Bard QTEC QH303D, Installation Instructions Manual

Manual 2100-435D
Page

32 of 38

FIGURE  22

HEAT PUMP CONTROL BOARD (HPC)

MIS-973

DEFROST CYCLE

The defrost cycle is controlled by temperature and time
on the solid state heat pump control.  (See Figure 22.)

When the outdoor temperature is in the lower 40°F
temperature range or colder, the outdoor coil
temperature is 32°F or below.  This coil temperature is
sensed by the coil sensor mounted near the bottom of
the outdoor coil.  Once coil temperature reaches 30°F or
below, the coil sends a signal to the control logic of the
heat pump control and the defrost timer will start.

After 30 minutes at 30°F or below, the heat pump
control will place the system in the defrost mode.

During the defrost mode the refrigerant cycle switches
back to the cooling cycle, the outdoor motor stops,
electric heaters are energized, and hot gas passing
through the outdoor coil melts any accumulated frost.
When the temperature rises to approximately 57°F, the
coil sensor will send a signal to the heat pump control
which will return the system to heating operations
automatically.

If some abnormal or temporary condition such as a high
wind causes the heat pump to have a prolonged defrost
cycle, the heat pump control will restore the system to
heating operation automatically after 10 minutes.

There are three settings on the heat pump control – 30
minute, 60 minute and 90 minute.  Models are shipped
wired on the 30 minute setting for greatest operating
economy.  If special circumstances require a change to
another time, remove wire connected to terminal 30 and
reconnect to desired terminal.  Refer to Figure 22.  The
manufacturer’s recommendation is for 30 minute defrost
cycles.

There is a cycle speed up jumper on the control.  This
can be used to reduce the time between defrost cycle
operation without waiting for time to elapse.

There is an initial defrost jumper (sen
jmp) on the control that can be used
at any outdoor ambient during the
heating cycle to simulate a 0° coil
temperature.  This can be used to
check defrost operation of the unit
without waiting for the outdoor
ambient to fall into the defrost
region.

Use a small screwdriver or other
metallic object, or another 1/4 inch
QC,  to short between the 

SPEEDUP

terminals to accelerate the HPC timer
and initiate defrost.  Be careful not to
touch any other terminals with the
instrument used to short the

SPEEDUP 

terminals.  It may take up

to 10 seconds with the 

SPEEDUP

terminals shorted for the speedup to
be completed and the defrost cycle to
start.

As soon as the defrost cycle kicks in remove the
shorting instrument from the SPEEDUP terminal.

Otherwise the timing will remain accelerated and run
through the 1-minute maximum defrost length sequence
in a matter of seconds and will automatically terminate
the defrost sequence.

There is an initial defrost jumper (sen jmp) on the
control that can be used at any outdoor ambient during
the heating cycle to simulate a 0° coil temperature.  This
can be used to check defrost operation of the unit
without waiting for the outdoor ambient to fall into the
defrost region.

By placing a jumper across the 

SEN JMP

 terminals (a

1/4 inch QC terminal works best) the defrost sensor
mounted on the outdoor coil is shunted out and will
activate the timing circuit.  This permits the defrost
cycle to be checked out in warmer weather conditions
without the outdoor temperature having to fall into the
defrost region.

In order to terminate the defrost test the 

SEN JMP

jumper must be removed.  If left in place too long the
compressor could stop due to the high pressure control
opening because of high pressure condition created by
operating in the cooling mode with outdoor fan off.
Pressure will rise fairly fast as there is likely no actual
frost on the outdoor coil in this artificial test condition.

There is also a 5 minute compressor time delay function
built into the HPC.  This is to protect the compressor
from short cycling conditions.  In some instances, it is
helpful to the service technician to override or speed up
this timing period, and shorting out the 

SPEEDUP

terminals for a few seconds can do this.