Photograph of attic air conditioning air handler, condensate drips on floor Cooling Coil or Evaporator Coil Ice-up or Frost Over
in Air Conditioners or Heat Pumps

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Cooling coil ice & frost formation diagnosis & cure in the air conditioner or heat pump; refrigerant piping line frost causes & remedies.

Diagnose & fix ice or frost blockage of the cooling coil or evaporator coil in an air conditioning / heat pump system.

We explain the causes of cooling coil ice formation and blockage. Some of these causes can be repaired by something as simple as changing a dirty air filter or cleaning a dirty coil or blower fan.

This article series explains the causes & cures for evaporator coil icing: the problems of ice and frost formation in air conditioning system air handler units, blower units, or AHU's, duct work, or other air conditioning system components.

A freezing or frosted A/C coil blocks air flow and leads to loss of cooling.

Our page top photograph of a thoroughly ice-blocked air conditioner evaporator coil was contributed by a reader who described: "I cleaned the coils & installed a new filter - obviously I have a low refrigerant problem. This is an 11 year old furnace/air handler with no history of other problems, but low on Freon."

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Causes of Ice or Frost Build-up on the Evaporator Coil or Refrigerant Suction Line on an Air Conditioner or Heat Pump System

Ice on the A/C refrigerant piping and thermal expansion valve (C) Bill Cauthen D Friedman

Discussed in this article: Frost or ice build-up on evaporator coils and its effect on cool air flow and mold: Freezing AC coils; Frost or ice formation at air conditioning compressor/condenser units; Ice and condensate problems in air conditioning duct work, why it forms, how bad it can get, how to prevention.

Page top photo of an iced-up air conditioning evaporator coil are courtesy Guy Benfante.

[Click to enlarge any image]

An air conditioning system will not operate properly and will lose cooling capacity if the evaporator coil becomes blocked with frost or ice. Even though there is all that iceOn the evaporator coil the cool air flow out of the system will be reduced as air flow across the coil becomes less and less as the ice area grows.

Our photo (left) of ice formation on refrigerant piping and on the thermal expansion valve in an air conditioner air handler unit was contributed by reader Bill Cauthen.

Article Contents

Cooling Coil Ice-Up on the air conditioner air handler or evaporator coil: Besides a reduced air flow through the HVAC system, a second result of the reduced air flow due to a dirty air conditioning filter can cause the evaporator coil (the cooling coil) in the air handler to become blocked by ice, stopping or significantly reducing cool air output from the system.

A visual inspection of the cooling coil in the air handler can quickly show whether or not it's ice-covered. If the coil is iced-up and blocked, turn off the cooling system entirely until the ice has all evaporated and cleared. Be sure that your condensate drain is not blocked and that the water from melting ice will be properly disposed-of.

If you replace all dirty filters on the system and remove ice from an iced coil and the coil ices up again when the system is turned back on, your AC or heat pump system has a frost problem that needs repair.

Why Frost or Ice Forms on an Evaporator Coil

AC Coil ice up (C) Daniel Friedman Bill McNeillFrost line on the cooling coil: When liquid refrigerant enters the evaporator coil temperatures may be as low as 10 degF at that point - that is at the top of the coil at the point of refrigerant entry. In normal operation of a refrigeration system, air movement across the evaporator coil provides enough warmth that frost or ice do not form on the coil.

In fact, as one sees in a refrigeration class, releasing liquid refrigerant into a coil over which air is not being blown will quickly result in frost formation on the coil surfaces, beginning at the point of entry of refrigerant into the coil.

At the point on the cooling coil (with no air blowing across it) where no more frost forms on the coil, we know that there is no more liquid refrigerant in the coil.

That is, at this point in its travel through the cooling coil all of the liquid refrigerant that has been introduced has boiled (evaporated) to a gas. Now as all vapor, the refrigerant begins to absorb sensible heat and its temperature will increase.

There are pressure increases at this point in the coil too, but they are insignificant.

In a refrigeration class demonstration, we learn that one could, given no other data, determine the proper refrigerant charge or better, the proper adjustment of an adjustable refrigerant metering device (Thermostatic expansion valve) by adjusting the refrigerant flow rate into the coil so that the frost line stops just before the end of the coil.

Normal cooling of building air at the cooling coil: In normal operation an air conditioning system is cooling air by moving it across a refrigerant-cooled "evaporator coil" or "cooling coil" in the air handler.

Dehumidification at the cooling coil: Cooling air passing over the coil also removes moisture from that air - a key factor in making indoor air comfortable in hot weather. (Photo at left of an iced-up cooling coil courtesy of Bill McNeill.)

Normally the moisture that's removed from building air forms condensate on the surfaces of the cooling coil, runs down that surface to a collector pan, and is drained away. [CONDENSATE HANDLING discusses disposing of air conditioning condensate.]

Why frost or ice forms on a cooling coil in an active or in-use air handler

Photograph of client using tissue to look for air conditioning air flow

Any or all of those conditions cause the level of refrigerant in the cooling coil to be too low; if there is some refrigerant but not enough the coil may become abnormally cold, freezing the condensate that forms on the cooling coil surface as moisture condenses out of air moving across the coil

This freezing condensate liquid can form frost and may build up into a coil icing problem or frost may appear on the cooling coil's refrigerant suction line.

When the surface of a cooling coil or suction line drops below 32 degF (say from too little refrigerant in the system or too little flow of warmer air across the cooling coil) frost formation is likely on that surface.

Conversely, when the air conditioning system is working properly the surface temperatures on the cooling coil and on the refrigerant lines stay above 32 degF.

In some installations the evaporator coil tend want to drop below 32 F even in normal operation, but air movement across the coil keeps its temperature higher, and thus avoids freezing.

On some commercial refrigeration or air conditioning systems where lower temperatures are common, a defrost cycle is designed into the equipment.

If an icing problem is occurring on commercial cooling systems, in addition to checking the refrigerant charge and air flow, the service technician will also check out the defrost cycle timer.

Reader Question: poor cooling traced to iced coil traced to dirty air filter

Our problem began with a poorly performing unit. The Ice only formed after the visit by a licensed hvac tech
recommended by our ins. co. who wanted to install a completely new system.

I discovered the ice this A.M. and on climbing into the attic to check the coil I found the thermostat control cable had been intentionally crimped in an attempt to break the wire inside.

It's an old unit but now I'm on my own trying to eke out the truth. BTW the tech never mentioned the possibility of a dirty filter. It was covered completely with dust. Hopefully the coil is dirt free since there is no access. - Jim 5/29/11


too bad your service tech missed the clogged filter - you might want to mention to him or her that that's what you found - it might help another customer.

I wouldn't assume the tech tried to sabotage your system, more likely it was someone without experience. Unfortunately it's too easy to recommend replacement of equipment when some careful diagnosis might be what should be done first.

Question: will a wet air filter block airflow and cause coil icing?

(May 14, 2014) Anonymous said:

If air filter gets wet does it block air flow causing frost to build up on coils?



Yes a wet air filter will severely block air flow,

Watch out: and worse, if the filter is located where it may be drawn into the air handler where it encounters moving parts it would be an unsafe condition, even risking a fire.

Reader Question: Low Refrigerant & Coil Icing: cooling coil icing & bubbles in the refrigerant sight glass:

(Aug 5, 2012) Rob C said:

Hi, AC just stopped cooling (2 year compressor / condenser unit). Tech said freon very low. Charged system with 6 lbs of R22, checked quickly for obvious leaks and left. The site glass indicator had a solid flow of Freon with micro bubbles. Noticed that the larger suction line never got cold. The smaller liquid line was warm for a short while.

AC vent air was never cool and less than 24 hours, the glass indicator shows low freon (gurgling - large bubbles) and the evaporator coil freezes.

Do not notice any obvious oil leaks. Can a compressor be the source of a freon leak and prevent the large suction line to never get cold to the touch?

(Sept 1, 2012) juan said:

I added R-22 2lb 4 days ago, now it is doing the same, coil is frozen up all the way to the suction line at the outdoor unit.

my question is if there is leak in the system or restriction in the liquid line, or air flow problem besides a leak in the system, replaced air filter, indoor fan motor seems that it is not running properly,but the fan capacitor is okay.

(Sept 7, 2012) mike said:

the small capillary lines are freezing up on mine....just like the pic at he top,,,they freeze almost instantly when the system is tuned on..just seems to be the small lines where they split at the joint at the larger line....any suggestions?? thanks in advance

Reply: low refrigerant can cause ice or frost on the coil

Rob bubbles in the sight glass and gurgling usually means low refrigerant. If the tech gave the system a full-up charge then you've got a big refrigerant leak.

The compressor motor is hermetically sealed on most residential systems - not likely that that can is leaking - if it were you'd see oil deposits at a seam or fitting.

Reader Comment: control board error may cause cooling coil icing

(July 24, 2012) said:

Another cause of icing is the control board not sending the signal to the compressor to shut down meanwhile the blower has soppped and the louvers has closed. Replace the control board or install a new seperate thermostat

Thanks so much Mr. Hansen for your important contribution to the diagnosis & repair of coil icing. Use the CONTACT US link at page top or bottom if you are a professional in the field nad if you want want to provide listing & contact information. No costs or fees are involved.

Question: frozen pipe outside my air conditioner unit - bad switch not lost freon?

Pipe frozen at outside unit and the inside unit would stop running but the outside unit would continue to run

. I made an appointment for service. The tech came out and checked my freon level, he said it was fine and suggested that my problem may be the compressor.

I told him that I was out of town all week and did not notice any problem before I left. He then told me that severe weather came through the area a few days prior and I should call my insurance agent to file a claim. The insurance agent sent someone out to check it.

This turned out to be another heating and air company.

This guy said that the freon levels were good but switch needed to be replaced. He replaced it but three weeks later I have the same problem and have no idea what to look at myself to see what the problem could be. My air filter is new.

Both units come on and off together. No registers are closed. I checked for dirty coils outside then went into the attic, up there the coils had frost on them hours after I shut everything down. - Jason 7/30/11

I have the same problem as Jason, only it is intermittent (once every two weeks or so). Ha,ve had two contactors(?) replaced since last year. What can I tell repairman to test next? - Jackie 8/2/11


Jason if your inside unit (air handler, cooling coil, blower fan) stops running, I'm not clear how a bad outdoor compressor would make the inside air handler unit stop.

A bad switch can be the culprit and sometimes even a replacement part can be bad, or the conditions that burned up the first switch could burn up the second one, indicating that more careful testing and diagnosis to find a root cause are needed.

Jackie, I would be careful not to be too directive to the repairman; but I would press for an explanation of why the same part keeps being replaced; it's natural to suspect that a different problem is damaging the part that's being swapped out.

Question: icing on the outside condenser

(Oct 1, 2012) jb said:

my outside condenser keeps freezing up with ice, i cleaned the coils in the outside unit and the attic unit, i can feel strong air coming out of vents but not very cold.

when i turn the fan on for about an hour or two the ice is melted outside on the condenser and lines and blows real cold air, however the ice formation occurs daily about mid afternoon and thermostat just stays at 75 degrees, then i turn the fan on again to defrost.

also has new air filter and electric bill is through the roof


Strong but not cool air flow argues that the problem is not a blocked coil, but possibly a wrong refrigerant charge or a compressor or refrigerant metering problem

Question: new 5G Coil was encased in ice as was the refrigerant piping - humidity sensor was bad

(Feb 21, 2013) Angelica said:

The new 5G coil was encased in ice and well as the indoor and outdoor pipes. After shutting system off and letting it thaw, it was determined that the sensor that reads the humidity was bad and the tech stated the unit was continuously running trying to de-humidify and that caused the freezing.

New sensors were to be ordered. Ie. The freon levels were good. Please give your opinion. Thanks.

(May 5, 2014) jhun said:

what happened the compresor is always backfrost


The risk, if that's what you are asking, is the movement of liquid refrigerant into the compressor - which can destroy it.


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