Central Air Conditioning Icing



Central air conditioning icing can be caused by low charge, restricted refrigerant flow, poor air flow, failed thermostat, failed controls, or a combination of these factors.


First, make sure the thermostat is working, and that the cooling contacts open and the compressor stops when the temperature setting is satisfied.


Make sure the compressor contactor is de-energized when the thermostat is not calling for cooling.

A failed thermostat, a wire in the wrong place, or a shorted wire, could keep the contactor energized and the compressor running while the evaporator fan is off, and cause central air conditioning icing problems.


If the thermostat is working ok, check for air flow problems.

Inspect the evaporator coil, and if it's dirty, clean it thoroughly.

Inspect the filters. If they're dirty, clean or replace them.

Inspect the blower.

If the blower is dirty, clean it, verify that it's the right size, and that it turns in the correct direction.

Check the blower motor, make sure it is running at the correct speed and in the correct direction.


Make sure there is nothing in the ducting to obstruct air flow, and that the return and supply registers are adjusted correctly and that air flow through them is not blocked.


Once you've verified that you have good air flow through the air handler, if your central air conditioning icing problem continues, you'll have to hook up gauges and check your pressures.


Our System Evaluation Manual has a cycle diagram, and an explanation of the presures and temperatures to look for.


You might also be interested in taking a look at our free 7 page Air Conditioning and Electrical Troubleshooting Chart, which is reprinted from a U S military training manual.


A suction pressure that's equivalent to a temperature of 32° or lower can cause central air conditioning icing problems.

With a low evaporating pressure and temperature, if you have high superheat and low subcooling, you very likely have a low charge.

If you have low suction pressure and nearly normal discharge pressure, with high superheat and high subcooling, you very likely have a restricted metering device or filter drier.


Our System Evaluation Manual explains how to analyze this.


If the pressures and temperatures look normal, you'll need to check the controls.


Make sure that the control wiring in the thermostat is connected correctly.

Cross wired evaporator fan and compressor leads in the thermostat can cause central air conditioning icing problems.

It's possible for the evaporator fan to cycle off and on while the compressor runs continuously, so verify that the connections in the thermostat are correct.


Make sure the evaporator blower relay isn't failing intermittently.

If its contacts are opening and turning off the evaporator fan while the compressor is running, the coil can ice up, and there's a possibility the compressor will fail.


Check the compressor contactor, and make sure its contacts are not welded closed.

If they are, the compressor will run continuously, regardless of the thermostat setting, whether the evaporator fan is running or not.


By the time you've checked all these possibilities, you'll have found the problem.


Good job!


Time for a diet soda?


I hope this page has helped, and please, feel free to contact us with any specific HVAC questions you might have, including questions about air conditioning on Guam, or refrigeration on Guam.

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