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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 8670
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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I have a Trane heat pump and the outside unit fan runs when

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I have a Trane heat pump and the outside unit fan runs when heating but is not running when cooling. The coils outside are getting VERY warm with no fan running.

Phil :

All work to be done by licensed professional. WE KEEP GOING UNTIL YOU HAVE THE INFORMATION YOU NEED, I come and go. This is step ONE.

Phil :

Hello, welcome to Just Answer !

Phil :

The problem is in the electronics controls that operate the outside fan motor...its not in the thermostat as long as the compressor runs... and by the fact that the outside coil gets *very warm we know the compressor is running....

Phil :

..further, the compressor should go off on its high pressure safety cut out when the outside fan fails... *do not let it run in that condition, it can destroy the compressor*

Phil :

This is not something you will be able to trouble shoot and repair yourself.. its not something you can reset or a minor thermostat issue.

Phil :

Let me know if you have any questions, we keep going as long as you need to, there are not time limits.

JACUSTOMER-rwt511ag- :

The only control outside is the defrost board so you think the problem is with that?

Phil :

Hello again, yes it can be the defrost board easily... the defrost board shuts the outside fan off when the unit is heating mode...and... when the coil gets too much frost on it... then, at that time...the defrost board, reverses the 4 way valve into cooling, blows the hot gas to the outside coil, and kills the outside fan motor.

Phil :

( Otherwise in heating mode, the outside fan runs whenever the compressor runs. )

JACUSTOMER-rwt511ag- :

Is there any other part you can think of that could cause this problem or you feel it pretty much has to be that board?

Phil :

Chances its the board are in the 90% range. or higher.

JACUSTOMER-rwt511ag- :

OK thank you very much for your help.

Phil and other HVAC Specialists are ready to help you
Thanks for the good rating... if you need further assistance now or weeks later, feel free to post back on the same dime so to speak.
Customer: replied 5 years ago.
To check something out I flipped the breakers off on the air handler unit. When I was done and flipped them back on the fan on the outdoor unit is now coming on with heat and cool. No idea why but maybe it reset something? Thought you should know.
Thanks, as you probably suspect, the fan not running when it should is not a resettable condition... the fact that it did start working points once again at an electronics defect. It might be a good idea to have a new defrost control card on hand.

Customer: replied 5 years ago.

We don't need AC here much so I have been a bit resistant to buy a new defrost board. I did notice though when I have the problem tripping the breaker to the outdoor unit off and on does not fix it. Only tripping the breaker to the air handler off and on fixes the problem temporarily. Still think it is the defrost board outside?

Thanks again,


Hello again Sean, If the compressor runs in cooling mode, without the outside fan running... the compressor can be ruined. I would not take the chance myself.


Having said that, the compressor will go *off * if it is allowed to run without the outside fan, its controls are wired through a high pressure cut out switch... the compressor will only run a few seconds if the fan goes off in AC mode.

Pay close attention to this last paragraph I have typed above.

It could be that your comment that the outside unit was getting real hot was due to the sun shinning on the sheet metal, and that the fan had actually not quit as we both had assumed... **but the the entire outside unit was off. ! That sheet metal can easily get so hot that you can't hold your hand on it.

If thats the case, then resetting the system by cycling the power at the air handler would work if the outside had gone off on its high or low pressure cut out switch.

A dirty outside condenser coil will cause the unit to go off on its high pressure cut out switch... if the unit is over a few years old. .. and its been going off in hot weather, then the condenser coil should be cleaned.

**A totally clogged condenser coil can look clean ** the dirt embeds deep between the fins where you cant see it. The way to clean the condenser coil is to *blast* water, forcefully between the fins of the coil unit you see water coming out the back side of the coil by looking down through the fan. A good job takes about half an hour.

Do the job with power off to the outside unit, and don't get any of the electricals wet.

Let me know if that makes any sense on your end or not, we can go from there.