70ºF (21.1ºC).
The defrost control board includes the combined functions of the
time/temperature defrost control, defrost relay, diagnostic LEDs
and terminal strip for field wiring connections. See "Defrost
Control Board" in "Adjusting Defrost System."
The control provides automatic switching from normal heating
operation to defrost mode and back. During compressor cycle
(call for defrost), the control accumulates compressor run times
at 30-, 60- or 90-minute field-adjustable intervals. If the defrost
thermostat is closed when the selected compressor run time
interval ends, the defrost relay is energized and defrost begins.
Defrost Control Timing Pins
Each timing pin selection provides a different accumulated
compressor run time period during one thermostat run cycle. This
time period must occur before a defrost cycle is initiated. The
defrost interval can be adjusted to 30 (T1), 60 (T2) or 90 (T3)
minutes. See "Defrost Control Board" in "Adjusting Defrost
System." The defrost timing jumper is factory-installed to provide
a 60-minute defrost interval. If the timing selector jumper is not in
place, the control defaults to a 90-minute defrost interval. The
maximum defrost period is 14 minutes and cannot be adjusted.
A test option is provided for troubleshooting. The test mode may
be started any time the heat pump is in the heating mode and the
defrost thermostat is closed or jumpered. If the jumper is in the
test position at power-up, the control will ignore the test pins.
When the jumper is placed across the test pins for 2 seconds, the
control will enter the defrost mode. If the jumper is removed
before an additional 5-second period has elapsed (7 seconds
total), the heat pump will remain in defrost mode until the defrost
thermostat opens or 14 minutes have passed. If the jumper is not
removed until after the additional 5-second period has elapsed,
the defrost will terminate and the test option will not function
again until the jumper is removed and reapplied.