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When heat is running the a/c coils freeze over with frost can you tell me the problem

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  • luke bussenschutt May 11, 2010

    sounds like the deforst system is not working correctly

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The outdoor coils getting some frost on them is normal. However the system should periodically (every 30-90 mins of run time) go into "defrost" mode. Essentially it reverts back to cooling and shuts off the outdoor fan. This heats the coil outside to melt the frost. You may see some steam at this time also. Again normal. If this is not happening or not happening often enough the coil will ice up. Inside the outdoor unit is the defrost board. There is usually a pin setting for either 30-45-60 or 90 minutes. Make sure you kill the power to the unit before sticking your hands in there. Check to see what setting your is on and lower it one peg. If it's been colder and/or more humid you may need to cycle sooner. Good luck I hope this works for you. Please don't forget to give me a Fixya rating.

Posted on Feb 23, 2009

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1 Answer

There is a loud humming coming from outside unit, the fan is not rotating.


Your question does not include what type of system you are asking about, but the assumption is that it is a heat pump. The loud humming is the compressor running by itself while the fan is off.

This is the typical way a reasonably new heat pump operates:
There is a coil temperature sensor that initiates a defrost period to eliminate the frost/ice build up on the outdoor coil. In the defrost mode, the outdoor fan shuts off and the reversing valve in the heat pump reverses the direction of refrigerant flow to warm the outdoor coil. The same coil temperature sensor senses the temperature of the external surface of the coil and tells the defrost control when the frost is gone initiating another 'reverse cycle' to put it back into heating mode. In the heating mode, the outdoor fan runs again.

If you have the heat pump installed where snow drifts accumulate, you will have a problem extracting heat from the outdoor air, and coil frosting and freezing will be exaggerated. If you have excessive ice build up due to a defosting issue, the ice can actually stop the fan from rotating, but you would hear loud bangind and rattling for hours first as the fan blades come in contact with the ice.

If you do not have drifts, but continually have an excessive amount of ice build up, either the sensor is possibly faulty or the defrost control board is faulty. The sensor's clip could easily have been knocked loose from the coil by ice and is no longer sensing coil temperature and if so the result would be the lack of defrost initiation.

Feb 13, 2015 | Heating & Cooling

1 Answer

The air conditioning fan outside the house is showing frost on the coils, what is the problem


Are you using your Air conditioner as a Heat Pump (Making heat in the house)- or as an Air Conditioner (making COLD air in the house.)

If as a heat pump, then remember-- that the WARM coil is now inside the house-- and the COLD one is outside-- So, if it is COLD outside, there is not enough HEAT out there, to keep the coil from going below freezing-- The unit may have to be turned off, and allowed to defrost, if you want more heat from the Heat Pump-

Let me know if this helped

Mack B

Jan 15, 2011 | Ruud Heating & Cooling

1 Answer

I have a trane outoor heat pump,twx736a100a2,coil freezes over.I had service tech. come to repair it twice,replaced relay,outdoor thermastat,checked freon,$500!..It worked fine for entire cold month of...


THERE is not a whole lott left not quite sure what all you have had done by post but i will let you know that the unit will run on a timed period based on outdoor defrost control board and once that time elapses and unit is still runnning it will check the defrost thermostat on the coils if the coils are cold enough (ie: frosted) the thermostat will send a signal back to board to go into defrost. the reversing valve actually switches back over to a air conditioner while mean while your indoor supplemental heat comes on rather it be electric or gas if they have it wired right. so that being said you could have a bad defrost control board if they have already replaced the thermostat outside. also if unit is low on refrigerant will freeze up ...lastly i want to point out not sure what your location is but when it gets down to 30 degree temps the heatpump will frost up it shouldn't be a block of ice but will frost up and look like snow on in completely normal. i tell people here in texas with heat pumps anything below 35 to go ahead and run the emergency heat just so you have warmer air and a stable home temp... yes it does use more electricity initialy ,but if you can warm a house in 2 hours with e-heat and it takes the heat pump most of the day running what is really costing you more? hope this helps and good luck to you please vote

Jan 04, 2011 | Heating & Cooling

1 Answer

My carrier heat pump has frosting on the outside on it's coils. I know that it's cold outside but is that normal?


Yes it is normal especially with high humidity. The moisture freezes on the coils just like on the evaporator coils in a freezer. The heat pump will defrost itself by shutting off the fan outside and reverse the flow of refrigerant to melt the ice when it build up then switch back to normal heat. That's why at times you may feel cold air coming out of the vents in the house. The outside coils are defrosting.

Dec 16, 2010 | Heating & Cooling

1 Answer

I have 4 ton carrier heat pump / puron. It is a new system been installed about 3 months. Live near Dallas Texas and lately the temp has dropped into the low 40's at night. The heat seems too be working...


The water is from humidity in the outdoor air which has condensed and then frozen to your heat pump outdoor coil. The heat pump will defrost occassionally, depending on how much frost has accumulated to the coil and will then generally drip off the unit and onto the ground. This is normal. To keep the water off the drive way may mean having to move the heat pump to a place where the condensation can run away from the drive area. Or provide some other means of redirecting the water.

Nov 20, 2010 | Heating & Cooling

1 Answer

When heat pump is in operation, outside coils freeze up.


Its common for frost to accumulate on the outside coil during a call for heat.

This is because your heat pump switched the flow of refrigerant and thus its now pulling heat from the outside and disposing it inside. Yes, as crazy as it sounds, its taking heat from outdoors and putting it indoors.


Depending on the brand system you have and what kind of bells and whistles it comes with will ultimately depend on how and when the defrost cycle initiates and terminates.

If your experiencing more than just frost, It may be time for the system to be checked out to ensure the defrost cycle is operating as designed or the system has not run low on refrigerant charge.

If you need more heat in the meantime, you can switch your thermostat to emergency heat and (provided you have heat strips) you will stay comfortable until you can have the system checked out by a licensed professional.

If the system appears to be working correctly and you get a huge utilities bill, then you will know the culprit.

Jan 13, 2010 | Ruud Heating & Cooling

1 Answer

I have a heat pump and it freeze up


If you are referring to the outside unit freezing up, the issue is in the defrost cycle. The defrost cycle works something like this: The unit reverses the flow of freon causing the inside coil to blow cold air into the conditioned space and the outside coil is now working like an a/c unit, it gets warm to melt the frost/ice. The inside unit electric heat strips come on so you do not notice the cold air being blown in. The fan on the outside unit stops running to allow the coil to get good and warm.
Now the unit should go into the defrost cycle every 90 minutes or so. It should not terminate the cycle untill the coil is clear of frost. There is an "Klixon" type termostat that measures the temperature of the outside coil. This is most likely the problem. It is prematurely terminating the defrost cycle. I'd start there first.

Hope this helps, Let me know if I can be of any further help.

Dec 29, 2009 | Intertherm P3RC-030K Air Conditioner

1 Answer

Whirlpool Heat and Air Window Unit ACE184XRO- freezing up


This is only low freon. If the bottom coils freezes and frost over, you don't have enough freon to travel as liquid back to the compressor and boils right there and past this only cool gas .I'll advise you to get this fixed before you loose a compressor due to locking up by not cooling the compressor properly. Get in touch with an Ac man. this should only cost around $100:00

Sep 05, 2008 | Whirlpool ACE244XS Air Conditioner

1 Answer

Amana heat/air conditioning pump


You are going to have to increase the amount of heat absorbed by the evaporator coil on the indoor section of your system. First change the filter. Second turn the power off to the outdoor section and leave the indoor fan running until all the ice on the evaporator coil has thawed this could take a few hours. Third you need to verify that you are getting adequate airflow across the indoor evaporator coil and the outdoor condenser coil both fans should be spinning at full speed and airflow should be unobstructed by dirty coil fins or debris Hopefully after you've done this you will have cold air blowing inside and the larger of the two lines entering the unit outside will be cool to the touch but not frozen or even frosted regardless of run time and the small one should be the slightly warm but not hot. If the larger line continues to freeze then it could indicate a number of problems but essentially your condensers cooling capacity is outrunning your evaporator coils ability to absorb heat from air stream circulating in your home.Hope it works out for you GL!

Jun 10, 2008 | Amana PTH123B25AJ Heat Pump Air...

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