Trane Demand Defrost Control

Demand Defrost Control

There are two versions of the defrost control board:

• Mnemonic No. 07255 = PSC Motor
• Mnemonic No. 07256 = ECM Motor

The demand defrost control measures heat pump outdoor ambient temperature with a sensor located outside the outdoor coil. A second sensor located on the outdoor coil is used to measure the coil temperature. The difference between the ambient and the colder coil temperature is the difference or delta-T measurement. This delta-T measurement is representative of the operating state and relative capacity of the heat pump system. Measuring the change in delta-T determines the need for defrost.

The coil sensor also serves to sense outdoor coil temperature for termination of the defrost cycle.

• Defrost controls have a selectable termination temperature. As shipped, defrost will terminate at 47°F. For a higher termination temperature, cut Jumper J2 to achieve 70°F. (See attached .pdf for full Data Sheet)

Fault Detection
A fault condition is indicated by the flashing Fault LED light on the defrost control board located inside the heat pump control box.
In normal operation, the status LED will flash once each second when idle or twice each second with a call for heating or cooling.

PIN Identification
1. TEST_COMMON (Shorting any of the other pins to this pin causes the function of the other pin to be executed. Leaving this pin open results in the normal mode of operation).
2. FRC_DFT = Forced Defrost (Short TEST_COMMON to this pin speeds up all defrost. Remove the short after defrost initiates.

Defrost Control Checkout
Normal operation requires:
1. Status LED on board flashing 1 time/second in standby or 2 times/second with a call for heating or cooling.
2. 24VAC between R & B
3. 24VAC between Y, Y0 & B with unit operating
4. Defrost initiation when FRC_DFT pin is shorted to TEST_COMMON pin.