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Douglas, HVAC Technician
Category: HVAC
Satisfied Customers: 3089
Experience:  Manufacture Rep for Major Brand, Technical Trainer, NATE
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My heat pump had ice build up on the outside coil, in heating

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My heat pump had ice build up on the outside coil, in heating mode. The filter was probably the problem. changed filter, restarted the unit but the outside fan was not running. Should I just let the unit run, is there some time out feature before the outside fan starts up? I also know low freon, leaks, etc can cause iceing but am not equiped to take pressure readings. Don't want to call a service company if all I need do is let the unit go through a cycle. Also don't want to damage the unit. 20 yr old Trane WCXO30-SF-1A Has been working fine up to filter becoming clogged by my neglect.
For the most part, a dirty filter can cause problems but not generally a defrost issue. Defrost is controlled by a defrost control in the outdoor unit. This control of course is just a switch of sorts putting the unit into defrost mode. If the coil sensor is loose or has failed it may not go into defrost. The charge comes into play which produces inconsistant icing and ultimately does the work to defrost the coil so it can be refrigeration related to also.

in the outdoor unit, there should be a set of test pins to shift the unit into defrost mode while it's running.
Customer: replied 7 years ago.

Is the out side fan supposed to be running? Both coils are located outside in the unit. The outter most coil would be the evaporator in heat mode with the innermost coil actting as the condenser. Or I may be all wrong and the outter coil hasnothing to do with heat mode.


I just need to know if I should turn the system back on if the outter fan is not running.

in defrost, no the fan should be off, but in normal heat pump mode, yes the outdoor fan should run. If it's not, that may explain the heavy ice.

The entire outdoor coil is the evaporator and your indoor coil is the condenser in heat mode. If the outdoor fan were not running in heat mode, the outdoor coil will drop in temperature quickly and form ice.

You may have either a bad outdoor fan relay, fan capacitor or just a bad fan.
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Customer: replied 7 years ago.

For defrost mode requires three conditions to be met.

heat mode

deforst termiant switch closed

compressor run time 50-90 minutes


So if I just started the unit the last condition could not be met and defrost mode is not what it is in. If so then fan is supposed to be running , it isn't, and of course would explain ice. Correct?


I need to have a service person check my fan, correct?

and thank you you have been helpful and of course I am going to accept. I use this service last night and a VET donated her time. I do not expect you to and will gladly reemburse.

Just to be clear the run time accumulates. So say the unit runs for 30 minutes and satisfies the thermostat. When it restarts the time starts from 30 and eventually hit's the time period so if it ran say 45 minutes last cycle, it may only need to run 5 on the next cycle.

On the defrost control is a fan relay that opens when the unit is in defrost but should be closed otherwise (normal heating or cooling mode). They usually dont fail open but anything is possible.

I would at this point have it checked.... too much to go wrong on a few guesses.
Customer: replied 7 years ago.

Sorry about the delay I lost my connection. So now the question is am I in defrost mode and having disconnected power the unit would not start up in that mode would it.


It seems it the fan and I need to get a service guy out here. if you agree we will finish here and I can get on the phone,.

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