Partial cold weather A/C check

For those that want to put some extra effort into a cold weather inspection or just use this information for home troubleshooting.

Here is a procedure that will indicate the refrigerant level is not below the “low pressure switch” setting (which indicates very low refrigerant) and some basic control voltage to the condensor contactor is working without actually running the compressor.

It takes two people to do this in most cases:

  1. Pull the 220V disconnect at the condensor unit and put your ear near the unit, 2 feet away is close enough.

  2. Have someone adjust the t-stat to call for cooling.

  3. If you hear the sound of a large contactor energizing then you know the control circuit is allowing the compressor to receive voltage which it will not allow if the refrigerant is really low.

  4. If you don’t hear the sound, don’t assume it is low on refrigerant, it could be many other things wrong, including two or more problems. Now you have a very good reason to recommend a service call on the unit.

  5. Set the t-stat back to where you found it (heat mode)

  6. Re-install or reset the disconnect.

Bruce all that will tell you is that the thermostat is indeed pulling in the 24 volt contactor to the closed position provided you have shut down the line voltage.

The low pressure switch does not determine the level of freon as in a liquid state, the LP switch is located on the suction side of the compressor and senses vapor pressure return to the compressor when it is in operation. You could in essences have one lb of freon in a system that requires five for proper operation and still have a closed LP switch in the off position of the unit but would open within the first few seconds that the compressor started to operate.

Your process for checking the contactor is fine but you need to re-think the LP switch. BTW usually only high end units and heat pumps have LP switches installed from the factory.