Ok...I know better...but since i was dealing with low voltage...i figured no harm...WRONG! I should of turned off the breaker when replacing my thermostat...enough said. I replaced my thermostat (simple comfort 3001) with a honeywell RTH6300b because the simple comfort display was jittering off and on, but still working. Anyway, when disconnecting the wires i noticed a small spark. DUH!....After replacing it with the honeywell, no power. I mean the display was there, but it has batteries so that's that. But no fan would come on., I then switched back to the old one (no batteries), and nothing. Not even a display. The wires out of the wall are red to R/RH,white to W,yellow to Y,green to G and blue to B. That's how it was connected to the old one as well...(the letters to the codes.) Connecting to the honeywell I did the same except that the blue wire, I was instructed by the manual to tape off because I have gas heat/ac. Not needed. My question is, blown transformer or is there a fuse that might have blown? Also location of both to check. Air handler i know, in the garage up high (oh boy)...but where abouts?
Thanks, chuck

labman

The first step is to check for 24 volts between the red and blue wire. If it is dead, the transformer may be too unless it is protected by a fuse. The fuse is likely near the transformer, either in line, or on a nearby control board. They are usually in the furnace. Follow the red and blue wires back to it. One little spark shouldn't burn up the transformer. With luck, it is a fuse.

mymechi

Thanks, I'll try and locate to see if it is indeed a fuse (which i hope) and let you know!
chuck

mymechi

That was the ticket! Blown 3amp fuse on control board inside air handler.
Thanks "LABMAN"...you r da man! chuck