I have an air conditioning unit that was here when I bought the house. It was in the 70's here today, but I have an old house and windows don't open so the house was hot. I decided to turn on the ac and put it at about 65 degrees. I left the house for about 1.5 hours and when I came home the house was hot. I went outside and the lines that run from the ac to the house were white with snow/ice.

I turned off the ac and checked and changed the filter. I have had the ac service people out twice this year. First time was to replenish the refrigerant, which was low. Second time was because there was still water leaking from somewhere in the basement. He put more putty around the hoses on the inside unit and the water stopped leaking out. On the second trip he rechecked the refrigerant and it was still ok from the charge he did on the first trip.

I am wondering if that might have caused the freezing. Also, wondering if running it in cooler weather...ie., the temp outside was cooler than the temp inside could create this situation. Does this sound like there are additional problems?

I did what I thought was appropriate: At the first of the year I did have a service person check it twice this year and he said all I needed was freon on the first trip, and that everything seemed ok on the second... I'm concerned as I have a fixed income and can't really afford a lot of additional charges and I hate to say it but I am afraid of being taken advantage of because I don't know that much about the subject.

Any tips on making sure the service man is not just trying to sell me a new ac if that is what it comes down to? Thanks.

1 Answer 1


The problem you describe is considered normal performance for many residential A/C units. By design, these only function correctly if the outdoor temperature is warmer than mid 70s and the indoor temperature is above 68F. Below these temperatures, the evaporator coil gets colder than freezing, becoming worse as the coil accumulates frost, eventually causing the system to become covered in ice and stop cooling until the unit is manually shut down and thawed.

In a situation such as yours where A/C needs to perform even during cool temperatures, it is necessary to install three accessories which will enable the unit to operate under these conditions:

(1) A compressor crankcase heater (sized for your compressor) enter image description here

(2) A head pressure control (such as Totaline p251-0083h, approx $35 new, Ebay)enter image description here

(3) A evaporator control limit (ECL) switch or Freezestat (approx $20 new, Ebay)enter image description here

Calibrating the head pressure control perfectly requires a set of gauges, but it usually works correctly if the temperature probe is placed anywhere near the discharge of the condenser coil. Other than that, these are electrical items only, requiring no refrigerant expertise. This setup is sometimes referred to as a "low ambient kit with freezestat." An experienced tech should be able to install all these within a couple hours.

  • Thanks for this info. I thought that might be the case Question: Did I do any harm to the ac by letting it freeze. Luckily I caught it before the ice was covering the larger unit...only the lines running from the outside unit to the house. Again...thank you. Rose
    – R.Doe
    Oct 20, 2015 at 14:32
  • That depends on factors such as: (1) what kind of metering device does the system have, (2) what kind of compressor does the system have, (3) is the system somewhat overcharged/undercharged, etc. If it still runs then don't worry about it. There is nothing practical that can be done to reverse this kind of damage anyhow. If this happens frequently, then the compressor will likely fail prematurely.
    – user39367
    Oct 20, 2015 at 15:34

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