Defrost Thermostat

Defrost thermostat signals heat pump that conditions are right for defrost or that conditions have changed to terminate defrost. It is a thermally actuated switch clamped to outdoor coil to sense its temperature. Normal temperature range is closed at 30_ ± 3_F and open at 65_ ± 5_F. Defrost thermostats are used in Legacy RNC and Legacy Line models, a coil temperature thermistor is used in Preferred and Evolution series units.

FEEDER TUBE

STUB TUBE

DEFROST

THERMOSTAT

A97517

Fig. 8 – Defrost Thermostat Location

Check Defrost Thermostat

There is a liquid header with a brass distributor and feeder tube going into outdoor coil. At the end of 1 of the feeder tubes, there is a 3/8-in. OD stub tube approximately 3 in. long. (See Fig. 8.) The defrost thermostat should be located on stub tube. Note that there is only 1 stub tube used with a liquid header, and on most units it is the bottom circuit.

Heating Sequence of Operation

On a call for heating, thermostat makes R-Y, and R-G. Circuit R-Y sends low voltage through the safeties and energizes the contactor, which starts the compressor and energizes the T1 terminal on the circuit board. The T1 terminal energizes the defrost logic. This will energize the OF2 fan relay start the outdoor motor. The T1 terminal must be energized for defrost to function.

When the cycle is complete, R-Y is turned off and the compressor and outdoor fan should stop. There is no compressor delay built into this control.

Defrost Sequence (HK32EA001)

The defrost control is a time/temperature control that has field selectable settings of 30, 60, and 90 minutes. These represent the amount of time that must pass after closure of the defrost thermostat before the defrost sequence begins.

The defrost thermostat senses coil temperature throughout the heating cycle. When the coil temperature reaches the defrost thermostat setting of approximately 32ºF, it will close, which energizes the DFT terminal and begins the defrost timing sequence. When the DTF has been energized for the selected time, the defrost cycle begins, and the control shifts the reversing valve into cooling position, and turns the outdoor fan off. This shifts hot gas flow into the outdoor coil which melts the frost from the coil. The defrost cycle is terminated when defrost thermostat opens at approximately 65_F, or automatically after 10 minutes.

NOTE: The defrost thermostat must be located on the liquid side of the outdoor coil on the bottom circuit and as close to the coil as possible.

Defrost Control Board

Troubleshooting defrost control involves a series of simple steps that indicate whether or not board is defective.

NOTE: This procedure allows the service technician to check control board and defrost thermostat for defects. First, troubleshoot to make sure unit operates properly in heating and cooling modes. This ensures operational problems are not attributed to the defrost control board.

HK32EA001 Defrost Control

The HK32EA001 defrost control is used in all Legacy RNC Line heat pump models 213ANA and 213ANC. Its features include selectable defrost intervals of 30, 60, 90 minutes, and standard defrost speed up capability. This section describes the sequence of operation and trouble shooting methods for this control.

Cooling Sequence of Operation

On a call for cooling, thermostat makes R-O, R-Y, and R-G. Circuit R-O energizes reversing valve switching it to cooling

T1 - ENABLES DEFROST

TIMER. MUST BE

ENERGIZED FOR

DEFROST TIMER TO START

C - COMMON

O - REVERSING VALVE

OUTDOOR FAN

RELAY

DEFROST THERMOSTAT

HK32EA001

 

MUST BE CLOSED BEFORE

 

DEFROST TIMER BEGINS

 

Y OUTPUT TO PRESSURE

SWITCHES AND CONTACTOR

SPEEDUP

position. Circuit R-Y sends low voltage through the safeties and energizes the contactor, which starts the compressor and energizes the T1 terminal on the circuit board. This will energize the OF2 fan relay which starts the outdoor fan motor.

When the cycle is complete, R-Y is turned off and compressor and outdoor fan should stop. With Bryant thermostats, the O terminal remains energized in the cooling mode. If the mode is switched to heat or Off, the valve is de-energized. There is no compressor delay built into this control.

THERMOSTAT INPUTS

A05332

Fig. 9 – HK32EA001 Defrost Control

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Bryant R-22 service manual Defrost Thermostat, Defrost Control Board

R-22 specifications

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