Came home yesterday to find that the house was burning up and the thermostat had automatically switched itself to heat... in the summer! We didn't know what was going on, and assumed some sort of bizarre computer chip issue, so we turned it back to A/C on the usual temperature, and left it alone.

Went down to the basement today to find that there was water pouring out of the A/C unit. It was running at the time. We turned it off, and the water stopped. Checked the drainage line for the drip pan... it is clear and drains fine. So, I opened up the plenum to take a look at the coils. They are covered in ice, which I'm assuming is not usual.

We replace our filters regularly. Other than that, I don't know much else. Any idea what could explain all this?

  • 1
    This answer might be helpful. Too much or too little refrigerant, or reduced air flow through the evaporator can cause ice.
    – Tester101
    Commented Aug 18, 2014 at 13:39
  • What are the make/model of the thermostat, heater, and A/C unit?
    – Tester101
    Commented Aug 20, 2014 at 11:58

1 Answer 1


Inadequate refrigerant causes the evaporator (and sometimes the condensor) to form ice or frost on the outside.

The usual fix is to have a HVAC tech repair the refrigerant leaks and recharge the system.

You can save some of the service call cost by looking for the leaks and pointing them out. Soapy water works well applied with a spray bottle or small paintbrush.

  • Thanks for the info. All of our Googling seemed to say that ice would be caused by insufficient airflow over the coils, or low levels of refrigerant. However, nothing could explain why the thermostat switched from A/C to heat automatically. Any ideas what would cause that? Commented Aug 18, 2014 at 11:22
  • @JohnChrysostom could be that the system has a built in defrost mode, that is switched on if the coils ice up. It's common on heat pumps, but I guess could exist in air handlers as well. Not sure how the thermostat would switch (especially if it's actually a physical switch), but if the thermostat was designed to work specifically with your system it is possible.
    – Tester101
    Commented Aug 18, 2014 at 12:57
  • @JohnChrysostom: Only a thermostat malfunction, or a human changing the switch, explains changing the mode from cool to heat. Tester101 correctly brings up the point that a defrost cycle could possibly look like the system is in heating mode briefly once in a while (turning on backup heat for a few minutes) but that should not happen in cooling mode.
    – wallyk
    Commented Aug 18, 2014 at 16:49

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