Bard QH422, QH243, QH602, QH482, QH362, QH302 Defrost Cycle, LOW Pressure Control Bypass Timer

DEFROST CYCLE

The defrost cycle is controlled by temperature and time on the solid state heat pump control.

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, 60 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 21. 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.

FIGURE 22

LOW PRESSURE CONTROL BYPASS TIMER

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.

Manual	2100-420H
Page	27 of 34