This is an ancient furnace at my aging mother's. Basic 24v thermostat, two wires only, the original from my childhood used a mercury switch. There are some commercial, line voltage, 2-wire thermostats on the market that are also just a simple SPST switch. The nice thing about them though is they have a single dial with the intended temperature and the temperatures are printed as large, black on white numbers. In other words, they'd be much easier for her to use. Without having to disassemble one, is there any reason one of them shouldn't be used? It'll be carrying only a fraction of the voltage & current it was designed for & there doesn't seem to be any added or different functionality unless I'm mistaken or uninformed.

  • Hello, and welcome to Home Improvement. It's hard to say for sure, but a line voltage switch doesn't have to worry about contact oxidation, and may fail if run at 24V. And, you should probably take our tour so you'll know how best to participate here. Nov 30, 2019 at 19:44

1 Answer 1


Plain old mechanical thermostats don't care, but...

They are simple switches, whether they are line voltage, millivolt, or 24V. However, obviously, they have different current ratings; I wouldn't lose any sleep over mixing up millivolt and 24V, but those must never carry line voltage.

However, your thermostat with the single dial and the "large, black and white numbers" is not a plain old mechanical thermostat. You are talking about some sort of smart thermostat there. If this thing is battery powered, that's a possibility; but more likely it relies on a "C" wire or on current leakage through a large resistance heater (which would be pretty easy to do for a line voltage 'stat). These won't work on the wrong voltage.

Why are you bothering? There's a huge variety of 24V thermostats

I gather you spotted one you like, but if you do some research I bet you can turn up a comparable one that runs on 24V. Probably ten of them. Some that use batteries, some that use C-wire power, and others that allow leakage power through a clunky old 24V solenoid like you'll have on an old furnace.

As far as price, line voltage doesn't buy you anything. There are very inexpensive thermostats available for all types.

  • 1
    Thank you for your reply. My apologies. The one years ago was mercury; the current thermostat on the same furnace is a bi-metal coil with a simple switch, like ignition points. The line voltage ones I've seen that would work for her low vision & dexterity appear to also be bimetalic coils with a simple switch. I've found no 24v that are similar. The furnace control is ancient & exceedingly simple. No circuit board, just a transformer, thermostat, motor relay & gas valve, all at 24v and all it would seem connected in series. Nov 30, 2019 at 22:14

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