Causes and Fix for Frozen Inside Air Handler Coils (Air Conditioning Unit)

July 21, 2010 | by Fred (email) |

There’s really nothing worse on a hot, humid Summer day than to have your central air conditioner give out.  Unfortunately, that’s just what happened to us about three weeks ago right after our hardwood flooring installation.  The temperature in the house started climbing steadily beginning around 6:00pm, rising from a relatively cool 73 degrees up into the mid-80s.

While I’m not an air conditioner tech, I can check the basics of an air conditioning system (air filter, blown fuses, ice build-up, etc).  The skill comes in handy when you contact an HVAC technician. It’s much better to be able to tell them, “I’ve got ice built up on the inside unit” than to simply say, “It’s starting to get hot in the house.”  They’ll be able to give you some potential causes of the problem and expected costs for repair.

So I started checking what I knew to check. Step one was to pull out the air filter, which was soaking wet and was stuck on something inside the unit.  In fact, it was so stuck it wouldn’t come free and I thought continuing to struggle with it might rip it apart.  I decided to open the upper access panel that houses the interior air coil and here’s what I saw.  Problem identified.


We’re no stranger to frozen outdoor air conditioner compressors. In the Winter, it’s not uncommon for an outside compressor unit to freeze up if the defrost cycle on the unit is set to run too infrequently. Freeze-up can also take place if freezing rain or snow accumulates on the fan blades, or is allowed to sit on the exterior of the unit.  Fixing an outdoor frozen heat pump can be as easy as setting the defrost cycle to run a little more frequently, which did the trick for us.

Of course, in the Summer the situation is reversed. The system extracts heat from the inside of the house and moves it outside. In this scenario, the air coils inside the interior air handler get very cold, with evaporated freon moving through them.  Air passes over these coils and most of the time, you get cool air conditioning in your home.

Cause of Ice Frozen Air Conditioner Air Handler

So what causes an interior air handler to freeze up instead of operating normally?

There are two common causes for freeze-up, and one is less expensive to fix than the other.  The first cause is a freon leak in the system. When a freon leak is present, the coils can cool unevenly with some parts of the coil staying extremely cold for a long time.  This leads to build-up of ice on the coil from the moisture extracted from humid air passing over it.  Once the ice starts to build up, it easily persists because it acts as an insulator on the coil, preventing air from passing over the coil and warming it up.

The second cause is poor air flow over the coils and fins, usually caused by an extremely dirty air filter or dirt build up on the coils or fins.  When air flow is restricted, the coils and fins become too cold leading to ice build up.

Since we had just finished that hardwood project, we figured the dust blown around in the house was probably the culprit, and at least worth checking before making a call to the technician.

How to Fix a Frozen Air Handler

The easiest way to melt the ice is to allow the fan to run without the air conditioning on for 24-48 hours.  You must let ALL the ice melt before turning the A/C back on, otherwise the problem will quickly return.

Unfortunately, we were in the middle of a terrible heat wave and didn’t want to wait 24 hours for a solution. So we used the faster method: a hair dryer and two 500-watt halogen lights. The process took 2 hours to get the air handler completely dry.

Note: You should always turn off the breaker to the HVAC unit before working on it.  Also, using a hair dryer and/or heat lights could be a fire risk if you aren’t careful.  So be careful and do this at your own risk!

It is imperative that you don’t disturb or break the coils inside the unit.  One broken coil will mean a much more expensive fix.


The unit dried:


We turned the unit back on and nearly instantly the cold air came rushing through the house.

In the three weeks since the event, we’ve had no recurrence.  It looks like the air filter did the trick, saving an expensive HVAC technician call and confirming that we probably do not have a freon leak. Had the unit frozen back up, we would’ve called our local air conditioning contractor to come take a look.

31 Responses
  1. Bill says:

    Good article. I’m sure this well help many people out there. HVAC is a very foreign thing to most I believe.

    • Billykelley says:

      One other thing yo do is Ck the squirrel fan make sure it isn’t caked with dust – if so just get at it the best you can and take each blade use a tool wire brush or screw driver to get he dust out- vacuumed the dust up—best not to take fan out to clean due to the complex wiring—

  2. Patrick B says:

    Thanks a lot for this article/ help Thanks to you I found out what I need to do

    I will follow your recommendations

  3. Daytona Limos says:

    Thanks for the info our shop AC froze up today and it has never done that before.

  4. Kate the Great says:

    Fantastic info – what did you need to change/clean to stop the ice from forming again? The usual in-ceiling filters, or do you have a whole-house filter or a filter-drier?

  5. Fred says:

    Kate, we just replaced the whole house filter and kept an eye on it. We didn’t end up with a refreezing problem. If it keeps occurring it can be caused by low freon, but I would definitely replace the filter first.

  6. Megan says:

    Thank you! We’ve been having this issue off and on for a year, each time just defrosting it and replacing the filter. You’re the first person who has put pertinent info out there about this problem. Thank you so much! Looks like now we can look into getting our leak fixed… 🙂

  7. Andre says:

    Thanks for the info. My filter was really dirty and my unit was frozen just like the one in the photo. I’ll cross my fingers, but the advice seems dead on. There must be a lot of techs wishing you had kept this info. to yourself, but us financially challenged people appreciate it. Thanks again.

  8. Tammy says:

    My unit on the inside is sometimes leaking on the sides. Right now I have bowls there to catch the water. It was leaking last summer then stopped during the winter when the a/c wasnt working as much but I just noticed the other day that it’s leaking again but not all the time..Do u know what’s the problem & how can I fix it before it starts to get really hot & leaks alot.Thank you..

    • Tony says:

      It sounds like a partially clogged drain line. My usual fix is to put a shop vac on the line outside and that usually clears up most of the clog; however, if the problem persists you may have to use a pvc safe drain cleaner to remove the scale from inside the line. The scale comes from dust rinsed off the coils and sometimes, depending on where you live, airborne algae from outside hitting the water in the p- trap. Hope this helps.

  9. eve says:

    Hello, if the freon is low how can you tell and fix it? ours froze just that way out of the blue and my filter wasn’t that dirty that why i’m thinking it may be freon, is that very costly to put in?

    • Fred says:

      You will need to have a licensed HVAC Technician test it. Usually house calls run between $100-$120 to check things out. Testing freon requires special tools and techniques. Short of buying a testing kit and getting a book (which will require time and money), I would go with the pro for something like this.

  10. RICK says:


  11. Jeff says:

    well written article, thanks. I jave a unique situation-with our new system off but power from the breakers to it, what caused my evaporator freeze up? It in FL and 49-72’F so no AC usage. The handler was making gurgling sounds only after it froze up which stopped with the breaker turned off.

    • Joann says:

      Fred I really appreciate this inflammation I was have the same problem AC keep running all night never turning off automatically. took the cover off the coil was frozen used a dryer to melt the ice waiting to see if it will work. Do not have the funds right now to call a HVAC repairman. running the fan for a few hours to help.

  12. Renae says:

    Thanks for sharing! I have melted ice. I turned on Air Handler and I can hear unit outside turning on, but when I checked the AH inside I heard some air and I felt the cold air, sounds like a leak?

  13. bello says:

    no blockage,no leakage,and the indoor still freezes up what could be the challenge?

  14. Wanda says:

    My ac freezes up all the time. I’ve put about a thousand dollars trying to keep it going. One ac company charged me 300 to tell me that I need a waterhose to spray out outside unit. Another charged me to co.e add coolant. Filter is new but about every 2 or 3 days it freezes. How hard is it to remove coils? I don’t have husband or brothers, it’s just me and my dog. Can I do it myself?

  15. Cmh says:

    Hi Wanda,
    I’ve had the freezing up problem on the inside unit, but it is usually from a dirty filter or dirty coils. Investing in having the coils cleaned is a good idea (especially if you have animals- the fur coats the coils if your return air filter isn’t a good fit). I personally feel the cleaning should be left to a reputable AC company as the coils are easily damaged and can create a leak of very expensive freon. my unit is older and I’m just hoping it gets through this summer season… It’s just not worth putting additional money into it at this point.

  16. Cmh says:

    Thank you for the explanation in this article. Thanks to you I was able to fix my own problem and avoided the cost and inconvenience of having a repair technician come out.

  17. Brandi Bieber says:

    We just got our hardwoods replaced two weeks ago and find ourselves sitting in the middle of a heat wave with a frozen AC. Thanks for the article/tips and my husband is getting my hair dryer as I type!

  18. Scooter says:

    With my ac shut off the coils in the air handler are frosting up the filter is clean but its doing this when the system is shut down

  19. Scooter says:

    When my ac system is shut down the coils in the air handler frost up when i turn the system on to ac the coils clear. In a couple of minutes but it blows warm air after they clear one line is cold the bigger one the smaller one stays cool thank anyone.

  20. Joe says:

    I’ve got the same problem – but it seems to occur after a brownout happens. My AC guy mentioned about installing a delay timer that, if a brownout should occur, the AC will stay off for at least 5 minutes to allow the thermstat a chance to reset. You ever hear or run into this?

  21. Jason says:

    Had the same issue today cleaned the coils no luck… So I took panel off to look a blower seen the capacitor and thought that might be bad so I got a wooden spoon turn ac unit on and used spoon to manually start blower fins and it ran fine … Going to get new capacitor tomorrow… Hope this helps someone else

  22. Mary says:

    Thanks! Fixed my middle of the night problem, which is the only time this happens! I wonder why is that?

  23. Glenn says:

    Great advise, thank you! and thank you again.

    We just returned back home from the airport with guests and the AC was not working. We live in Florida AC has the same value as O2 – required to live.

  24. Pat Cashell says:

    My A.C. SYSTEM IS ONLY 8 YEARS OLD. THE AIR HANDLER IS IN THE ATTIC. IT FROZE UP AND THE TECH. SAID EVEN THOUGH IT WAS STILL UNDER WARRANTY IT WOULD COST OVER $1,500. for labor to replace the evaporator. That seems high for labor. After he left I turned the a.c. on and it worked fine. I have gotten a few tips now. Let me know what you think about that charge. I think it may just need Freon…

  25. Jay says:

    The idea in this article is awesome! BEFORE I read it, I had noticed that my heat pump was running endlessly (at night) and there was very little airflow. The little air coming iur the vents was cold, just reduced.
    It had been working great during the day, and we have had no work or cleaning done recently, so I suspected there was some other kind of obstruction.
    I am not aHVAC service person but have moderate knowledge of how te heat pump works (the reversible freon operation), and have seen my share of frozen outside evaporator coils, necessitating the shutdown of the unit to await the thawing process. It was not hard to reason that the inside unit could do the same reason (although “why” is another issue).
    I did what was done in this article, but a little differently: I decided that, if I turn the unit off, it will defrost Over a long time. But after it was off, I thought, “why not runt the heat pump in “heat mode” and force the heated freon INTO the inside unit”?
    I did so, and the air flow increased – blowing frigid air the whole time as the “heat mode” air blew over the block of ice in my attic!
    About 45 minutes later – violà – warm air!
    All that was left to do was turn the unit to “off” one more time and place it back into cooling mode!
    Hope this idea helps someone else…just be sure that the drain on your collection pan under the inside unit drains well or you will have a flood from the melted evaporator ice cube!

  26. Jack says:

    We had a new system installed in April. Installer has been out twice for icing problem. He insists that it’s the filter. We’ve been changing them every two weeks per his suggestion. Isn’t a Freon leak indicated by now? He says no leak.

  27. Absolutely! If your heat pump is blocked by ice or doesn’t seem to be defrosting enough to allow it to run normally, there are a few things you can check. Make sure that the air filter is in good condition. Replace the filter if it seems to be full of buildup.

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