lefos123 t1_ix8h5td wrote on November 21, 2022 at 3:42 PM

For us. The entire furnace uses 24V AC to do what it needs to do. There is a converter to go from 120->24V. Looks like a doorbell transformer. If you have one of those, you could short the thermostat 120v line and use the 24v for the thermostat like a normal hvac would work.

But, there’s a reason someone ran the thermostat at 120V. It could be laziness, but for brand new installs that’s not super common.

Thermostats are light switches. They will connect two wires when they want heat. Or disconnect them when they don’t. So long as you have that, you should be good.

Overall, can’t help without a lot more info / wiring diagrams. But it may be easier to call someone to handle the conversion. At the end, you want at least 3 wires. The two for the 24v AC, and a third to send the on/off signal back to the boiler.

WeaknessNegative392 OP t1_ix8i0t6 wrote on November 21, 2022 at 3:48 PM

Sorry maybe should’ve added a bit more detail for context but this isn’t a new install. The entire heating setup is from the late 80s. I just bought the house and wanted to update the thermostat to a programmable one in hopes to cut down on unnecessary heating when no one’s home. But 90% of the stats out there now are 24v. I did find one 120v stat but it said right on it not for hot water baseboard. But I will definitely look into the transformer/converter, thanks for your help!

lefos123 t1_ix8icjr wrote on November 21, 2022 at 3:50 PM

I wish you luck on this one. Not sure what is up though. Our house is New England 1975. Same heating setup. And it’s 24v. We do have a 120v thermostat but that’s for electric baseboard heat in a bonus room.

So when whoever installed this originally. I’m surprised they went 120v

assholetoall t1_ix8p020 wrote on November 21, 2022 at 4:36 PM

Confirming the New England houses I've lived in from the 40s, 80s and 90s all used forced hot water from a boiler with a 24v AC control system which includes the thermostat.

Two did also have electric baseboard that ran on 240v, but it was very separate from the 24v system.

Way2Saucy4U t1_ixbzjx8 wrote on November 22, 2022 at 8:22 AM

Unless I’m misunderstanding, I don’t think this idea will work.

In most programmable thermostats, the “R” terminal on the thermostat is both the source voltage to power the thermostat and is also the same power to each energized output.

Think of the thermostat as a switch to each output. The thermostat makes decisions about when to switch ON the signal voltage from the furnace power source back to the furnace devices/controls which operate the heating functions (ex. relay, contactor, etc…)

You definitely can use a transformer to step down the 120V power but the 24V thermostat outputs will also be 24V back to the furnace. In order to make this idea work, each thermostat output would individually then need to be transformed back to 120V since the current controls internal to the furnace or controls associated with your heating setup are looking for a 120V signal.