For some reason the thermostat / heater in the middle of the circuit are not working correctly. Here is an image of the wiring diagram. enter image description here

These are three small heaters (500w), each thermostat is rated for 2000W. I can't figure out why I'm only getting 120 V in my second thermostat (black wire). The other two thermostats, when I measure the load ENTERING thermostat, and LEAVING it says (240).

Also, if I disconnect thermostat in BOX 3, the heater completely stops. However, in BOX 2, the heater keeps working. I think it's running on just 120 V, instead of 240, but that should tell me something where the problem is, I just don't know what it is.

Essentially in all 3 boxes, all RED wires, are just wired into one nut together, the BLACK is passed through, with one PIGTAIL out, into the thermostat. Then, the wire going out of the thermostat, is going into the BLACK wire of the baseboard heater.

NOTE: All grounds are wired together in all boxes, going to the heaters etc. I didn't put them in the diagram to avoid confusion.

EDIT: I tested resistance between RED > GROUND(120.4O) and BLACK GROUND (0.746MO).

Box Resitance GROUND RED Resitance GROUND BLACK

  • The wiring you show is correct, however I suspect the wiring is not as you show it. I wonder if heaters 2 and 3 are in series somehow. With 6 wires in a thermostat box it can get confusing. It boggles my mind how a 2-wire 240V smart thermostat could possibly be burned out, since it's designed to have 240V across its terminals all day and all night...... unless you are dead-shorting it across a 120V or 240V supply. Mar 4, 2023 at 20:44
  • @Harper-ReinstateMonica I have added a picture of the box. I have full access to all 3 boxes, I can see and trace the wires, they really are as my diagram suggest. There must be something going on with the wire between thermostat 2 and heater 2. Something is being shorted somewhere, but how do I find it? What are the debugging steps for this?
    – Amir
    Mar 4, 2023 at 21:51
  • First measurement between Ground and Red =120V >> is correct ....Second measurement between ground and black as the output from Thermostat- No voltage = correct, if you bridge the black to black at thermostat , you should read 120 V
    – Traveler
    Mar 4, 2023 at 22:19
  • Those are resistance measurements...
    – Ecnerwal
    Mar 5, 2023 at 14:22

2 Answers 2


Heater 2 has an unintended connection (short) between black and ground, based on what you have reported. Specifically, somewhere between the black at the heater and the black at the thermostat. The black to the thermostat is not implicated.

Go hunting for defects in the wiring (circuit off, of course.) Those will almost always be in junction boxes (possibly in the one built into the heater, typically, here) though things like a nail driven the wrong place is possible, but uncommon, unless the problem started right after you did some nailing in the vicinity.

With power off check the resistance between that black wire and ground. Anything other than infinite is a sure sign of the problem.

  • Disagree. If that were true, the breaker would trip immediately. The OP didn’t report that.
    – DoxyLover
    Mar 4, 2023 at 20:37
  • 1
    @DoxyLover The thermostat appears to be blowing and saving the breaker from bothering, based on the question. Read the text in the pink box. With thermostat disconnected, power is flowing from red, through the heater, presumably to ground via the black that's between the other end of the heater and the thermostat. The thermostat blows if connected. If the thermostat was removed and the blacks joined, then the breaker would blow.
    – Ecnerwal
    Mar 4, 2023 at 20:39
  • @Ecnerwal I've done as you said, tested resistance between red/black/ ground. Do the numbers tell you anything? What next can I trouble shoot?
    – Amir
    Mar 4, 2023 at 21:44
  • From red to ground you see the resistance of the heating element, and from black to ground you see a few milliohms, if I read your meter correctly. Or 3/4 of a megaohm, which would imply the terminal is shorted to ground and the black wire barely connected. Since you don't see a problem at this end of the cable, dismount heater 2 and access the other end of the cable, and the terminals there.
    – Ecnerwal
    Mar 4, 2023 at 22:08
  • @Ecnerwal I think I'm making progress. I dismounted the heater #2, and checked continuity (BEEPING SOUND) between RED>GROUND= NO SOUND, BLACK>GOURND= BEEP. Then where the thermostat is, I did the same thing BLACK (this is the wire coming from the heating element)>GROUND= BEEP. I did the same test on heater 3, and there is no beep. Does this mean, the wire going from Thermostat 2 to heater 2 to is somehow broken? Somewhere between them there's a nail through it or something?
    – Amir
    Mar 4, 2023 at 22:28

I think you have a ground fault in that heater. It is shorting to ground, which is why it's powered up even when it's powered off, and is why the thermostat burns out.

How do we detect ground faults? With a GFCI! Fun science facts:

  • 240V heaters work on 120V, but at 1/4 heat output.
  • AFCI breakers (that require circuit neutral*) contain a weak GFCI! (good enough for our purposes).

Do you have any GFCI breakers (or neutral-requiring AFCIs) in your panel? If so, take the two hot wires that feed the heaters (presumably black and some other color, probably "white remarked with black tape"). Place both on the hot/neutral of the GFCI, for testing.

Alternately, just disconnect the wires in the panel and measure resistance from either wire to ground. Should be infinity.

What was the point of all that? Now you have a way to positively test success/fail. Now you can troubleshoot normally and know when you beat it.

If the thermostats give you a hard time about 120V, bypass them.

* As long as it's not the latest models from Siemens or GE that don't take a neutral wire. Those don't contain a GFCI and won't work for this.

  • Hm I don't have any GFCI breakers. I am not sure I follow what to do next to trouble shoot? Why does my wire in the thermostat say 120, instead of 240 like the other two? and How is the electricity flowing without the black wire connected at all to the heater? Are you saying it's going from the RED > and then back through the ground? So it's creating a circuit like that? If I detach ground from the heating element, just to test, and it no longer heats, does that tell me there is a short somehow between RED/Neutral?
    – Amir
    Mar 4, 2023 at 21:47
  • Don't DO that. It will make the whole case of the heater electrically hot, and is quite dangerous.
    – Ecnerwal
    Mar 4, 2023 at 22:06
  • @Amir cut the breaker and measure resistance from red or black to ground. Should be infinity, if not, troubleshoot that. You never, ever, ever disconnect ground. Ground is never the problem. You know, it really sounds like this is beyond your experience technically, maybe it's time to call a pro. Mar 5, 2023 at 0:23

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