Baseboard heater wiring q
 

Alright so long story short my place used to have a single baseboard heater in the basement but it was removed (or never fully installed?) well before I bought the place. There was a thermostat for it on the wall though. We have a freezer down there now and I'm hoping to repurpose that circuit so the freezer can have a dedicated breaker like I assume it's supposed to.

The problem is the wiring isn't making sense to me. The panel has two breakers marked for this heater which I believe means it's actually wired for 240V? Since that's how all the other big 240V stuff like the A/C, oven & dryer are set up. With the cover off I can see that one breaker goes to a black wire and one to a white wire.

Downstairs is where I think it gets weird. The box for the thermostat has one cable coming in with the usual one black, one white, one ground. Coming out of the wall where I can only assume the heater was is one cable with the usual one black, one white, one ground.

How can that have worked if there's only one cable in the thermostat box?

Seems like someone put a white wire into the hot, without taping it red / blue or such at the panel.

You said the thermostat has three wires in the third paragraph, then you said it only has one at the end of your post, which one is it?
Could invest in a crappy tire 20$ meter, and find out if you actually have 240 at the wall

Yeah sorry it's 3 wires in 1 cable.

I was able to chase the wire from the thermostat through the utility room and found a junction box in the ceiling behind the furnace that appears to have the fuckery needed to make this work. So I can change that to have the power just go to where the outlet will be.

The question is now whether it's 120 or 240. I have a DMM so I guess I could just switch the breakers back on and measure it.

I got the meter out and it's 240V across the breakers which I expected, so with the junction box rewired it'd be 240V at the wall. So I just need to determine what I need to do to change it to 120V at the panel. From that I understand I should just need to remove the white wire from the second 15A breaker and connect it to the bus bar with all the other neutrals and I'm good to go. But I'd like to confirm that before doing anything.