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BillyHvac, Journeyman HVAC Tech
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Tempstar Heat Pump, Fan on the Heat pump continues to run when

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Tempstar Heat Pump, Fan on the Heat pump continues to run when in defrost mode.

This morning I was in the house and the heat was running, The heat pump then suddenly got louder (its been doing this for a few days when its been in heat), I happened to have my clothes on ready to go to work, so I went outside and looked, the frost was melting as it is supposed to, I watched it go from cloudy to clear and start falling away. However the fan was still running.

After I went back inside I heard it go quiet again (assuming it went out of defrost), and then a few minutes later the entire system shut off (by the thermostat, it had hit proper temperature to turn off).

So the system is definitely working, defrosting et al, however I am unsure if the fan should be running when defrosting, and should I have it checked out or leave it alone?

so in short the fan is not stopping in the outdoor unit when defrosting? It continues to run even though the defrost kicks?


Customer: replied 3 years ago.

Yes, that is what seems to be happening.

Thank you.

This will become an issue as it gets colder outside.

You have a bad defrost board that is not turning the fan off during defrost. The board with installation and service call should be around $150-$200.

This is not critical in medium temps, but will be as it gets colder.


BillyHvac and 2 other HVAC Specialists are ready to help you
Customer: replied 3 years ago.

I have a followup, the board was replaced yesterday afternoon. When testing the new board it has the exact same symptom of the old one.


I did a little testing myself and after the unit had shut off I still heard a buzzing, I pulled the reversing valve lead off the defrost board and sure enough the buzzing stopped.


I reconnected it, went inside and kicked on another heat cycle, the unit did a proper defrost, fan didnt start, compressor ran, about 45 seconds later the fan kicked, a blast of warm air (from the coils) then i heard the reversing valve go back to heat mode.


I shorted the test pins after 5-10 minutes and the unit went into defrost, the reversing valve hit, and the unit got really loud, the fan still running.


Is it possible that my unit isnt supposed to kick off the fan if it goes into defrost mid cycle?


The Model is: NHP036AKC2


As I sit here typing this I heard the unit get quiet again, probably went out of defrost.

Can you please tell me how the unit is wired?

Thermostat wire colors to the outdoor unit?

The reverse valve should not be energized while the unit is off.

Customer: replied 3 years ago.

Its wired correctly best I can tell.


The reverse valve was only energized when the board was calling for defrost or in cooling mode, in cooling mode the valve stays energized the entire time.


At least thats what the Electrician who came out said (for background he is the one who installed it 8 years ago)



I called him again this morning, he thinks we may have gotten a bad board and/or the sensor is bad. I suspect something more.


Regardless if the thermostat initiates a cooling cycle then O is energized (I think) and the defrost board reverses the valve and keeps that energized regardless of whether the thermostat is calling for cooling or not.


When the board is in defrost mode it was energized as well, to me it seemed like this morning it started a defrost cycle just as the thermostat was satisfied, so when i realized the RV was energized I went inside and forced the thermostat to kick another heating cycle, which made it run what seemed like a normal defrost.


So it seems like the reversing valve is good (when he was out yesterday he manually reversed it mid cycle, by energizing the orange wire to the board) The sensor seems good by the fact that when it defrosted normally as soon as it hit the temp it un-reversed and the fan kicked on, The contactor seems normal since things are turning on and off (I dont know much about this)


So could I have just gotten a bad board with the exact same symptoms as the first one?


My system does not have pressure switches, and as such has a jumper wire between the pins.


I cant think of any more detail to give, it befuddles me that it could be that the replacement board was bad, but it has the exact same symptoms...


What else could fail that would cause everything to operate normally except the defrost cycle when the unit is mid cycle?


If the unit is wired correctly:

O terminal is reverse valve and ONLY has power during cooling or defrost.
Defrost is actually just a cooling cycle to move heat out to coil...the only difference is the fan shuts down so as not to circulate cold air through the coil while we are trying to melt it.

Thus defrost is cooling just without the outdoor unit fan running.

In heat...the reverse valve has NO power.

The only thing that can turn the fan on is a defrost board. The compressor is turned on separately by the contactor.

The fan motor gets power straight from the defrost board. So if the fan gets power or not is because of the defrost board.

Really the only intent of the defrost board is to shut the fan down in defrost.

I would have hime try another board in case....but if the result is the same there is another issue.


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