My unit runs fine for a while and then suddenly stops.
When it runs, I can measure 25VAC at the thermostat.
When it stops, I have no 25VAC so I can not even run the fan in manual when it stops. After sitting for several hours, the system restarts and runs fine.

I have checked the main electrical breakers, but it's fine.
Also, when it runs, I added more freon to make sure that's not the problem.
Air flow in the home is fine when it runs too.

Seems like something with the control system. Any suggestions?
I'm using a simple thermostat (mercury switches).
If the compressor was overheating, would it turn off the 25VAC transformer power to the thermostat? Is the 25VAC the root cause or is something else causing the 25VAC to be turned off? What's most likely and what action is best to take?

Thanks,
Dave in SC

Update: A friend looked at the unit and suggested that we clean the outside coils as they appeared to be dirty. He had some coil cleaning solution (seems to be NaOH - caustic soda from what I can tell). We sprayed it on there and let it sit for 5+min then rinsed with water. Repeated the cleaning and it seems to be much better. Upon turning the unit back on (we turned off the power before cleaning), I could feel an improvement in the air flow along the coils (placing hand on the coils). It ran all night without turning off.

We looked at the cooling coils close to the blower unit . . . they were not freezing up and the water was running out of the case. Seems like that part is OK.

tkrussell

The control transformer should be on all the time. it may be defective and stop delivering voltage after it heats up. Or there can be a loose or broken wire, loose connection anywhere in the control wiring system.

dave.croucher

make sure your furnace filter is clean.i recomend the cheap ones as they let lots of airflow through.also most units have safety switches that will interupt the 24v cicuit if the pressure gets too high or too low.cause:dirty coils,dirty filter,low on refridge.

dodgeramit

New Update: The wiring terminal located inside the blower unit was very rusty. We eliminated the terminal by directly connecting the wires using wire nuts (following the same wiring connections that the wiring terminal was providing). Since this change, there have been no more problems. 24V is being provided all the time to the thermostat (I'd also checked those connections and all the screws are tight and the contacts were cleaned). So the root problem seems to be bad connections at the wiring terminal due to rust.