Ice Build-up on Censing unit (Heat Mode)

Has anyone ever come across this? At today’s inspection I ran the Heat Pump in “heat mode” for about 20 minutes before I noticed ice build-up on the exterior condension unit.

I know this happens in the cooling mode as a result of low refrigerant, but what about in the heating mode?




Hi Kevin, heat pumps are not my forte but a google search brought this.


Kevin ice will form on a heat pump not uncommon that is why the have a defrost cycle but in a 20 min period of time you have a problem. What was the outside ambient at the time of inspection.

Low 50’s.


Hi Kevin,

That is a fairly high temperature for the outside coil to be freezing up, especially as it was only on for 20 mins, remember as well that the ammount of frosting will also depend on the ammount of humidity in the outside air.

The exterior coil in heat mode will always freeze up when run at low temps with high humidity, and is designed to go into defrost mode automatically every hour.

That one of yours probably is low on refridgerant, as it is freezing up to fast and at too high a temp I would recommend that the system be serviced

BTW for comparison my 4 year old outside coil down here in the humidity capital of the world seldom freezes up unless the outside air temp gets around the low 40’s with higher humidity.



Kevin with the outside ambient being in the low 50’s that is the ideal temp for a heat pump to operate in for testing purposes. Hot gas vapor being discharged to the indoor A-coil condensing to a warm liquid being sent back to the outdoor unit which is acting as the A-coil complete with its own expansion valve. With a properly charged unit with expansion valve operating properly I would not want to see any ice on the outside unit with the ambient being in the 50’s. I would write that unit up as not performing its intended function.

Thanks for all the replies, they were very helpful. NACHI members are the best!!

I think I will spend some time today “brushing up” on Heat Pumps :slight_smile:


Cool Gerry you slipped in here while I was thinking about what to say trying to keep it simple to understand

Hi Charlie, it’s the “keeping it simple” bit thats hard, as we all know a heat pump system is witchcraft :mrgreen: . Of all the subjects that I teach Heat Pumps are the most confusing to most people :shock:

Lets get it right AC systems and Heat pumps just shouldn’t work at all based on what most people understand about boiling points and evaporation :wink:


Confused of Clearwater

I would agree once worked with a guy that spent two
years in a vo tech for HVAC and was unable to wire his first thermostat for a heat pump. He could not do the work so later on they made him a boss???

The other possiblity is that the system had the condensing unit replaced without replacing the evaporator. Mismatched systems is getting to be a problem with companies that want to sell that unit without regards to system operation in both modes. At least when 2010 gets here with R22 phaseout this problem should be reduced.

If anyone is interested I can e-mail a pdf file of a test report that Bristol Compressors made on mismatched systems. Just e-mail me at mechacc at subject: Mismatch System

If anyone can tell me how to upload a pdf file on this forum I can do that as well.

Thanks alot you. Its true mismatched system always create issues which is hard to understand or resolve. essay help online