What do the old type of furnace thermostats use for power?

The are usually wired in. We replaced our with a programmable just fine. You may need to get more “instruction” than the little book your new one comes with…That’s where the Internet comes in for rescue/help!

One of the wires should be hot. The thermostat doesn’t read the furnace temperature, just the air temperature. So essentially, the wires are power to the furnace, the thermostat is a switch in the line. I have always wondered about whether the digital thermostats need a separate power source as well, but I don’t think they do, I think they should work with the same two wires. Open one up at the store and read the manual before you buy it. I suppose you might get yelled at, so you could buy it, open it, then return it if it’s too much hassle. Man I hate how they package expensive electronics and such so that you can’t actually look at it to see if it will work for you, and the store staff can’t answer the questions….

The same power supply that powers your ancient switch will power a new, digital thermostat without any modification.

Like the others said it is just a switch. It tells the furnace to be on or off. The digital ones I have dealt with actually have a battery inside to power the LCD and electronics. The ones at the theater I worked at were hard wired into our electrical system. But that isn’t really that common unless it is a new installation.

Moral of the story is make sure you get one that has a included battery. They usually take AA ones.

And if you set it up right you can save a ton on your heating bill. We have ours set to turn the temp down at 10PM since the kids will be asleep and then it warms the place up at 5AM. At 9AM it drops back down until 1PM. At 1PM it starts heating again so it is warm when everyone gets back.

Thanks everyone for the GAs. Hopefully this will be one home improvement job that doesn’t turn out to be a fucking nightmare.

@evelyns_pet_zebra ;) You may swear at the tiny instructions (;

…you may have trouble finding the english words on the instruction page…That is why I suggest finding the diagram online once you buy the new one. Then, it will only take maybe 15–30mins to install, and another 30mins to figure out how to work it for programming and daily use.

All very good advice. One thing you need to be aware of, though: most furnaces produced within the last 20 years or so have an integrated circuit board in them (your thermostat wiring is actually powered by a small transformer mounted on the board). Learn from my mistake—DO NOT short-out (touch together) the two thermostat wires while they’re live. Although it’s only 24 volts or less (no risk to your safety), shorting the wires can seriously damage the transformer, and in turn, the circuit board. You’d be looking at somewhere in the neighborhood of $100 for repairs, depending on the model of furnace. I strongly recommend disconnecting the t-stat wiring at the furnace before you install your new ‘stat. You’ll avoid said nightmare.

yeah, the old kind use a mercury bubble that when tipped by the temp sensitive spring would complete the circuit and switch on the power to the furnace or a/c. The new digital ones need a battery to run. The original wires there should work.

The two wires are basically a switch, but I don’t think either one is carrying AC power, so neither would actually be considered “Hot”.

All I know is that I ended up with a new digital thermostat when I had to redo the AC unit (it made it 13 years), and the new thermostat has given us nothing but trouble. I want my old one back!

@filmfann I don’t mean to nitpick, just trying to inform, but basically “hot” means that’s the direction electricity flows from in a circuit. The switch is part of the circuit, just like a light bulb (or the furnace igniter) would be part of a circuit, so one side has to be hot. Whether or not the thermostat draws power from that hot line is another matter, as @johnpowell says, some are hard wired and some take a battery. I think that the transformer that @frostgiant mentions occurs before the thermostat, so for a thermostat to take power from it, it would need to be designed for the same voltage (and probably DC current wait, is that redundant?).

@gussnarp You are probably correct about the circuit flow. I tend to think of HOT as a wire with voltage, looking for ground. This is just a sensor looking for a short.
You are also right about DC current being redundant. My friend Ken likes to talk about his time in the Army, working for the Department of Redundancy Department.

@filmfann Well, you’re right, it is a wire with voltage looking for ground, but the switch in the thermostat is interrupting it, so we’re sort of saying the same thing in different ways.