I tried to switch to a new smart thermostat in my home. When I was adding it, something happened. The new thermostat didn't show any power (And yes, the C wire was plugged in). Not knowing what else to do, I tried to back out and put in the old system, but I'm still having the same issue of nothing seeming to work. To try and diagnose things further, I turned off the power via the fuse box, and measured the resistance between each of the 5 sets of wires. I noticed that the R, C, and Y all appear to be shorted to each other, while the G and W wires appear to not be connected to anything. I know that there is AC power magic, so a DC short doesn't necessarily mean anything, but I am somewhat worried that the C and R appear to be shorted to each other.

Also of some note is I can't detect any AC voltage between C and R, even when everything is powered on

Bottom line, does this mean I broke something, and if so, what do I need to replace? My suspicion is that I blew out a transformer that will need to be replaced, but I would like to confirm this if possible. Thanks!

  • Do you have ~24VAC between R and C? If not, check for voltage at the secondary terminals of the transformer (will require poking around in a live HVAC system). – Tester101 Jan 27 '18 at 12:35

Open the furnace so you see the control panel.

Manually depress the kill-switch that powers down the system when the panel you took off is absent.

Meter across the secondary terminals of the transformer. 24V? You're problem isn't the transformer.

No voltage? Meter across the primary terminals. Got voltage? You need another transformer.

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  • No voltage across the 24V line. I was almost certain the main line was working. Replaced the transformer, and it worked. Thanks! – PearsonArtPhoto Jan 27 '18 at 22:33
  1. Make sure your all your furnace panels have been replaced, there are safely switches that prevent the furnace from operating with the panels off.

  2. Check your furnace control board to see if it has a blown user-replaceable fuse.

  3. Take a picture of, then disconnect all the thermostat wires from the furnace control board. Connect R to G and see if the fan turns on.

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  • Panels were replaced, no fuse I can find, and fan didn't turn on. – PearsonArtPhoto Jan 26 '18 at 23:55

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