Single pole thermostat wiring question

Been changing out old single pole thermostats with no problem until I got to this one. The old thermostat is labeled "Load-black" "Line-red". The black load on the old thermostat was wired together with the two in the left of the picture, for a total of three wires on one nut. The red "line" was wired to the black wire on the right. (For reference, the picture uploaded sideways. To the left is "up" and right is "down")

The cadet heater is below this thermostat. The two wires already twisted together originate from the top of the box, whereas the single wire on the right is from the bottom. I thought it was safe to assume that the wires coming in from the top were the hot "line" and the one going down would be load to the heater. But the way the old thermostat was wired the black load went with the two wires that go up and the red line went with the single wire going down.

I thought I'd be smart and use the voltmeter with a probe on the ground wire in the box and the other on the wires in turn. Both gave me a reading around 120-140. Shouldn't only the true "line" be hot??

Edit: I just pulled off another one from another part of the house. In this one the red is going to the two wires coming from the top of the box and the black is going to the single that heads down. Could the first one I posted about be wired backwards? Is it even possible to have one of these thermostats wired backwards??

You have another tstat on same circuit, turn it off and check again, or check voltage between the two if you get 240, just hook them up.

nealtw said:

You have another tstat on same circuit, turn it off and check again, or check voltage between the two if you get 240, just hook them up.

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You're saying it doesn't matter which way I wire it? I have a total of six thermostats across three different breakers.

Eisenfaust8 said:

You're saying it doesn't matter which way I wire it? I have a total of six thermostats across three different breakers.

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If the breakers are 240 breakers (double wide) then they are 240 volt heaters and both wires are hot.

If that is the case I prefer 2 pole but a single will work.

nealtw said:

If the breakers are 240 breakers (double wide) then they are 240 volt heaters and both wires are hot.

If that is the case I prefer 2 pole but a single will work.

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I tested the wires as you said and the read is 240 between the two.

Do you mind explaining the concept of these? I assumed one wire was hot into the thermostat and when the temperature triggered it the circuit would complete then charging the line down to the heater. Thanks

The white wires are also hot from the other side of the breaker and are connected thru the heater and comes back from the heater on the other black and meets at the switch.
The two wires twisted together are the line and the continued line to the next t stat. The single wire goes to your heater.

The problem with a single pole is when the t stat is off you still have the white wire feeding 120 v to the heater.

With a double pole you hook up the white and the blacks to the switch and off is off.

nealtw said:

The white wires are also hot from the other side of the breaker and are connected thru the heater and comes back from the heater on the other black and meets at the switch.
The two wires twisted together are the line and the continued line to the next t stat. The single wire goes to your heater.

The problem with a single pole is when the t stat is off you still have the white wire feeding 120 v to the heater.

With a double pole you hook up the white and the blacks to the switch and off is off.

Click to expand...

Thanks for the speedy and helpful response as always.

The old thermostat is labeled "Load-black" "Line-red". The black load on the old thermostat was wired together with the two in the left of the picture, for a total of three wires on one nut. The red "line" was wired to the black wire on the right.

Click to expand...

If the old therm was wired so that the red wire (line) was wired to the one black (hot line in) wire on the right then the two black wires are the load indicating you are feeding more than one heater with that thermostat. This concept may only apply to this therm depending on how the rest of the heaters and therms are layed out (wired).

This could be the dead end of a circuit for heaters. In this case one of the Romexs is the line in (both black and white are hot total 240v), then the two (twisted) blacks go to two different heaters.

In another scenario it could be that the two sets of Romexs (blacks together) could be the line (hot-one of the two blacks are the line then daisy chaining to another therm somewhere else) feeding the thermostat in turn feeding the one heater from the box. But then there could also be another heater just after that one attached to the internal wires of the first heater coming from the therm.

It is important to ascertain at each box which is the hot (line in) before connecting the therms and not assume that everything was set up the same way.