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Our A/C evaporator coil freeze up last year, so I called out a technician.
He said that it was charged with the proper amount of Freon, and my problem [paraphrased] "something had narrowed the inside of the coil", and said that "cleaning the coil would be a waste of money since that would cost ~$900 (uncharge, take out, clean, reinstall, recharge), and a new A/C is only $1800".

My BS alarm was going off at the time and I'm going to get a second opinion, but in the mean time:

Q1: Can a coil go bad in the manner he described?

Q2: I know in the system there should be only Freon and lubricating oil, but can you figure out if there's any contaminates ?

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Do some research to find out if the coil is under warranty. If it is, it will make your question mute. You'll just have to pay to have the new one installed. It would cost < $1000 –  Edwin Mar 22 '14 at 19:29

2 Answers 2

Mine was freezing over and not cooling the house. I had recently replaced the filters so I made sure all the vents were open. I noticed there was no airflow so I checked the A/C coil up in the attic... I pulled the panel off and it was completely blocked by lint. I used a wide vacuum brush and sucked all the crud off and now there's airflow again and the house is cooling down as I write this. Cost, sweat! lots of sweat!

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I found out I was using too-fine of a filter. Was great at catching every little virus in the air, but sucky for airflow. Swapped them with some more regular filters and haven't had an issue since. –  Alex Moore May 12 at 0:15

Q1: The coil can get plugged with contaminants. Maybe if it was improperly filled, or water got in through a leak. Both situations tend to be rare.

Q2: I'm sure diagnostic equipment exists, but most techs would either just flush it or pull new copper line.

Some other things that tend to cause frozen coils:

  1. Restricted airflow due to a dirty or plugged air filter. Make sure the filter is clean. Also, maybe try a basic pleated one that has good air flow, versus an expensive dense allergy type filter that slows the air down quite a bit.

  2. High humidity indoors. If it's humid indoors, don't set the thermostat too low to start with. If it's set low, like 65f, and the house temp is 85f and humid to start with, it'll run continuously pulling warm humid air over the cold coils. That can lead to freezing. In that scenario, set the thermostat to 78f, wait till it stabilizes at that temp for a few hours, then lower it a bit more. See if the freezing returns or not.

  3. Low airflow due to narrow ducting or blower issues. You'd need a tech to check this out.

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