While replacing thermostat, lost power.

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While replacing thermostat, lost power.

Postby pvstansel » Sat Aug 31, 2013 4:24 pm

My thermostat was acting up (old mechanical mercury unit). And I decided to replace the unit. During the installation I noticed that the power to the thermostat went away (verified by a meter). Note that the power existed prior to me attempting to replace thermostat. And now no power after my R&R. Yes, Guilty as charged. So I need to regroup.

I replaced the fuses on the compressor.
I've tried hardwiring AC (RC Y and G)
I reset the AC and FAU breaker, and lastly all the breakers on the panel.
Still no power to the Thermostat.

The Honeywell ST9120C4016 Fan Control board on the FAU is an older board - thus no fuse on it.

My house is dual zone. And the other zone is working fine.
So an option for me would be to swap the Fan Control board from the working unit to the other.
But given my earlier luck, I'm cautious.

Any input on how to figure out why no power to to the thermostat?
pvstansel
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- While replacing thermostat, lost power.

Postby pvstansel » Sun Sep 01, 2013 11:52 am

Never mind. Got it solved.

When wiring the new thermostat, I blew the transformer.
Got a new transformer.
Figured out the wiring on the new thermostat (BTW, the marketing on the thermostat units teases you... installs in 5 minutes But if you are replacing a very old thermostat with a very new thermostat, beware that the wiring is not always straight forward)

Also installed a 5amp fuse coming off the transformer before the control board.

System is back up and running.
pvstansel
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- While replacing thermostat, lost power.

Postby Freon » Tue Sep 03, 2013 3:18 pm

To all reading this story, ALWAYS turn off the power to the air handler or furnace before doing any electrical work. This story had a happy ending... only a blown transformer. However it could have been a control board and that is expensive.
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