I need to know if this is a probabale cause…
I replaced the mercury thermostat on my gas FHA furnace with an elecrtronic timer unit. After I fininshed the furnace wouldn’t start. I tried jumping the wires (R to the W) and no action. Called the hotline for the t-stat and all the connections checked out. The thermostat checks out OK with the ohm meter (r to W connects with the demand for heat) both at the thermostat and the control unit in the furnace. The transformer on the control unit was warm (not hot) and has no voltage out (yes, 120 in).
Could I have shorted out the transformer while changing the thermosat? I’m pretty sure I had the power off but may have flipped the switch during the operation. There was a couple of hours while I ran back to HD to get a two stage t-stat with the wires loose. It just seems weird that it would fail at the same time I was changing the t-stat.
DW isn’t impressed with my HVAC skills tonight…
Freezing in Duluth! Ted
Replies
Yes, I just had a helper pull a thermostat and he didn't kill the power to the furnace. Not sure what two wires touched on the therostat side but he fried the transformer. Of course it was past quittin time on Friday and I had to call in an emergency repair. Cost me $475. Good thing he is a really good helper.
Did you disconnect wires from transformer output before checking? And let it cool down 15 minutes or so? Some xformers have a thermal relay that will cut them out under short conditions. (Unfortunately, others have an internal fuse.) Most are designed to tolerate a momentary short.
happy?
powered down the whole thing for long time (2 hours) and tried again - no luck. I'll pull the transformer and try for a replacement. Are there big differences in transformers?
This one says 24 vac out...
Current capacity is the only other thing, and for an old-fashioned non-electronic furnace the current capacity is standard, so pretty much any furnace transformer should work.Be sure to ohm out the wires with the xformer and tstat disconnected and make sure there's no short between wires. Then connect & power the xformer, with tstat disconnected, and check again for voltage -- if voltage is zero or below 22V or so pull power ASAP. Finally, if you get this far, pull power again and connect tstat. After powering up, check xformer voltage a final time.
If ignorance is bliss why aren't more people
happy?
Thanks for the help. The wires at the thermostat had some 'rash' where the wire was unbundled and I think they got shorted when I put the new t-stat in place. Replaced the transformer after taping all the wires and testing to make sure no shorts. I have hot air and a happy DW. Also discovered the value of the old Heatilator fireplace. It does smoke with the doors open but keeps the house warm. Warm in Duluth, Ted