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So my central AC unit started doing something that I have not experienced before.

Once the AC stops, cold air comes out of the intake vent, like really cold air. This has lead to ice forming on the inside of the evaporator coils. Once this ice melts water drips down onto the insulated duct work of the intake.

Any clue what might be causing this?

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AFIK the main cause of freezing on the evaporator coil is low refrigerant. This may sound counter-intuitive but it is a well known accepted fact.

There may be other possible causes. I think the blower is supposed to continue for a short period after the compressor cycles off and your unit may not be doing that. Call a competent a/c tech. This is not a DIY project.

From original poster:

a tech . . . discovered that the switch to turn the outside unit on and off was bad. It would sometimes stick open and other times closed. The result was that although the inside unit turned off, the outside unit continued to run.

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    Also caused by too little return air flow. Check filters and return grills for obstructions that starve the air handler. – Kris Jul 10 at 12:41
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So the intake was cold because the evaporator coils were freezing. Jim speculated that this was because there was a leak with the Freon. That wasn't the issue.

After employing the services of a tech, he discovered that the switch to turn the outside unit on and off was bad. It would sometimes stick open and other times closed. The result was that although the inside unit turned off, the outside unit continued to run.

This was causing the freezing on the evaporator coils. Once he replaced the switch, no Freon was needed and the unit has been cooling without freezing since.

This probably could have been a DIY fix if one was comfortable working with the outside unit.

  • You should deselect my answer, and maybe select yours. I agree that changing the "contactors" could be a DIY job, but I have never done it. How long did it take the tech to diagnose the faulty contactor unit? – Jim Stewart Jul 15 at 19:43
  • Probably about 15-20 mins. He tried adding Freon and it did not work, because it was frozen up. Then he told me to turn off the AC. That is when he noticed the inside unit was off and the outside unit was on. He took off the panel, tapped the switch with a screw driver, and it turned off. Replacing the switch seemed easy enough, it is just the high voltage would make me proceed carefully and you have to have the part. – Pete B. Jul 15 at 19:48
  • You turn off the power to the condensing unit either at a switch next to the unit or in the panel. – Jim Stewart Jul 15 at 20:03

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