supplementary heat for all heat pump installations in areas where the temperature drops below 45° F. It is also a good practice to provide sufficient supplementary heat to handle the entire heating requirement should there be a component failure of the heat pump, such as a compressor, or refrigerant leak, etc.
Since the temperature of the refrigerant in the outdoor coil on the heating cycle is generally below freezing point, frost forms on the surfaces of the outdoor coil under certain weather conditions of temperature and relative humidity. Therefore, it is necessary to reverse the flow of the refrigerant to provide hot gas in the outdoor coil to melt the frost accumulation. This is accomplished by re- versing the heat pump to the cooling cycle. At the same time, the outdoor fan stops to hasten the temperature rise of the outdoor coil and lessen the time required for defrosting. The indoor blower continues to run and the supplementary heaters are energized.
DEFROST CONTROL
During operation the power to the circuit board is controlled by a temperature sensor, which is clamped to a feeder tube entering the outdoor coil. Defrost timing periods of 30,60 and 90 minutes may be selected by setting the circuit board jumper to 30, 60 and 90 respectively. Accumulation of time for the timing period selected starts when the sensor closes (approximately 31° F), and when the wall thermostat calls for heat. At the end of the timing period, the unit’s defrost cycle will be initiated provided the sensor re- mains closed. When the sensor opens (approximately 75° F), the defrost cycle is terminated and the timing period is reset. If the defrost cycle is not terminated due to the sensor temperature, a twelve minute override interrupts the unit’s defrost period.
4.Disconnect outdoor fan by removing the outdoor fan motor wire from “DF2” on defrost control.
5.Restart unit and allow frost to accumulate.
6.After a few minutes of operation, the unit’s defrost thermostat should close. To verify this, check for 24 volts between “DFT” and “C” on board. If the temperature at the thermostat is less than 28° F and the thermostat is open, replace the unit’s defrost thermostat, as it is defective.
7.When the unit’s defrost thermostat has closed, short the test pins on the defrost board until the reversing valve shifts, indicating defrost. This should take up to 21 seconds depending on what timing period the control is set on. After defrost initiation, the short must instantly be removed or the unit’s defrost period will only last 2.3 seconds.
8.The control is shipped from the factory with the compressor delay option selected. This will
9.After the unit’s defrost thermostat has terminated, check the defrost thermostat for 24 volts between “DFT” and “C”. The reading should indicate 0 volts (open sensor).
10.Shut off power to unit.
11.Replace outdoor fan motor lead to terminal “DF2” on defrost board and turn on power.
AIRFLOW MEASUREMENT AND ADJUSTMENT
After reviewing Duct Work section, proceed with airflow measure- ments and adjustments. The unit blower curves (see Specifica- tion Sheets) are based on external static pressure (ESP per in/wc). The duct openings on the unit are considered internal static pres- sure. As long as ESP is maintained, the unit will deliver the proper air up to the maximum static pressure listed for the CFM required by the application (i.e. home, building, etc.)
In general, 400 CFM per ton of cooling capacity is a rule of thumb. Some applications depending on the sensible and latent capacity requirements may need only 350 CFM or up to 425 CFM per ton. Check condition space load requirements (from load calculations) and equipment expanded ratings data to match CFM and capacity. After unit is set and duct work completed, verify the ESP with a
NOTE: Never run CFM below 350 CFM per ton, evaporator freezing or poor unit performance is possible.
SUGGESTED FIELD TESTING/TROUBLE SHOOTING
1.Run unit in the heating mode (room thermostat calling for heat).
2.Check unit for proper charge. Note: Bands of frost on the condenser coil indicate low refrigerant charge.
3.Shut off power to unit.
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