Does the Defrost Board Have 24VAC?

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This installation was on a 4-story home, and the tech had not yet gone to the basement to troubleshoot the “infinity” water source heat pump, as he was told he by the homeowner. He was in the attic at the infinity furnace above the 4th floor. He was getting his dcv in the proper range on his A and B green plug on the air handler, 3 to 4 vdc. His indoor board ohm reading was in range, 28k to 32 k. The tech ohmed out the A and B thermostat wires on the water source heat pump from the upstairs. His ohm reading was not in range of a normal infinity board. His reading was around 64.4k. The tech then made the long walk to the basement where the “geothermal heat pump” was located. Bingo.

Blue electronic control circuit board with multicolored wires and components inside a device enclosure

Close-up of a blue circuit board with green terminals, resistors, and a gray connector block

Close-up of electrical wiring and connectors inside an equipment panel, with red, blue, and black cables.

Close-up of a wiring diagram label inside an electronic device, with a screwdriver pointing at it.

Simply lift straight up off the heat sink and lay it to the side where the copper and electrical are located. Finish removing the remaining wires and start the new board installation. Remember to add new heat sink paste to the heat sink. If this is not completed the new board may not last long.

Install the new board, assemble the unit, restore power, and check for normal operation.

The heat pump works in heating and cooling mode but it simply will not go into defrost mode.

Why won’t the heat pump go into defrost mode? I find this question quite often in the winter months after the tech has changed multiple defrost boards. This is a very simple problem to check and troubleshoot. Does the unit have 24VAC? Do not say yes because the unit will operate. Yes, the heat pump will work when the “R” or 24VAC is missing. The “R” powers the defrost circuit and the unit will run without it. Much like a straight cool unit, all that is needed to run is “Y” and “C”. Start by checking 24VAC at the defrost board on “C” and “R”.

I highly recommend using needle point leads so as not to damage the wiring harness. Start by checking 24VAC at the defrost board on the plug. 0VAC will be read when this problem occurs. I recommend starting here and working backwards towards the 24VAC source, for example, the transformer in the indoor unit. Working backwards will lead you to the source. This could include a broken wire under a wire nut or a broken thermostat wire.

So, to recap, 0VAC between “C” and “R” = 0 defrost. The unit will run in both heating and cooling modes.