Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Electric Baseboard Thermostat - HELP!!

I have 240V electric baseboard heaters, and was looking to replace the thermostat with a wifi version. I purchased the MYSA thermostat. I opened the existing thermostat (pictures below) and am a little bamboozled by the wiring. The picture shown is of 2 separate cables into the box, but this is because there are 2 baseboard heaters on this thermostat/circuit. In the other rooms, there is only one cable coming into the box (black/white) connected across the load. When looking at the thermostat, the 2 black wires and the two white wires are connected to the line terminals, and the load terminals are shorted out by another wire (in this box, it is a white wire).

Any idea/guidance on what to do with the MYSA system? Is there another wifi unit I should be using? I contacted MYSA tech support and had very little help.




  • Jamie Hall
    Jamie Hall Member Posts: 22,947
    Someone did you no favours. Those white wires, if they are actually 240 volt, should have a red or black marking on them, easily visible and permanent, so that they can't be mistaken for neutrals.

    Second, in the first picture, you show two wires on each of the line terminals, and one on each of the load terminals. Either one of the two wires on the line terminals actually feeds another thermostat somewhere, or again someone did you no favours. Where it says line terminal, it means line terminal. Though it may not alter the function (depends on the application), reversing line and load on a switch can give rise to nasty surprises for the unsuspecting.

    On the switch jumpered from one load terminal to the other -- are you sure? if there really is a jumper between the two load terminals, when you turn on the thermostat you have a dead short hot to hot, which should blow both breakers or fuses. I wonder what is really there?

    All that said, if you can correctly identify what is actually line and what is actually load, and that it can handle the connected load, the Mysa should just wire in. But you will have to be absolutely certain that you have correctly and positively identified what is line and what is load. I wouldn't trust the existing wiring connections for that.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • hot_rod
    hot_rod Member Posts: 21,878
    The top 2 wires are the 240 line voltage, probably going to another stat from this one.
    The bottom two go to the heater.

    A volt meter would be handy to assure what’s what. And make sure power is off when you work on it.

    Got a pic of the new stat?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • timjor
    timjor Member Posts: 2
    I can confirm the wiring is as I stated - the "LOAD" terminals are shorted together by the wire link. I have a thermostat that only has a single cable coming into it, and it has the black and white wires as show going into the LINE terminals, and a white "link" across the LOAD.
    I took some measurements with my meter (and took pictures). With the thermostat "OFF", the LINE voltage measures 240V (across the black and white wires). When the thermostat is "ON" the LINE voltage measures 0V. I also attached some pictures of the panel, showing the voltage should be 240V, as the black and white wires are connected across 2 breakers.

    Any ideas here?

  • pecmsg
    pecmsg Member Posts: 4,697
    You’re breaking 1 leg of 240. That’s normal unfortunately. 
    EBEBRATT-Ed Member Posts: 15,345
    You see how your old thermostat has an "off" position. This is required with electric heat, unless you can lock you circuit breakers in the "open" position" This is a NEC requirement. Your bryant Circuit breaker is a "BR-240 which is probably the wrong circuit breaker BR240 means type BR (which is a available breaker) the 2 means it is a 2 pole breaker and the 40 means it is 40 amps which is a problem with the #10 wire you have is not allowed on a 40 amp circuit (at least for electric heat)

    Something is weird about your set up. Check inside the baseboard for a sticker and make sure the baseboard is 240 volt or it could be 120 volt.

    I don't know what someone did but something isn't right

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,600
    edited November 2022
    The MYSA you purchased has 3 wires Correct?
    You have one pair of wires (+ground) entering the box where the thermostat is located. As you stated there is 240 VAC present when the thermostat is not calling and there is 0 VAC when the thermostat is calling.

    The problem is that the 240 Vac from the breaker panel goes directly to the baseboard heater. The two conductors in the thermostat box are the switch legs. The 240 Vac goes away when the thermostat contacts close. There is no difference in voltage with closed contacts. So, only 2 wires can not power the thermostat AND switch on the heater element. You need to run a third wire. as shown int the illustration on the left.

    If that is not possible, then could you relocate the thermostat to a location where the wire can be accessed? You can try it with an electrical fish tape or perhaps wire mold to make the connection of a 3 conductor wire to the thermostat location

    You just need to find the junction box on the Baseboard heater where the 240 Vac from the circuit breaker enters.

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • Jamie Hall
    Jamie Hall Member Posts: 22,947
    Permit me to add,,, before you light yourself up... that breaking one side of a 240 volt circuit does NOT remove power from the unit being supplied. True, it won't heat --current can't flow -- but the electrical contacts in it are still hot. Which is why, so far as I know, such an arrangement is not permitted by code.

    In you instance, before you do any more fiddling, find the breaker or breakers which control both 240 volt legs and make sure they are off. Then get yourself a non-contact voltage tester and wave it at the wires. If it lights, you didn't get the power turned off -- try again.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Powe
    Powe Member Posts: 30
    edited November 2022
    I am not an electrician so I recommend that you contact a licensed electrician to verify this and perform any work.

    The short answer to your question is that the MYSA thermostat won't work. It requires 240 volt power to power the thermostat itself. As far as I can see, you only have one side of the 240 power supply in the box. The "phantom" 240V you read when the thermostat is off, does not exist when the thermostat is calling for heat.

    Your existing thermostat is in violation of code where I live. You would have to check with an electrician or inspector to determine if it violates code in your jurisdiction. A thermostat that has an "OFF" position MUST switch both sides of the power. Yours only switches one side and therefore creates an electrocution hazard if someone relies on the "OFF" position while working on the baseboard heater. It sounds like your other baseboard heaters are wired the same way as you state that the other heaters only have two wires, and they are connected to the LINE terminals. For that to work, there would have to be the same jumper on those thermostats (assuming they are the same model as this one).

    With the existing thermostat off, check the voltage from each of the top LINE terminals to ground (i.e. the box). You will get 120 V from one terminal and 0V from the other.

    In fact, this thermostat (and it sounds like all your thermostats) are wired such that they are switching the same wire twice. It's a double pole thermostat and the jumper makes it such that the two poles are wired in series. The power comes in and is switched through one side of the double pole, then it is jumpered to the other side of the double pole and switched AGAIN before going back out to the baseboard heater. It is nonsense and definitely NOT code (here at least).

    So your MYSA thermostat won't have power and will not work.

    Also, the wire on the bottom left LOAD terminal is wired incorrectly. The wire should be under the brass retainer clip like all the other wires.
    Single pipe steam
    Weil-McLain EG/PEG -50
    175K/145K in/out (454 sf)
    EBEBRATT-Ed Member Posts: 15,345
    The only time a thermostat without an off position is allowed (for electric baseboard) is if the panel containing the circuit breakers has permanently installed circuit breaker lock outs. Not the add on type. This is NEC code because as @Jamie Hall pointed out a SP thermostat does not remove power from the baseboard although it will shut down the heat output