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I noticed my house wasn't getting under 75°F this weekend when I had the thermostat set for 70°F. Since I'd done a lot of drywall sanding inside recently, my first idea was to replace the filthy air filter.

  • My first indication that the air filter wasn't the root cause was when the return didn't have enough suction to hold the air filter in place while I was replacing it.
  • I went outside to look at the compressor unit and discovered the exposed portion of the refrigerant line running into the house was frozen, but the fan was still running.
  • The evaporator drain line still had a steady stream of water coming out of it.
  • After turning everything off and waiting for the line to thaw, I turned the thermostat back on. Cooler air came out of the vents, but it didn't seem to be blowing with much force, and it wasn't as cool as it should be.
  • I went back outside and discovered the refrigerant line was already frosting up again. At this point it was already dark, so I turned everything back off and slept with the ceiling fan on. :/

Other random notes that may or may not play a factor:

  • The AC unit had been sitting idle the past 3 days since we had nice weather in the mid-70's last week.
  • I mowed the lawn next to the condenser unit the day I noticed it stopped working. However, I was bagging the grass clippings, so nothing should have been blown into the unit.

Any ideas on the root cause of the icing and lower air flow? I wouldn't think the two would be connected normally. Is it something as simple as low refrigerant levels? I plan on going to look more closely at it after work today. What are some other things I should pay attention to?

UPDATE: I checked the system after work today and decided to give it one more chance after I couldn't find anything wrong with it. After a few hours, it had brought the inside temperature down from 80°F to 75°F. I periodically checked the refrigerant line for freezing, but it never got much colder than 60ish degrees.

I think the root cause was the air filter, and that I didn't allow enough time for the line to thaw last night.

UPDATE 2: Maybe I drew conclusions too soon - I just checked the refrigerant line and discovered frost on it again. The inside temperature was at 72°F while the outside was at 66°F. I think pumping warm air to a cooler outside temperature may have contributed to the icing, but I'm not completely sure. (And if you think I'm crazy for running the AC when I could just open the window, you'd think twice too if you saw how many bugs swarm my porch light and windows every night. Dozens of them somehow find their way past the screens if I open the windows.) I plan on running it again tomorrow during the day and monitoring the refrigerant line temperature every few hours. If it freezes up again, I think I'm going to cave and call a repair guy.

UPDATE 3: I totally forgot to follow up on this. It froze again, so I called an HVAC repair guy. He checked the system, added refrigerant, and problem solved. I think the bill came out somewhere around $150 for the service call fee and refrigerant costs.

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    The lower air flow might be causing the freezing. I would look for the cause of the low air flow, clogged ducts, closed dampers, closed/blocked vents, dirty filters. You will also want to call out a serviceman to check refrigerant levels.
    – Tester101
    Sep 12, 2011 at 17:07
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    What you are seeing might be normal, see the first bullet in the HVAC section of this answer. 66°F seems a bit warm to see this type of reaction, but AC units are all a bit different and I'm sure there is some fluctuation to the 60°F limit (keep in mind that during an inspection you will likely only run the AC for a short time, not all night so that could also account for the temp difference).
    – Tester101
    Sep 13, 2011 at 15:52

4 Answers 4

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You may be looking at the problem backwards, the freezing lines could be a symptom of the problem not the cause. Start by looking for blockages in the system that would cause lower air flow / heat exchange.

  • Dirty filters.
  • Clogged ducts.
  • Closed dampers.
  • Closed/Blocked vent covers.
  • Dirty coils.

Basically if the system cannot exchange the heat/cold, it will not function properly and could lead to over heating/cooling.

Frozen lines could also be caused by high or low levels of refrigerant, so you'll want to get a service person out to check the refrigerant levels.

Here is a really good image to visualize how it works.

enter image description here

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    I forgot to follow up with this question. You were right about the low refrigerant levels. The AC repair guy I ended up calling just refilled the refrigerant and that fixed everything.
    – Doresoom
    Feb 20, 2013 at 14:21
  • Flooded evaporator systems start freezing up when they're low on freon. The ice blocks the fins, ice-melt leaks into the plenum, etc. As you found, this is a time to call out the HVAC service tech.
    – Fiasco Labs
    May 24, 2013 at 1:09
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    +1 for the link to Principles of Air Conditioning. A great explanation of how the system works.
    – Joe Shaw
    Sep 11, 2013 at 15:17
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    New, clean filters won't help if the underlying fins are dirty and thus bad at transferring heat to/from the air. Check to see that both the condensing coil and the evaporating coil are reasonably free of dirt and dust.
    – Wayfaring Stranger
    Apr 25, 2015 at 11:50
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My A/C's frozen up before due to insufficient airflow over the evaporator coil. Turns out those $20 allergen-blocking air filters can slow down the air too much, so I switched to a cheaper one that doesn't block the air flow as much.

My A/C guy also told me that running the A/C when it's cooler outside than inside can cause it to freeze up in certain conditions like low airflow or low freon.

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If the small pipe is frozen, the gas is insufficient. Call a service man to recharge some gas again.

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    This answer doesn't add anything that hasn't already been covered by other answers.
    – Tester101
    Nov 10, 2014 at 13:35
  • This is a symptom of low refrigerant, but only when it happens on the ingress side of the evaporator. If the whole thing including the egress is freezing up then it's probably an an air flow problem.
    – Alex Cannon
    Sep 29, 2021 at 19:05
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My guess is a dirty "A" coil or blower wheel. There also may be a leak in the system. If it is the coil or blower wheel, clean it and there won't be a problem any more. If there's a leak adding freon will only temporarily fix the problem. The system should be leak free and refrigerant shouldn't need to be added.

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