House A/C went nuts

[1982fxr]

In Phoenix, we don't run heat. Come home from work and AC is going full blast. Thermostat is off and even if it was on it is set at 76. House is down to 60.

Put new batteries in thermostat, still won't shut down. Had to go trip the breaker. It's a 20+ year old roof top unit.

I'm a tenant so I don't have to worry about fixing it but wtf?

 

[Fierljeppen]

It's probably a stuck relay on the control board or thermostat. Either way, 20+ years for an AC unit in Phoenix is impressive.

I'm expecting to shovel a lot of heavy snow in Chicago tomorrow, so I'm a little light on sympathy at the moment.

 

[Mr_fixit]

A mouse chewing on the wires? A squirrels nest ? Hang a picture lately? Any remodeling done lately?

 

[Done That]

Sounds like welded contactor if not the tstat, but not sure why it had a cooling call to engage if off unless rodent s etc as stated above.

 

[LS6 Tommy]

Not a welded contactor. The evaporator fan wouldn't be running, the evaporator would most likely be iced, it would possibly cycle on the low pressure cut out and the house would not be cold.

Take the T-stat off the sub base. If the unit continues to run, it's not the T-stat, Something is shorted. Take the R wire off the T-stat sub base. If the cooling runs normally, there's a short between the R, G & Y wires or (very unlikely) at the control board. If the evaporator fan stops, but the unit continues to run the compressor, there's a short somewhere in the R & Y circuits. Again, it could be the T-stat wires themselves or a problem with the control board.

Tommy

 

[AP514]

+1 for a Control Board short....

 

[acmikee]

do what LS said i suspect bad t-stat or wiring
with the t-stat removed check V between R and Y if 0V stat ok if 24V reinstall stat and go to indooor unit and remove the R wire if still running bad/shorted board if it shuts off bad wiring