What's the possible problem with a frozen air conditioner condenser that won't turn off except with the breaker?

There are 3 possible problems with this unit and will probably need the attention of an HVAC Technician for repair. Here's the 3 possible problems:

Commercial 3-phase Compressor Contactor

1. The first thing is the condensing unit will freeze up if the air handler blower is not running when the condenser is on. The condenser is pumping liquid refrigerant to the air handler evaporator coil where air is needed to blow across the coil so the cold refirgerant can absorb heat. If the refrigerant is not absorbing heat because the blower is not blowing air across the evaporator coil then the condenser will freeze. This often happens when everything is working fine but the filter is plugged up, ductwork is collapsed, or the blower motor fails. In this case the blower is working fine but the condenser will not turn of so the thermostat is cycling the air handler blower on and off but the condenr is not turning off. It is possible you have a defective thermostat but i would check two other things first. I would check the compressor contactor to see if it was stuck in the closed position. Under normal operation when the thermostat calls for cooling (heating in a heat pump) a 24 volt signal is sent to the compressor contactor inside the air conditioner condenser unit. The energizes and closes the contacts of the compressor contactor which is basically a switch controlled by the thermostat. This allows high voltage to pass to the compressor and the condenser fan motor. When the switch or compressor contactor closes and opens a slight arcing of electricity happens and over time this causes pitting of the contacts. It can also cause the contacts to stick so the even when the thermostat disengages the compressor contactor or stops sending the 24 volt signal the unit will continue to run because the contacts are stuck and will not release. The compressor contactor needs to be replaced and this should be done by a qualified HVAC technician. When the compressor contactor sticks like this the only way the condenser will turn off is if you turn the main power off to the condenser as you have been doing when you need air conditioning.
2. The other possibility for your problem is the thermostat wires are bad somewhere. These would be the thermostat wires that go to the air conditioner condensing unit. Old thermostat or damaged thermostat wire will cause the condenser to run and not shut off. Old thermostat wire loses the insulation and the copper which the insulation protects touches and feeds continuous voltage to the compressor contactor. Same thing with a damaged thermostat wire. Leave the condenser on and turn the power off to the air handler to test this. This will kill the 24 volts which feeds the compressor contactor. If the condenser turns off then you have either a bad thermostat or the wire problem described here. New thermostat wire needs to be pulled from the thermostat to the condenser to fix this problem and I recommend you get a professional to do this job.
3. The next thing it could be is a bad thermostat. If you have checked all the other things and nothing happens carefully disconnect the yellow wire from the Y terminal on the thermostat. If the unit shits off then you have a defective thermostat and need to replace.

Whichever problem you have it is probably best to call an HVAC contractor that specializes in service to repair this problem. The work usually comes with a warranty and you will get the job done right by a professional who does this kind of work day in and day out. Another word of advice. Don't keep running the condenser unit when its frozen or the evaporator coil is frozen. You cause irrepairable harm to the compressor and that costs a lot more than replacing a compressor contactor, thermostat wire, or thermostat. Good luck.



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