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I'm trying to replace an old Mears M21 SP thermostat controlling the baseboard heater with a new one (Honeywell RLV3120A to be specific). Problem is, the old thermostat has 3 wires (Black, Red, and White - all connected to the respective wires in the box). The new one is a simple 2-wire setup (I assume merely acting as a resistor in the circuit). What would be the right way to wire the new thermostat? Thank you!

Below is a not-so-great photo of the box. There are overall, 2 red wires (connected to the red wire of the stat), 2 black ones (connected to the black), 3 white ones (connected to the white), the ground one (not connected) and some other two wires coupled together but not connected to the stat).

UPDATE: Ignoring the White and connecting the Red and Black to the new stat didn't work - the stat never turned on. I've done some troubleshooting and this is what I see:

The box has 4 sets of input wires (wires in each set are connected together)

  • 3 Black wires (not used)
  • 3 White wires (connected to the white wire from the old stat)
  • 2 Red wires (connected to the red wire of the stat)
  • A single wire that enters the box as Red but then is painted Black :-) (this one is connected to the Black wire of the stat)

Checking the voltage across all of those gives me the following:

  • There is no voltage between the weird Red/Black wire and any of the other sets of wires.
  • There is 240V between any pair of the remaining wire sets (Black/White, Black/Red, White/Red).

Has anyone seen the setup like this in the past? :-) What I suspect could be done is the following:

  • Ignore the Black set of wires (which was ignored before)
  • Ignore the weird red/black wire
  • Connect the new stat to the White and Red wires respectively.

This is basically similar to what one of the comments suggested except it stipulates that Red and White are the Power and Load ones. But I'd really appreciate some advise before I burn a circuit or do some other very bad thing. :-)

UPDATE 2: found this wiring diagram for another thermostat with heat anticipator. Judging by this,I should definitely see the potential between between Read and "Black" and none between White and Black. However, I see instead potential between Red and White and and no potential between either Red and "Black" or White and "Black..."

box wires

  • Can you post photos of the inside of the box please? – ThreePhaseEel May 31 at 5:12
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The M221sp is a single pole that uses the neutral for a heat anticipator feature, which is really almost pointless with a baseboard. The Honeywell you indicated doesn't have that feature.

After you connect the new stat you will have one less wire in the wire connector with the remaining whites. Sometimes when installers twist the wires so hard that the wires spiral I have found the internal metal cone in the connector becomes flared at the tip and won't thread onto a new connection well. It would be a good idea to replace the wire connectors or at least be very sure that the existing connector makes a good bite, it could be most problematic on the white splice since it is now smaller.

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  • Yeah, that spiraling the outside wires for a couple inches is more than even I do. Not least because that much twisting could have the same effect as tape: mask a poor connection inside the nut that will arc. That said, the worst mistake is tightening nuts too little so they don't make good connection, and again, arcing. – Harper - Reinstate Monica May 31 at 16:02
  • Thank you for the advise. Unfortunately there is no potential between Red and "Black" wires - could it be that it is that "Black" wire is used as heat anticipator? (See more details I've put in the original question and thank you for your help!) – Alexander L. Belikoff May 31 at 22:12
  • Sorry, without better "before" pictures anything I could offer would be just a guess. – NoSparksPlease Jun 1 at 14:31

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