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We have a thermostat that we need to replace. Our system is I think pretty typical, the thermostat I think controls the furnace and fan, and I suppose the furnace relays controls for the air conditioner. The wiring of the thermostat just has one R wire (Rh) which has a jumper connecting it to Rc. (I'm no expert, but I think this is the situation.) Thermostats are battery powered, I don't see any black wires or C wires.

The instruction manual says to turn off the breaker for the furnace before replacing the thermostat. But it's more convenient for me to simply unplug the furnace. (Or, as the plug goes into a remote-operated switch, I can easily turn the furnace/fan system off or on with a remote.)

My question is, can I just do that, unplug/switch off the power to the furnace, or is there some other wiring powering the thermostat wires that I would need to go to the fuse box and turn it off from there?

thermostat furnace

Edit: I unplugged the furnace and installed the new thermostat and it worked out fine.

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    Usually unplugging the furnace is all you need to do most thermostats are 24v or less on the ones I have worked on. A non contact voltage tester is an inexpensive tool every home owner should have if it shows the circuit dead there is no 120vac so no real shock hazard.
    – Ed Beal
    Apr 24, 2017 at 21:55

3 Answers 3

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So long as whatever you do de-energizes the wiring you're working on, you should be okay.

There's really just two concerns:

Shock hazard

You obviously don't want to get shocked/electrocuted, and only working on wires that are not energized is a good way to ensure this. This is less of a concern on the thermostat end, where you're only dealing with 24VAC, but there is 120V on the furnace control board.

Shorting out equipment

The other concern is when dealing with the wiring you can easily cause connections between stripped ends, which could do anything from simply turning the fan/furnace/AC on, to rapidly cycling equipment (which could damage it), to actually shorting something out -- such as the transformer itself -- and causing it to release the magic smoke, which renders it useless.

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  • does it sound like from what I described, if I were to unplug the furnace, it and the thermostat wires would be De-energized?
    – A L
    Apr 24, 2017 at 22:25
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    @AL What is your 'furnace' in this case? It doesn't sound typical, if it has a plug, as permanently installed furnaces are typically hard-wired. So long as all power is disconnected from the unit, it should be de-energized, but without seeing it I can't say for sure. You could use a multimeter to verify no voltage between any wires (especially C and R/Rh).
    – gregmac
    Apr 24, 2017 at 22:29
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    I agree with this answer but am curious about the plug thing. But yes, as this answer says, as long as the furnace is de-energized you'll be fine. The thermostat will still look like it is energized until you take out the batteries. And btw, thermostat wires are like doorbell wires. If everything is in good shape there shouldn't be more than 12 volts on them.
    – SDsolar
    Apr 25, 2017 at 1:42
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    @AL The only thing I see there is what looks like a transformer. That's definitely not power for your actual furnace -- the fan draws way more current than that transformer can provide. It might be power for the control board (thermostat circuit) or to run an air conditioner or humidifier, but the furnace itself should have its own breaker, and that's what you should turn off.
    – gregmac
    Apr 25, 2017 at 13:32
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    @SDsolar Most, if not all, modern HVAC circuits are nominally 24VAC (may actually be +/- a couple volts).
    – gregmac
    Apr 25, 2017 at 13:36
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1: You say Unplug the Furnace - that furnace looks like it has FIXED connections back to the breaker and so I do not see you unplugging the furnace from a 240V receptacle. So I am questioning exactly what you mean by Un-Plug the Furnace.

The Circuit Breaker is most likely in the breaker panel - sometimes they are with the Furnace / AC . Easy enough to turn off. The reason you do this is two fold.

A: Because there is a 24VAC power line coming from the furnace to the Thermostat to control the Furnace/AC .

B: You do not want the system to do whacky things as you are disconnecting the old and installing the new Thermostat . What I mean by whackey things is that it could possibly cycle things on and off or reverse the unit when it should not (i.e heatpumps and their controls) .

The smart thing is to turn the breaker off it should be around 60 to 100 amps and a dual ganged breaker (2 together) - although it might have 1 flip pole (probably will have 2 flip poles if it is older) - it will take up two spaces in the panel.

By doing this you will also protect that brand new thermostat of yours..

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Yes, unplugging the furnace will deenergize it. If the furnace has power and the red (power) and blue (common) wires touch it will blow the small fuse on the mother board. As for the plug on the furnace, I have seen furnaces wired this way. It's in-lieu of wiring a disconnect or switch for the unit.

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