Picture of Anthony Coker

Anthony Coker

Technical Services Manager North Georgia District

Diagnosing and Correcting a Heat Pump That Will Not Enter Defrost

Imagine that you receive a call from a customer stating that their heat is not working.  

Upon inspection you find that the heat pump is not going into defrost mode. You cannot initiate defrost mode by jumping speed up terminals. The outdoor fan continuously runs. You notice that the outdoor coil ices up and never goes into defrost mode. There is no voltage on T1 to Common or Ground. What could be happening?

The voltage comes from the Y terminal and goes through a few  safeties then the voltage goes to the T1 terminal. Please refer to the connection diagram above. 

If power is on the T1 terminal, then it should be capable of going into forced defrost or able to run normally by the timer and defrost thermostat. 

When there is no power on the T1 Terminal the outdoor fan runs continuously. With no power on T1, it will not allow the defrost process to happen which, in turn, would energize the relay and change the NC set of contacts to  “NO” thus shutting down the outdoor fan.

Typically, the reason this happens is because it is mis-wired at the contactor. The brown wire should be on one side of the contactor by itself. The rest should go on the other side.

An Example of a Properly Wired Contactor Vs and Improperly Wired One

The two images below shows a board with the T1 terminal on it so you can see the locations of this specific board. In the picture on the right, you can see it is mis-wired. The blue needs to be on the other side of the contactor coil.

When the 24vac is on T1 and there is a call for defrost then it can energize the relay to shut off the fan.  

If you have further questions or need assistance troubleshooting, refer to the units service manual or contact Mingledorff’s Technical Service team. 

Disclaimer: The technical statements, information and recommendations contained herein are believed to be accurate as of the date hereof, but Mingledorff’s does not make representations or warranties, express or implied, as to its accuracy, its completeness, or the results to be obtained. The information is being provided for informational purposes only and is intended for use by persons having adequate skill and expertise regarding the proper selection, use and application of the products and recommendations and at their own risk and discretion. 

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