Looking for some advice on how to troubleshoot my system problem.

Problem first appeared a few days ago when the screen on my programable thermostat went blank. Main power was good. No power in thermostat.

After checking the voltages in the air handler, I discovered that my transformer was open. I replaced the original 240-24 40 VA model with a 240-24 55 VA model with a reset switch.

Once I reset the breaker, the indoor fan ran for a few seconds and then shut down. A few seconds later, the reset switch on the transformer was tripped. I confirmed my connections, reset the switch and tried again; same results.

Returning to the thermostat, I switched it to the off position and reset the switch and tried again; OK. Switched indoor fan to ON position, OK. Switched to AC, fan quits after few seconds, then trips reset in transformer. Same results with heat. As soon as the call for the outside unit is made, something overloads the transformer and kicks it off.

I checked the outside unit and found that when called, the unit is not activated. I only noticed a slight click down near the entrance to the condensing coils. I manually engaged the contactor and found that the system appeared to run normally as long as I held the contactor in. Unfortunately, the system doesn't run long enough to verify whether I have 24 V on the contactor switch. I have looked around at the wiring and have not noticed any damage to the wires.

Any ideas?

Thanking you in advance,

Mark

PS; My wife would like some heat.

 

Mark,

Did you test your emergency heat? If it works then the thermostat wire to the HP is probably shorted. Trace the t-stat wire looking for the wire being cut or pinched.

 

Your condenser contactor may have a shorted coil, or a bad tstat wire to the outdoor unit like sgthvac said.

 

I just tested the Emergency Heat and it works fine.

The system has been in place a year and a half. There's been no disturbance to the air handler or outdoor unit.

Any ideas on which wire may be the culprit?

Thanks,

Mark

 

Any chance that the T stat is bad?

 

It's hard to say, if you have a meter that checks continuity, disconnect the t-stat wires at the outdoor unit and the indoor unit. Leave the t-stat wires to the t-stat connected to the indoor unit. Check continuity across the wires when you have them isolated. You should not have any continuity across any of the wires if you have them disconected on both ends. If do not have any readings, replace the contactor in the outdoor unit as HVACGuy stated.

Make sure you turn the power off before taking any wires off. I don't want to insult you, but I try not to assume anything.

 

Might start inside at the tstat pull all wires off it and try out each one to R and see if it all works as should.

 

Update:

Tried to isolate each of the wires; all seem fine.
Pulled all wires off of the contactor and used a live 24v circuit to check function. It appears to be shorted. There is a light sparking and it does not activate. I hope that I've found my problem. I'll pick up a new contactor tomorrow and give it a try.

Thanks alot for your advice.

Mark

 

Replaced contactor and all is well.

Thanks to all that gave advice,

Mark