No Cooling or Defrost Mode

by William Harmon
(Charleston, SC, USA)

I apologize for the long story but am perplexed.

Summer of 2012 heat would be produced when thermostat was set for cooling. I was able to get a new defrost board under wty. That did not fix my problem. I learned power has to be applied to reversing valve for cooling to take effect. To determine if reversing valve was good or not, I removed the black wire from the def brd that ran to the rvsg valve and jumped power from the little 24V circuir breaker to the rvsg valve. It clicked. So I left the jumper in place. AC worked. Left jumper in all summer and AC worked fine.

Winter 2012 came.... I pulled the jumper out. Placed wire back into def brd. Heat worked fine and defrost cycle ran all winter.

Summer 2013 came.... I put jumper back in... AC worked fine all summer. I know it wasn't good to keep pwr applied to the valve all the time but.... didn't seem to hurt.

Now... winter 2013. I pulled jumper out. Placed wire back into def brd..... heats fine but we had our first cold night (low 30s) and the def mode did not kick in.... condenser a block of ice all the way around.

Manually defrosted... heat works fine and we will have nights in the 40s and 50s for the next week or so, so we'll be fine until it gets into the low 30s or below.

I also swapped the digital thermostat with the same model. Made sure all settings were the same. No difference. Also checked all 24V wiring and connections. Some spliced connections were questionable, so I stripped and reconnected.

With the def brd already replaced and the rvsg valve known to be good. Any ideas where to look now?

Payne (Carrier)model PH3PNA048000AA
Def Board PN HK32EA003

Thanks in advance for any ideas.

Bill Harmon

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Jun 07, 2016
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intermittent no cooling NEW
by: Anonymous

What if the a/c works fine most of the time but looses 24v to the thermostat during mid afternoon
until mid evening. What would cause this concern?

Oct 24, 2014
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No Defrost
by: Ex-HVAC Instructor

If the reversing valve(RV) has to be powered for cooling there should be a thermostat wire on the "O" terminal. That wire should send power to the defrost board and thus to the RV anytime the thermostat mode is in "COOL." The defrost board also gets 24V power (usually a red wire) and 24V common (color varies) anytime 240V and 24V power is available to the unit. Since you know the RV shifts when powered, if the board is receiving 24V power and the "O" wire, is intact and is supplying 24V power to the defrost board when in the cool mode, either the defrost board is bad or some of the wires on the board are on the wrong terminals. Since the board was changed with the same results, it is likely a wiring problem.

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