I just purchased a home, and was in the process of replacing the thermostats with Nests. I flipped off all the circuits in the fuse box, and the thermostat on the first floor powered off. However, the thermostat on the second floor still had power.

After talking to the builder, the electrician is claiming because it is low voltage, it does not need to go through the fuse box.

Anyone know if that is actually the case? And if so, how should I install the nest if the wires remain live with no way of turning them off?


  • What make and model are the iar handlers/furnaces and the old thermostats in question? Can you post photos of your breaker/fuse boxes? – ThreePhaseEel Jan 10 '20 at 3:30
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    My thermostat is battery-powered (no mains connection at all, not even through a transformer), so I don't understand this problem. The only wire it has goes to the heating unit. – Mast Jan 10 '20 at 10:34
  • We do need more info, millivolt stats usually do have a battery and do not connect to the house wiring, i put this in my answer. Without more info it is all guess and your electrician may be 100% right, since the op turned off all the circuit breakers it would not matter if it was a rule of 6 panel, but a second panel fed from the main’s I have seen that more than once. – Ed Beal Jan 11 '20 at 16:17

There are two very different issues here:

Power to the Thermostat

I flipped off all the circuits in the fuse box, and the thermostat on the first floor powered off. However, the thermostat on the second floor still had power.

A thermostat is typically powered primarily by a transformer. However, many thermostats include battery backup. There are a variety of reasons, among them that unless a thermostat is wired up a particular way (see all the messages about Nest and C wire), it may only have power when the furnace or air conditioner is running. That is fine for an old mercury dial, but doesn't work for modern thermostats. Therefore, batteries. However, even with battery backup, a thermostat designed to always have power from the transformer will appear "off" when power goes out, using the battery just to save settings, clock, etc. A thermostat that is not designed to always have power will continue just fine when the power goes out - and even if pulled off the wall, using the battery to power everything.

Low Voltage Wiring/Devices

the electrician is claiming because it is low voltage, it does not need to go through the fuse box.

The electrician is correct about this, in a way. The great thing about most thermostats (there are some line-powered thermostats which are exceptions) is that they use low voltage and are therefore very safe to work with. While they indirectly go through the main electrical panel (by way of the transformer) the wiring actually present at the thermostat is considered separate from the electrical wiring at the main panel (the other side of the thermostat). That means it is reasonably safe to mess around with that wiring without the same level of precautions needed for 120V or 240V AC. The same is true for telephone and network wiring.

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    Network wiring yes, but telephone wiring has an occasional habit of carrying 75 V AC to ring the bell, which while not fatal, its above the magic number of 50V, and can be a surprise. – Criggie Jan 10 '20 at 19:32
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    A surprise yes. But not a big deal. Been there. – manassehkatz-Moving 2 Codidact Jan 10 '20 at 19:52
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    @Criggie It's 75VAC open, maybe, but once there's any load drawing current from the telephone line that value will drop considerably. In any case, furnaces almost universally use 24VAC for the control system, and that is also what powers the the thermostat, so definitely it's a safe voltage. – J... Jan 11 '20 at 18:20

Thermostats get power from a low-voltage transformer that is either inside your air handler or very close to it. That same transformer also powers the circuit boards and controls of the air handler. That transformer, along with every other electrical device in the house will be connected to a breaker.

If you turned off all the breakers and one thermostat was still on, it was either being powered by a battery, or there is a second breaker panel you forgot about that powers the transformer for the upstairs air handler.

The power should be cut off before installing a transformer in case you touch wires together that shouldn't be touched, but they operate at low voltage and it will not shock or hurt you. You could damage the AC system if the wrong wires touch, but its not "dangerous" for you.

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    ... or it's a line voltage t-stat and you're about to have a bad day. – Mazura Jan 10 '20 at 3:18

This is probably a matter of not knowing where your service panels are. Plural!

Find the meter and follow

You need to "follow it from the meter" as it were. Find the meter; easy. Then you'll have one of three things:

  • Additional compartments in the meter cabinet that open up (do not break any seals).
  • Very obvious conduit to another equipment box nearby
  • Invisibly, conduit going straight back through the wall behind the meter. On the other side of that wall must be a service panel, possibly up or down a floor.

The cable between meter and main breaker must be very short; so it can't be more than 6' away or so. Wherever you find your main breaker, you might also find some additional breakers. Typically one of those feeds a sub-panel; I'm guessing you initially found the subpanel and are unaware of this panel.

Beware the "Rule of Six" panel aka Split-Bus panel.

There is a type of service panel out there that has no main breaker. Instead, it has 8-12 breaker spaces in a special section called "Main Breakers" -- and a bunch of other breakers. All the breakers in the "Main Breakers" section are your main breakers. The idea was to save money by not having a 100A+ master main breaker (back when those cost hundreds of dollars), and instead just have up to six breakers that feed large appliances and subpanels.

Typically, these panels have one - e.g. upper left - marked "Main Lighting". This is the breaker that feeds the *other section of the panel", called the "lighting section" but intended for normal branch circuit loads. So if you searched a Rule of Six panel for "the one main breaker", you probably shut off this breaker (but left the other "5" on).

Rule of Six panels tend to get screwed up over the years. The 12 spaces are supposed to be no more than six 2-pole breakers. But Rule of Six panels generally have few spaces by modern standards, and so people run out of space in the lighting section. Then, they start replacing the Main section breakers with single-poles or double-stuff breakers, and you can find yourself with 10 or 15 "main breakers".

Generally, Rule of Six panels should be deprecated - but that's not as scary as it sounds. You simply install a subpanel right next to it. 100/125/150A breakers are now cheap, so get the one that matches your service, and have it feed the subpanel. Then move each of the circuits in the "main breaker" area to the subpanel. Now there is only 1 main breaker. That was easy! (Make the subpanel huge, so no one ever has a need to stick breakers in the "main breaker" section).

If nothing turns it off

It is absolutely possible to "hot-wire" a circuit directly off the main lugs so it is unprotected by a breaker. No electrician would do this; no, this incompetence is done by somebody who doesn't do electrical - like a furnace guy. If that's the case, that needs to be fixed post-haste.

Why would this happen? Well, you have 2 furnaces, right? I bet one was added later. Furnaces require dedicated circuits. The furnace guy was probably bright enough to know that, and normally just adds another breaker. But the panel was totally full, and furnace guy had to finish the job to get paid and not get fired.


The transformer(s) that power hvac systems have a primary winding that is 120v in most U.S. cases (some are 240). These are controlled from the service panel or a sub panel. The secondary side of the transformer is normally 24vac I have seen 6v to 32v systems all considered low voltage on the secondary the transformer primary is powered in most systems If it has a transformer (millivolt systems are an exception). So it is possible that that thermostat is running on batteries and doesn’t use power for a millivolt system if your nest won’t work it may be millivolt.

So we can’t say for sure but I would guess high 90% do require a transformer but not all thermostats turn off when the power goes out.


I just installed a nest on an oldish furnace.

If you read the documentation from Nest its says "it draws power from the leads leading to the furnace by briefly turning the furnace ON and OFF faster than the furnace is capable of registering. .... In most cases.

However, in my case I had to run additional wires so I could hook it to a 24v transformer because my furnace didn't support there mode of operation.

In addition, the nest has batteries that last more than a day so turning OFF the power won't change that. I want to say the batteries last a month, but I don't remember that part of the documentation that well.

If your furnace supports the default mode of operation then the thermostat gets power from the furnace. If not it is on whatever circuit the transformer is on.

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