I’m installing a nest thermostat and trying to turn off power to the thermostat before doing so. I’m using a voltage tester and can see the wires coming in have power. I have two systems, one for upstairs and one for the basement. I’ve turned off the furnace breaker for both, and even pulled the large pull out head for the system, and the thermostat still has power. I don’t have any other breakers indicating anything for the HVAC system. I then started flipping off breakers for the surrounding area, and still has power. How/where in the world is this thing getting power from? Am I missing something else somewhere?

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    Have you actually disconnected the old thermostat and are checking voltage on wires that used to be connected? Or are you checking voltage while still connected to the thermostat which could show voltage due to a battery in the old thermostat? What is the voltage measurement? Between which wires? Oct 28 '18 at 23:59
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    "I’m using a voltage tester" - one of those magical non-contact thingies? Yeah I thought so. It's not telling you anything useful. Use a multimeter.
    – brhans
    Oct 29 '18 at 1:30
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    Why do you need to turn off the power to install a thermostat, unscrew old, tape off so they don't touch, reconnect
    – user70085
    Oct 29 '18 at 11:17
  • @brhans Yes it is a non contact voltage tester. Are those no good? For normal wiring it has always seemed to work fine. Why would it indicate power on low voltage wires when there is none?
    – Ryan
    Oct 29 '18 at 21:52
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    They're too easily fooled by 'ghost' voltages. If the wire you're looking at is disconnected at both ends, but runs close to another wire which is live/hot then you can get some capacitive coupling between them which can make your open-ended wire look like it's hot to non-contact testers.
    – brhans
    Oct 29 '18 at 21:55

BRHANS is absolutely right. Use a multi-meter to check for actual "voltage". 24 volts A/C is the minimum voltage you should have. You said you have two systems. By chance do you have a zone control board for any other zones?? sometimes they have their own transformer and a separate control wiring. If you do NOT have a zone control board, get a multi-meter that can do LOW A/C voltage and verify the voltage. Check voltage at "R" to "C". with power ON you should have 24v A/C. If you still have voltage with the system off, you will have to check power at the HVAC equipment. I'm not sure the age of your furnace/ air handler, but some of the older equipment has a separate Transformer that runs the low voltage side only. This may have a separate power source as well. ( not very common) but I have seen this situation in the past. Hope this helps. and good luck to you.

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