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Facts: Existing heater circuit, 30 amps, 240V has two WALL HEATERS AND two thermostats in two different rooms. (For the furnace-focused, milliamp minds, you can move on to another question). ;)

Thermostats have been removed from their box locations.

In dining room I had a 240v thermostat,similar to the Honeywell TL8230A1003. This junction box contains three romex cables: 10-2 w ground for all. Line in is upper right black wire pigtailed from twisted blacks. Load #2 goes to living room thermostat and load #3 goes to the King EFW2440, 240V dining room wall heater. The thermostat was installed by an acquaintance a decade or so ago with neutrals not tied together but used as separate returns for the two black wires. I was concerned about this...Box may need to be redone as it is recessed 1/2" into the wall and there is no contact between box and drywall.(Will look into an extension ring but just want a bigger box). New 240V King EFW2440 wall heater under warranty just installed.Its wattage can be adjusted. Were the neutrals supposed to be used as two sets of wires in this installation?

(Insert picture here) doesn't work on my cell phone...brain has 0 volts again.

In the living room junction box, it had a similar 120v thermostat there that lights up. Think the two thermostats got switched by mistake at installation. There are two romex cables, 10-2 w ground. This thermostat controlled an old but good 1998 240v king electric EFW2440 wall heater, whose wattage cannot be adjusted per King. I will be changing this from a 120V thermostat to a new, 240V, 2-pole, Honeywell (same model). In this box the 120v thermostat red wires were nutted one each to each of the white neutrals also. Black to blacks. No o issues despite it being a 120V...
Is a 2-pole, 240v thermostat appropriate for this location? I wanted 2 poles to be able to fully shut off the power in summer. Wiring here seems straightforward.

(Insert picture of brain in frying pan with wires coming out). ;)

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    Those white wires aren't neutral. They should have been re-marked with red tape, but the original installer was lazy. – Harper - Reinstate Monica Feb 12 at 7:24
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You don't have any Neutrals - you appear to be working in North American 120/240 VAC and for a 240V heater setup you have hot, ground, and hot - whites should have been re-marked (red, or black, or any color other than white, grey, or green.) You should do that (I reach for red first in this case, but it's not required to use red, just one of the hot colors.)

You mention 10 gauge wires, and that sets off an alarm bell since typical 240V heater circuits are 20A, and the usual reason you see 10 gauge in that case is that you have (the bad kind of) aluminum wiring. If your thermostats are not specifically rated to connect to aluminum wiring, and the wires are not copper, you may have some additional work to do. OK, you did say this is a 30A circuit, which might make finding suitable thermostats difficult, but should mean the wires are copper.

So, at the junction box where power comes in, in the dining room, you need to join the black wires from the feed and the black wires leading to the living room, and a pigtail to connect to one line side of the dining room 240V two-pole thermostat. If the wires are aluminum, use one side of an alumiconn to connect the two aluminum wires, and the other to connect a copper pigtail. Then you also need to join the whites (remarked red) from the same cables in the same way, and a pigtail to the other line side of the dining room thermostat. The dining room heater connects to the load side of the dining room thermostat - again, use alumiconns to transition to copper if needed.

All the grounds should be joined, and connected to the box if the box is metal.

That gives you unswitched power to the line side of the thermostat, and unswitched power to the living room thermostat location, so that the heaters work independently.

In the living room, connect the cable from the dining room to the line side of that 240V two-pole thermostat, and the cable to the heater to its load side, again, using alumiconns if you need to transition from aluminum to copper.

In the summer, you might as well shut off the circuit breaker feeding this circuit. That kills the whole thing, while the two-pole thermostats only kill the wires from the thermostats to the heaters.

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