Heat Pump Defrost Board

control boardWe looked at the basic concept of how a heat pump operates. We reviewed the defrost cycle. Now we’re going to take a look at the brain of the heat pump; the defrost board. Defrost boards are chock full of terminals like R, C, Y, W, O, B, DFT, T1, RV, AUX, CC, ODF L1, L2 and Test. Fifteen terminals, give or take half a dozen depending on the model. Don’t let the mass of wires scare you. Defrost controls are just fancy timers…with fifteen wires hanging off them.

First, notice the controls are called De-Frost controls and not De-ice controls. Heat pump coils should have frost on them, never ice. Why? The defrost cycle is designed to melt lite frost, not a block of ice. If you have a coil that turns into a block of ice, you have a problem.
If the charge is correct, and the defrost thermostat is located and functioning correctly, and the coil turns into a block of ice, you need to shorten the time out period. If it’s set for 90 minutes and the coil is icing up, try 60 minutes.
Typically the time-out period is selected based on humidity in the area. If the heat pump is located in a valley or next to a lake the time should be set at 30 or 60 minutes. If the heat pump is located in a cold dry area, select a longer time like 90 or 120 minutes.
Remember, the colder it is out, the less moisture there is in the air. The less moisture there is, the less frost will form on the coil. The less frost that forms, the longer the defrost time can be.
Now we’re going to look at the defrost controller’s wire terminals.
(R) input-
supplies 24vac to the control. It’s connected to “R” at the indoor unit and “R” on the thermostat.

(C) input-
supplies 24vac common to the control. It’s connected to “C” on the air handler and thermostat.

(Y) input-
24vac from “Y” on the thermostat. It turns on the compressor for cooling and heating mode.
(W) or (Aux.) output-
24vac from the defrost control. It energizes the air handler’s electric heat to warm the supply air while in defrost mode.

(O) input-
24vac from the thermostat “O” terminal. It energizes the reversing valve, shifting it into cooling mode.
(B) input-
24vac from the “B” terminal on the thermostat. It energizes the reversing valve, shifting it into heating mode.
(DFT) input-
24vac from the outdoor coil thermostat. It starts the countdown timer (30, 60, 90 or 120 minutes) for defrost when the coil is below freezing, normally closes around 28degrees .
(T1) input-
24vac from the compressor contactor coil. Energized when the compressor is running. It enables the defrost circuit and energizes the condenser fan relay.

(Test) input-
Two pins labeled “speed up” or “test”. When these pins are jumped together the timing for all board functions is sped up by 256 percent for checkout purposes.

To run a unit through a defrost cycle do the following-
Power the unit down and disable the outdoor fan by disconnecting and taping the condenser fan common wire. Power unit up.
Make the unit call for heat at the indoor thermostat.
Let the unit run until the coil drops below 30 degrees.
Jump the “test / speed up” pins until the reversing valve shifts into defrost position (about 15 to 30 seconds) and immediately remove jumper.
Defrost should terminate after the coil warms to 80 degrees or ten minutes has lapsed.
Power unit down, reinstall the fan wire, power unit up.
If the unit didn’t enter defrost mode try this procedure-
Jumper terminal “DFT” to “R”.
Make the unit call for heat.
Jump the “test / speed up” pins.
The system should go into defrost mode, then seconds later cycle back to the cooling mode.
Remove jumpers and return to normal operation.
If the unit still didn’t enter defrost mode check for 24vac at terminal “T1” during a call for heat; this terminal needs to be energized to initiate defrost.
Note; the defrost control uses either the “O” or “B” terminal, but never both. Most residential heat pumps use the “O” terminal to energize the valve for cooling. The “B” terminal energizes in the heat mode. Remember, O and B are not interchangeable!

See, that wasn’t to bad.

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Patrick is Zen HVAC’s diagnostic and training guy.

Patrick started in the trade the day he left technical school and never looked back.

He’s served in various technical and training roles in the HVAC industry but specializes in system troubleshooting and diagnostics, retro commissioning, and technical training.

His moto: If I can understand it, anybody can.

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One comment on “Heat Pump Defrost Board
  1. Edward h Matheson says:

    Thanks, this is good info. I’ve been trying to find the exact wire diagram for my old InterCity(or Snyder General)ye024g with 34332101(or 01) control board. I’m putting in a universal White-Rodgers unit and have a couple questions. This control board has high and low press sensor inputs that didn’t go on the old board far as I can tell with my partly illegible diagram, and my other question, is com tied to earth ground or is it an isolated “ground”???

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