During the heating season, the defrost on my Heil heatpump cycles repeatedly(up to 6 defrost cycles at a time) for extended time period (up to 18 minutes) even when there is very little frost on the coils. Then heatpump runs normally for 90 minutes till next defrost which recycles needlessly again. Defrost board and outdoor coil temp sensor were replaced once many years ago. I don't remember if this fixed the problem at that time or not. In case it matters the 5 minute accessory time delay(to protect compressor from short cycling) is installed in the yellow control line from the thermostat.
Without knowing what the problem was with the old board, you're not going to be able to figure much out, I don't think.
Did you check that temperature sensor I mentioned? You will have to pull the top off to get access to it. Normally found connected to one of the tubes for the coil, silver in color, with 2 wires connected to it, near the bottom of the base of the unit.
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The temperature when this malfunction occured was between 30 and 35 F. I do not know if it is always 6 cycles. I believe in the past it was less cycles and shorter time ~ 15 minutes. This may be an indication that the problem is getting worse. I have the original defrost board that was replaced ~ 10 years ago. I was considering swapping it in just to see what difference it makes.
The board could be bad, but I would check to make sure the temperature sensor located on the outdoor coil is securely mounted. Sometimes a little corrosion occurs here. What is also important is what temperature is it where you are? Is it always 6 cycles?
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