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Old 05-26-2020, 02:49 PM   #1
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no power to thermostat, quick response greatly appreciated!


Hi all.

I took the thermostat apart to put up a new sheet of drywall, and then reassembled it. I took a video of it to make sure it went back together correctly.
Now there is no power to the thermostat.
No breakers flipped.

There are 9 slots in the thermostat, labeled: E AUX Y G O L R B C.
E and AUX had a black wire shunted across them. AUX had a white wire connected to it. Y had a yellow wire. O and L were empty. R had a red wire, B had a blue wire, and C had a copper wire connected to it.

I have 6 wires here, and not knowing what they all did, I tested all 30 possibilities with my tester and got no response.

I don't think I was rough with the wires in this process, but I did press them in a couple of times to get the thermostat case to close. It shouldn't have been enough to snap a wire of that thickness.

Now that I think about it, I did kind of swirl the wires together to push them through the wall. Something shorted out? Again, no breakers flipped.

What have I done wrong and what do I do now?

PS The urgency is that I have on A/C til I resolve this.
Thanks.

Last edited by wengang1; 05-26-2020 at 02:53 PM.
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Old 05-26-2020, 03:06 PM   #2
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Re: no power to thermostat, quick response greatly appreciated!


You probably blew the control fuse on the circuit board. Look for it in the indoor unit. Take a picture of the board if you can't find the fuse.
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Old 05-26-2020, 03:15 PM   #3
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Re: no power to thermostat, quick response greatly appreciated!


HVAC is one area where I really don't do anything. The air handler is in the attic. Where would I see the circuit board? Thanks.
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Old 05-26-2020, 03:18 PM   #4
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Re: no power to thermostat, quick response greatly appreciated!


Inside the air handler up in the attic. You have to remove the access panel to see it.
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Old 05-26-2020, 07:38 PM   #5
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Re: no power to thermostat, quick response greatly appreciated!


So, that was it. I removed the burned out fuse and took it to Lowes. It was labeled T3.15AL250V, which I take to mean it is a 3A fuse. Lowes only had 5amp fuses in that size (5x20mm), so I got one, understanding the risk of going with a higher fuse.

I installed it, with power to indoor and outdoor units off, and to be safe, cut 3 inches off the cable and stripped new wire, installed all, turned power back on and no display.

It's utterly miserable in the attic right now, so not sure I'll give it another go tonight, but 1) if it is blown again, what could this be (power was off the whole time this round), and if it is not blown again, what would be the next logical thought, damage to the board?
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Old 05-26-2020, 08:18 PM   #6
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Re: no power to thermostat, quick response greatly appreciated!


Make sure the fuse didn't blow again, then check the voltage at the t-stat between the C and R terminals. If you have 24V there your thermostat wiring or your thermostat is bad. If you don't have voltage there your board may be bad or your wiring may be bad.
You can start at the furnace and check the voltage where the wires connect first to see if there is power there.
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Old 05-27-2020, 01:34 AM   #7
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Re: no power to thermostat, quick response greatly appreciated!


Well, I went back up there, and noticed there was another breaker on the box that the access panel was cut out around, and it was off. I guess the first time around I turned that off and forgot I did it. Problem solved. Thanks all.
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