I'm a new homeowner and I'm trying to improve the heating in my new place!

The heating is done with baseboards and it is currently using th104plus thermostats. My goal is to replace it with Mysa!

In the junction box, I see 4 types of wires.

  • One bare copper wire is connected to the metal box too which I believe is the ground wire.
  • Two black wires (which I believe are line and load?) are currently connected to the th104plus.
  • Some red wires are just in the gang box, connected together.

What is interesting is after I took out th104plus, my voltage detector reads high AC voltage on both black wires as well as the red wires. Is this normal in thermostat gang boxes? I tried two different voltage detectors and got the same results.

How do I know which of those black wires is load and which is line? Do I even have a neutral wire that Mysa needs? I do have neutral wires in the light switches and they are colored white.

I live in Canada btw.

Update: I installed Mysa successfully. The instructions on the app were very very clear. I was just wondering why all the wires carry voltage.

image of the gang box wires

  • Does this thermostat control one baseboard heater or multiples? Is this the only baseboard thermostat in your house? Dec 3, 2021 at 18:10
  • Only one baseboard. I have baseboard heaters and thermostats all around the house. But I also have an AC that can reverse cycle to warm the place up. But I think those are completely separated and controlled differently.
    – Aᴍɪʀ
    Dec 3, 2021 at 18:12

4 Answers 4


The red and black both carry live, 120V each, but opposing phases totalling 240V. The thermostat you have switches one of the two, thus switching the baseboard on/off by closing/opening the circuit. Red/Black is the common colour code for this.

enter image description here

Based on the wiring in your picture I'd say the above single pole wiring was how you had it.

This is different from a 120V switched circuit, where you switch the live and the neutral is permanently connected.

Notably, with the thermostat off you still have live power at the baseboard from one of the lives so be careful. With the 240V circuit off you might think there is no power.

To cut off power entirely, on both phases (both lives) you need a dual-pole switch and wire it like this:

enter image description here

You need a high voltage replacement. It can be battery-fed but must have a high voltage and high current relay in it. The product description will tell you.

Images from https://getmysa.com/blogs/thermostat-talk/single-pole-vs-double-pole-whats-the-difference


There are two very different types of thermostats:

  • Low Voltage

These use low voltage, typically 24 volts from a transformer. Generally they use 18 AWG wire, though thicker wire is perfectly fine. Minimum 2 wires, but often 5 or more wires depending on heat, air conditioning, multi-stage heat pumps, etc.

A low voltage thermostat essentially controls signals going into a control board or power to a relay, but it doesn't control any 120V or 240V power directly.

While always best practice to turn off all power before working on low voltage thermostats, it is generally quite safe to work on them even when the power is on.

  • Line Voltage

These use 120V or 240V - the same power used to operate the heaters, which are generally baseboard heaters like you have.

A line voltage thermostat turns the power to the heater on/off directly.

You probably had low voltage thermostats in your previous location, but you have line voltage thermostats now.

You must turn off power to the circuit when working on line voltage thermostats.

Any replacement must be rated for both the voltage (likely 240V) and current used by your heaters. Your heater should have the current (Amps) and/or power (Watts) listed somewhere. If not, you can play it safe based on breaker ratings. For example, a 20A breaker should have heaters no larger than 16A and 3,840 Watts.

Another way of thinking about this:

A line voltage thermostat is like a typical light switch. It can be simple (a traditional bimetal thermostat) or it can be complex ("smart") but it gets power (if it needs any of its own) from the same source that it controls power to (the heater).

A low voltage thermostat is like a wired remote control (yes, that used to be a thing). It doesn't directly control the power coming into the HVAC (or TV or whatever in the case of a remote control). It just provides some low voltage signals which in turn tell some other parts to control the main power.

A modern/smart line voltage thermostat will actually use very low voltage internally to do all the "smart" stuff. But a low voltage thermostat does that too - it uses low voltage (probably 12V or less, definitely less than the 24V transformer provides) and DC rather than AC to run its smart stuff too.

  • 2
    The linked manual does show that the thermostat expects a supply voltage of 240v.
    – JPhi1618
    Dec 3, 2021 at 17:12
  • Thank you for taking the time! I did turn off the power before touching anything. Also, this is the first time I'm trying to install smart thermostats. What I don't understand is why all wires have high voltage?
    – Aᴍɪʀ
    Dec 3, 2021 at 17:22
  • 2
    Because they are like a light switch. I'll add more. Dec 3, 2021 at 17:32

Understanding the wires.

Wires are grouped into cables. You have 3 cables with 1 black-red pair each.

You have one heater coming off this thermostat. Obviously only one cable can go there. This is the "Load" side.

You have multiple thermostats all over the house. They are sharing the circuit (breaker), so "always-on" power must daisy-chain from this box to feed other thermostats. That explains the other two cables - one is coming from the circuit breaker and the other is carrying power onward to the next thermostat. These are supply or "line" wires.

(And a ground, that we don't need to talk about; they go to the metal junction box, exactly as the Electrical Code requires; and the switch is grounded via its mounting screws.)

Why are all the wires hot?

That has to do with the weird way the US distributes electrical power - and it's because of Thomas Edison's obsession with a DC electrical system, but that's another story.

Our system uses 120 volts -- twice. Most houses get two opposite phases of 120V. So we have neutral in the middle at 0V, and one phase at +120V and the other phase at -120V (this being AC power, it reverses constantly.

The upshot is by using both phases (no need for neutral), we can get 240V.


  • Thank you so much! This is very helpful!
    – Aᴍɪʀ
    Dec 4, 2021 at 4:35

The old stat opens only one leg to stop electron flow.

The when tested the reds are one hot leg coming in and one going to the element. When the stat is open (off) and the blacks are tested one black is hot, on the other black the tester is seeing the pressure of the electrons being pushed through the heating element from the red wire.

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