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New TSTAT, nothing works now

tommygags Member Posts: 68
Hi all. Changed my TSTAT yesterday and the new one didn't turn on, so I switched back to the old TSTAT and that didn't turn on either. I triple checked and it's wired correctly, but still nothing - TSTAT doesn't turn on. I checked and am getting voltage on the red and white wires. I touch them together and nothing happens. I touch the green and red together to turn on the fan and nothing happens.

I took off the plate to the air handler and I don't see a fuse connected to the transformer, so maybe I shorted the transformer? But if that was the case, wouldn't I not see any voltage on the multimeter when I touched the red and white wire on the TSTAT?

This system uses a c wire and the relay fuses are all good and second zone with a different air handler and TSTAT work. 5 wires total.

Any ideas?

Also, the ecobee came with a device to use if your set up doesn't have a c wire. Could I use this to bypass the transformer, or the transformer gives the air handler power and if that is shorted, nothing will ever work?

Air handler is: heat controller, inc,. 120v. Model AHG36-0A



  • Zman
    Zman Member Posts: 7,433
    Do you have 24-volt power coming out of the transformer? Do you have 120-volt power going into the transformer?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • mattmia2
    mattmia2 Member Posts: 7,156
    So this is electric heat? maybe the new t-stat wasn't set to close w(heat) and g(fan) at the same time and that tripped a manual limit switch or thermal fuse.
    HVACNUT Member Posts: 4,859
    Checking volts across R and W at the thermostat won't show true 24v. Check R to C.
    Is there a circuit board in the air handler? Could be a spade fuse.
  • mattmia2
    mattmia2 Member Posts: 7,156
    Did you check the fuse in that fuseholder that is sort of kludged in in front of the terminal strip with the 2 red wires with quick connects?
  • tommygags
    tommygags Member Posts: 68
    Thank you everyone! I was looking for an old car style fuse and couldn't find one. Thank you mattmia2! The fuse was located in there. Very happy that was the issue. Fuse was blown, new one fixed it right up. Appreciate it!!!
  • mattmia2
    mattmia2 Member Posts: 7,156
    Next time turn the service switch off before you start disconnecting t-stat wiring.
  • rick in Alaska
    rick in Alaska Member Posts: 1,396
    mattmia2 said:

    Next time turn the service switch off before you start disconnecting t-stat wiring.

    Great idea. I went to test a Tekmar rtu the other day with the system on, and one of the wires bounced back and hit the circuit board and smoked it. That was a very expensive repair I had to eat.
    Lesson learned.