What is wrong when thermostat is turned off and outside unit is still running for hours and the line on the unit is frozen?

Answer 1

This condition can badly damage your compressor outside by what is known in the profession as liquid refrigerant slug back. I strongly recommend you cut the breaker off to the unit outside until the reason for this situation is found and fixed.

As for the problems cause itself. There are a multitude of conditions that could exist that would cause this. I will name a few but no way can I go into all I've seen in the field.

1. 1, And most Common cause is the contactor points are arc welded shut and can't release to open the compressor and condenser circuit. If you do find your contactor points for outside unit are arc welded closed then look for what caused it to. Mostly you will just find the coils outside are really stopped up or fan motor runs intermittently and shuts off by internal overload until cool enough to cut itself back on for awhile which makes the compressor load sever and amp draw extreme.

Also rapid short cycling due to a low pressure or high pressure automatic reset switch.

Look for loose or inadequately sized wires that supply the units main power as well. Just because a unit run load amps may be adequately handled by smaller feed wires doesn't mean that wire can adequately supply the very large amounts of currents to start up. This creates low voltage and surprise ! ( extreme contactor points arcing ) during the few seconds of motor start up initation.

Also weak, or shorted run capacitors or also start capacitors if you happen to have it on your system.

1. 2 most common reason,,,, Shorted thermostate wires or improperly installed thermostat.

1. 3 most common reason,,,, Outdoor condenser unit is wired wrong and is bypassing the thermostat control function to shut it off for one reason or another.

Note: Be careful! (inductive loads) such as motors can knock you clean back to last year or worse so ( BE SURE THE MAIN POWER IS OFF COMPLETELY) You may need a good electrician to give you a hand here if you are even remotely baffled by what I've just told you.

Answer 2

If the outside unit is running when the thermostat is turned off and the lines are frozen, it could indicate a problem with the compressor not shutting off properly or a lack of proper airflow causing the unit to work harder than needed. This could lead to the evaporator coil getting too cold and causing condensation to freeze on the lines. It's best to contact a professional HVAC technician to diagnose and fix the issue.