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I'm trying to replace an old manual thermostat with a new digital thermostat for my line voltage baseboard heaters. If it's important, this particular thermostat simultaneously controls two separate baseboard heaters on opposite sides of a large room.

The hole in my wall has 6 wires, 3 black and 3 white (and a 7th ground wire, which I am ignoring for the purposes of my question). The wires come in black/white pairs from three corners. I am naming these corners α, β, and γ (alpha, beta, gamma). Thus the six aforementioned wires are αb and αw, βb and βw, and γb and γw.

The old double-pole thermostat seemed to function correctly. The wiring was as follows, where parentheses indicate a single solderless connector:

(L1, αb, γb)

(T1, βb)

w, βw, γw)

L2: unused

T2: unused

I thought it was strange that L2 and T2 were unused. When I wired my new double-pole thermostat I did as follows:

(L1, αb, γb)

(T1, βb)

(L2, αw, γw)

(T2, βw)

However, this new thermostat does not appear to be working. Did I wire it correctly?

I hope my notation is clear but I'm happy to edit if necessary. Thanks in advance for your help; much appreciated!

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It sounds like the a-cable and the y-cable do indeed go to the two controlled heaters. So we know for sure those should stay together.

That would make the B-cable the supply.

Your old thermostat was passively powered. It either connected sides together, or it did not. As such, it didn't matter which side each one connected to, there was no input or output side.

This new switch needs power. It needs power all the time. So it won't do if it severed power to itself when turning off heat.

Instead of trying to wire this thing in one swell foop, you should do it in steps.

Hook up power to the thermostat first. Get the thermostat to power up and pass a diagnostic.

Since we're fairly sure power is the B-cable, figure out which wire sets (L or T) need to connect to that.

Once the 'stat powers up, the rest is straightforward.

  • Thanks Harper, you really helped me better understand my system. To clarify, L1/L2 are "line" and T1/T2 are "load". My understanding is that it is the line that should be connected to the power. Since you think power is the β wires, I guess this means they should be connected to L1/L2 (and therefore I've wired it backwards)? – nimble agar Oct 1 at 17:20
  • @nimbleagar I can't vouch for how they designate the wires; they often are quite arbitrary and UL doesn't care. UL approves the instructions (and Code requires you to follow the instructions). Normally you're right to research first and not guess, but in this particular case of connecting B to L1/L2, the worst thing it'll do is not work. Since it is is what you haven't tried, I'd try it. – Harper - Reinstate Monica Oct 1 at 17:24
  • Also, if I want to just hook up power to the thermostat like you suggested, I'm a little confused how I would do that. Don't I need to connect at least two wires to complete the circuit? – nimble agar Oct 1 at 17:24
  • You need to connect 2 wires, yes. Not 2 cables. Connecting both wires of B will suffice, since there is 240V between them. I can't type the greek letters, I'm on mobile. – Harper - Reinstate Monica Oct 1 at 17:25
  • Ok, thanks for fast replies! I will give these a try now. – nimble agar Oct 1 at 17:26
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Two long for a Comment: It looks right after a couple more sips of coffee. I did not follow your wiring , not enough coffee yet. But I think your old thermostat is only breaking 1 side for each heater, this is a trick we used to do to double the ability of a single thermostat. We would tie the whites together with one of the hots (usually the white hot and Mark them black) we would then use the supply or other hot to both of the line sides of the stat. Next we would put 1 heater on t1 and 1 heater on t2 this way a double pole stat could control 2x it’s listed ampacity (but each would be be below the max) this worked great since most rooms have multiple heaters. Yours are both in parallel on one contact I would have split them up so the stat would last longer. As you did and yes it looks like you did it correctly. Please verify your new stat is listed for use without a neutral. Some can use the ground and some require the neutral, and some only need the 240v but it looks good.

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