You need a transistor like doorbells to take in 120v down to 24v

Transformer*

Is that something I can put in myself? Or do I need to call in a professional. And would it get wired at the thermostat end or on the furnace end and have to run new wires the whole length of the run?

Kind of an odd configuration. Are you sure you have 120VAC at the thermostat?

Your oil boiler should have a controller on the front (called an aquastat).

Can you post a photo of the boiler? And the thermostat wiring.

thanks.. must have spelled it wrong on my phone and it guessed that

I dm’d you a picture of the wiring. Can’t figure out how to add it to the post 🙄

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.

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!

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

Reading your updates, my first question is why do you think it is 120v and not 24v?

I have had forced hot water in three houses (built in the 40s, 80s & 90s) and all three used 24v for their original boiler (though the one from the 40s was converted from steam to hot water at some point using the same boiler).

If you have access to a multi-meter, that will give you a definitive answer, just make sure it is in AC mode (not DC).

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.

It's a fairly simple thing to do, if you're comfortable working with electricity and have basic tools (like a multimeter).

I would install it at the boiler and use the existing wires, making sure to label them as 24V on the stat end.

It's hard to say without you providing additional details (i.e. how you figured out it's 110V and not anything else).

But in general, thermostats are wired to receive power from the furnace or ac. If your furnace has 24V and you have extra wires in the wall - you can just buy a new thermostat and connect it to the new power wires. If you don't have extra wires, you may be able to use a kit that allows you to send multiple control signals over the same wire (you may need to add a c wire, or something else - depending on your current wiring). Worst case scenario - you can always buy a 24V adapter for the wall plug and power thermostat from it.

I bet I know what you have. I had the same thing. You have a line voltage thermostat switching on a taco circulator. Don't you?

The best thing you can do to save money is rip that ancient furnace out and replace with a new unit. It will save you a lot more money than a smart thermostat, especially given the price of oil.

However, it can be converted to a 24v by connecting the circulator to a taco zone controller and then using any thermostat you want to switch the input on the controller. You will need a pro to do it though as the aquastat is usually interfaced with the zone controller. I don't exactly remember why though.

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

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

Using a transformer to step down the 120V signal voltage will power the 24V thermostat but then the energized outputs will also be 24V back to the furnace. Each thermostat output would individually then need to be transformed back to 120V since the current controls internal to the furnace are looking for a 120V signal.

Also, some thermostats use triacs instead of relays for energizing the outputs. You could potentially damage a thermostat if you put 120V on a 24V rated triac circuit.

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.