I have a smart thermostat (Mysa) I'm trying to install.

I have 2 hots coming into the gang box (black and red). These are connected to another black and red that goes into another box with another thermostat, and then i have a black and red going to the baseboard heater.

When I connect the Mysa thermostat (hot black to line in, load to black on the baseboard heater, and L2 to the red (which is also hot?) The baseboard heater remains on, like the mysa is not controlling it. If I switch the Line in on the mysa to the red hot then it doesn't turn the heater on.

Can i not hook this up with 12/2 wire? (I dont understand why both wires coming into the gang box are hot, where is the neutral?)

Thank you enter image description here

  • Just to be clear; the black and red coming into the box on the top left are both hot, the black and red on the top right are to the baseboard heater, and the black and red in the bottom right are to the gang box on the other side of the wall - there there is a another thermostat controlling a different baseboard heater. – sci-guy Feb 20 '20 at 21:54
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    That information should be in your post, not in a comment. – isherwood Feb 20 '20 at 22:15
  • I did not see the comment when I wrote the answer. The hot and the cable going to the stat in the next room both are tied together and connected to the line terminal. Now it depends if you have 2 or 4 terminals. – Ed Beal Feb 20 '20 at 22:23

It turns out the thermostat itself was bad and was creating a short between L1 and load, so whenever we turned on the breaker the heater was on at max. Verified this when switching around units. Will file a warranty claim.

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    Um, it's supposed to short L1 and Load. Just it should stop doing that after the room warms up... As for "max", most electric heaters only have 1 speed. – Harper - Reinstate Monica Feb 21 '20 at 13:59

Please turn the breaker off unless verifying the hot wires. Both are hot because this is 240v I found several Mysa thermostats so you will need to provide the model #. Here is how 240v baseboard heaters work (there are 2 possible wiring combinations.

240v hot on black to red 1 switch contact set or a single pole single throw switch: the hot wire usually the red is switched you do this by identifying the hot wire and put this on the L or L1 terminal or wire the other 2 going to 2 heaters? Are on the other contact. If only 1 heater and this feeds another thermostat you need to figure out witch goes to the heater and the one going to the next thermostat, the one that goes to the next thermostat goes with the hot on the L or line terminal, the one that goes to the base board may be a T or output terminal (if only 2 screws the one not used) . All 3 blacks tied together This will control the power to the baseboard if the thermostat doesn’t require a neutral. Standard single mechanical switches work this way. The negative with this kind of switch is that there is always 120v going to the heater but it will only heat when turned on.

If your thermostat is a double pole single throw or 4 contacts you still connect the red the same way. But you now connect all the blacks similarly to the red bit on the other side not in use , black hot should be on the L 2 or line 2 screw or wire the base board in the room should be connected to the terminal that is left.

The hot cable 1 wire on each side of the switch, to the baseboard each on the other contacts.

Do not connect red to black that will be a direct short when the switch closes. The advantage of a double pole switch it can control both hot’s for 1 baseboard or control 2 baseboards with 1 switch 1 baseboard on each side so I guess that would be a 3rd way to connect them up.

Now turn the power back on and if the thermostat is 240v no neutral it will work. If the myasthenia has a white it will not work as you have 2 Hot’s and a ground. This is normal for 240v baseboards.

  • Ths is how I wired the stat: but the heater just remains on always. help.getmysa.com/hc/article_attachments/360019134431/… – sci-guy Feb 20 '20 at 22:32
  • Ok that is a single pole thermostat, I see both L1 & L2 , but I also see a N on the device is this a 240-N thermostat? North American power is split phase 2 hot’s , it makes a difference , some may work on both but we don’t know as you did not provide the model number. This may be a 240 to neutral single pole switch and this will not work with split phase. I have seen some that will but right now I think you have a European model. – Ed Beal Feb 20 '20 at 22:55

I just installed one of the new Mysa models last week which is working properly and as I understand it, the two black wires act as the single pole to control whether the heat is on and the red (white in old model) serves as a neutral by using the out of phase leg of the 240V.

I'd turn off the breaker then mark/untwist/disconnect all your wires to verify this (by validating which pair are hot) but...

It's very likely that A) your single black wire is coming from the panel and should therefore be connected to the LINE black wire on your Mysa and B) the twisted pair of black wires lead on to two baseboards and those should be connected to the black LOAD wire on your Mysa. The remaining wire on your Mysa should C) be connected to the three reds twisted together which should be the panel connected to both of your baseboards.

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