Take the 2-minute tour ×
Home Improvement Stack Exchange is a question and answer site for contractors and serious DIYers. It's 100% free, no registration required.

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.

share|improve this question
5  
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 '11 at 17:07
    
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 '11 at 15:52
add comment

2 Answers 2

up vote 11 down vote accepted

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 explanation of the Principles of Air Conditioning, and an image from that to visualize how it works.

enter image description here

share|improve this answer
    
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 '13 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 '13 at 1:09
1  
+1 for the link to Principles of Air Conditioning. A great explanation of how the system works. –  Joe Shaw Sep 11 '13 at 15:17
add comment

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.

share|improve this answer
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.