Heat Pump Interminently Blowing Heat When In A/C Mode

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  #1  
Old 07-08-18, 06:30 AM
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Heat Pump Interminently Blowing Heat When In A/C Mode

Hi everyone. I am at a dead-end here trying to diagnose my problem and am hoping someone can help me solve this mystery.

I have a 10 year old Lennox heat pump that has started having an interment issue where it would start blowing heat while in A/C mode. It would work fine blowing cold air for a period of days to weeks and then randomly we would notice that heat is coming out. Turning the a/c off for a few minutes and then back on again corrects the issue for a period of time and then the problem happens again. It seems like it happens mostly in the middle of the night but that may be just a coincidence. When the heat is coming out, the thermostat still says it is in cooling mode.

After researching the issue, I tried a few things:
  • Replaced Thermostat
  • Replaced Capacitor
  • Replaced Defrost Control Board
  • Re-stripped all wires going into the board

None of these fixed the issue. I ruled out the compressor being the problem because the air coming out is maybe 100 degree heat, not just outside warm air. The copper pipes coming out of the unit are red hot when this issue is happening. (was I correct in ruling out the compressor?)

This lead me to think the issue was a faulty reversing valve. I called a tech out and he tested the reversing valve and said he didn't see an issue with it. He also tested the refrigerant and said that was at correct levels.

Now I am at a dead-end and tried everything I could think of. I still think the issue could be the reversing valve randomly getting stuck, and the tech just wasn't able to recreate it at the time due to the fact that the issue is internment.

Is there any other components in the system that could cause this issue or anything else I should look at?

Thank you for your help
 
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Old 07-08-18, 07:46 AM
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Do you have auxiliary/emergency heating? Usually this is electric heat. If so, could it possibly be turning on? Normally, when the heat pump is running in cooling mode, the large copper pipe should be cold and the small copper pipe should be warm to hot. Are you saying that both pipes are hot when this happens?
 
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Old 07-08-18, 08:14 AM
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Originally Posted by Bob14525 View Post
Do you have auxiliary/emergency heating? Usually this is electric heat. If so, could it possibly be turning on? Normally, when the heat pump is running in cooling mode, the large copper pipe should be cold and the small copper pipe should be warm to hot. Are you saying that both pipes are hot when this happens?
Hi Bob,

Yes we have aux heat. The large pipe is red hot and the small pipe is warm when this issue is happening. Cold air is being exhausted from the top of the unit. The tstat does not show aux mode while this is happening.
 
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Old 07-08-18, 08:38 AM
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I'm not an expert on heat pumps, but it sounds like you're running in heating mode rather than cooling mode when this happens. I'm thinking that your diagnosis of a faulty reversing valve is a good possibility.
 
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Old 07-08-18, 12:26 PM
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Correct. If the large pipe is hot.... you are running in heat mode. You need to determine if the reversing valve is the problem or the supply from the thermostat.

You need to check for 24vac on the reversing valve when A/C mode is called for. If the 24vac is not present on the valve.... it will be in heat mode.
 
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