Dr. Kool‘s

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Freeze-Ups, Evaporator Coils

If your AC isn't cooling, it's a good idea to check your outdoor condenser. Is it running, but frozen up? If so, you'll see ice on the suction line—that's the fat copper line that's insulated most of the way. Your best view may be through the horizontal grille above the compressor.

Why the Freeze-Up?

Low Freon, or anything that interferes with air flow, can cause a freeze-up. The first two possibilities can be corrected by the homeowner:

  • Stopped-up air filter.
  • Registers (air vents) in the closed position.
  • Low Freon.
  • Stopped-up evaporator coil.
  • Outdoor unit running when it shouldn't run. (This is usually caused by a stuck contactor, but for a heat pump, it could be a defective defrost board.)
  • Indoor unit not running when it should be running. (This is usually caused by a intermittent fan relay or faulty blower motor. On rare occasion, it could be a defective thermostat.)
  • Indoor blower motor not running. (On rare occasion, the indoor blower motor will run at partial speed.)
  • Duct obstructions (internal).
  • Evaporator coil failure (rare).

Marginal Freon level, along with a dirty air filter, can hasten a freeze-up. Once an evaporator coil freezes, air flow completely stops, and the ice will continue to build.

Stop the Condenser

You must stop the outdoor condenser immediately. If one thing doesn't work, then try another.

  • Turn it off at the thermostat.
  • Pull the disconnect at the condenser.
  • Trip the breaker at the electrical panel.

Melt the Ice

Most of the ice is hidden in your indoor evaporator coil. You can either wait for the ice to melt, or you can hasten the process. At the thermostat, set your AC to the "Off" position. Here are some possible approaches.

  • Set your blower to the continuous "On" position, and let it run for at least two hours. If you have electric strip heat, this is the only way.
  • If you have a gas furnace, set the thermostat to heat, and fire the burners for 7 minutes. Turn off the heat, and run the blower for 15 minutes.
  • If you have a heat pump, set the thermostat to heat, and then carefully raise the temperature 1-2 degrees above room temperature. This will run the heat pump without bringing on the heat strips. Watch your thermostat, and keep the setting 1-2 degrees above room temperature. Do this for ten minutes. Then shut off the heat pump, and run your indoor blower for fifteen minutes.

Correct the Problem

Changing a stopped-up air filter, or opening closed registers, may completely solve your air flow problem. If registers were not closed, and your air filter is clean, you will probably need to call an AC technician.

If you think the underlying problem has been solved, go ahead and run your AC, and then check at your condenser. (1) Warm air should be blowing from the top of the condenser, and (2) The suction line should soon reach 65F, and be sweaty. If air from the condenser is not warm; if the suction line remains warm, or if it starts to frost up; you may be low on Freon. Do not allow your unit to freeze-up again—call for help.

Get it Fixed

If your AC is not cooling, never allow it to continue to run. When the repairman arrives, he may find it frozen up, or he may find the compressor has overheated. Both of these conditions are hard on the compressor, and will delay the repair job.

Kool, but not Frozen,

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Dr. Kool

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