My AC unit has the thermostat indoors, a unit in the crawl space and the outdoor unit. Everything was working until I had a contractor come out and add insulation in my crawl space. Now the heat/air will not cut on. I have checked:

1) Fuses in fuse box
2) Outdoor breaker box
3) Thermostat has batteries (working)
4) Unit is set to cool

What else could it be that is preventing anything from cutting on when I turn on my AC from the thermostat? (heat doesn't work either, so it's not limited to AC)

1 Answer 1


The thermostat cable could have been pulled loose or broken at some point. When the thermostat is turned on, it should supply voltage to a relay on the AC unit and that normally makes an audible "click". Checking the voltage on the thermostat wires leading to that relay should show 24vac (alternating current).

If there is no voltage, you can check the wire for breaks along it's path. Because it is low-voltage, it doesn't have the same rules on splices and how its run as the main electrical wiring so it can get damaged more easily. An existing splice could have been pulled loose.

  • Hi - I verified that the thermostat wire running into the side of the “housing” in my crawl space is “hot” (has charge) using non contact voltage tester (the pen type where it’s green for no charge and red for charge) May 28, 2019 at 20:17
  • Well at this point in your troubleshooting, you're really going to have to measure actual voltage to see if each wire is good. Just because one has voltage doesn't mean the others are not broken. I admit that this level of troubleshooting does get a little advanced.
    – JPhi1618
    May 28, 2019 at 20:23
  • Ugh! I was hoping it was going to be something simple, like “they flipped a switch”. Lol. May 28, 2019 at 20:32
  • Well, my A/C does have a switch in the attic that turns it off, so that's possible. Sometimes a switch is installed close to the unit that only controls the low voltage transformer. Since you are sensing some voltage on the thermostat wire, that's probably not the case, but by all means look for switches.
    – JPhi1618
    May 28, 2019 at 20:33
  • Will do. Thanks for the input. If I don’t see a switch I’ll call in the pros. May 28, 2019 at 20:36

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