Defrost Cycle

The defrost cycle is controlled by temperature and time on the solid state heat pump control. See Figure 20.

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 temperature sensor mounted near the bottom of the outdoor coil. Once coil temperature reaches 30°F or below, the coil temperature sensor sends a signal to the control logic of the heat pump control and the defrost timer will start accumulating run time.

After 30, 60 or 90 minutes of heat pump operation at 30°F or below, the heat pump control will place the system in the defrost mode. Factory default setting is 90 minutes.

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 temperature 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 8 minutes.

The heat pump defrost control board has an option of 30, 60 or 90-minute setting. All models are shipped from the factory on the 90-minute pin. If special circumstances require a change to another time, remove the wire from the 90-minute terminal and reconnect to the desired terminal.

The manufacturer’s recommendation is for 90-minute defrost cycles. Refer to Figure 20.

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

Use a small screwdriver or other metallic object, or another ¼ 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 15 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 terminals. Otherwise the timing will remain accelerated and run through the 1-minute minimum defrost length sequence in a matter of seconds and will automatically terminate the defrost sequence.

There is an initiate defrost jumper (sen jump) 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

¼inch QC terminal works best) the defrost sensor mounted on the outdoor coil is shunted out & 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. The board’s LED will have a fast blink rate when in the compressor time delay. 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.

FIGURE 20

DEFROST CYCLE

	SW	SW
	1	2	TIME (SEC)

	OFF	OFF	30
LOW PRESSURE BYPASS TIMER SWITCH	ON	OFF	60
LOW PRESSURE BYPASS TIMER SWITCH	OFF	ON	120*
*(FACTORY SETTING 120 SECONDS)	OFF	ON	120*
*(FACTORY SETTING 120 SECONDS)	ON	ON	180
	ON	ON	180

OFF

ACCUMULATED DEFROST TIME TIMER

(FACTORY SETTING 90 MIN.)

MIS-2668 A

Manual	2100-549G
Page	33 of 59