I have M21 SP thermostats through the house here (built in 1978). Single pole switch.
Of course the two heavy leads (Blk and Red) are for the source and the load for one side of the 220 V circuit.
There is also a lighter gauge white wire. Where should that be connected in this configuration? Right now it is on the other side (the unswitched side) of the 220 V line.
Or is this some sort of "ground" and really should be connected to something neutral (such as the earth ground or at least the neutral)?

https://www.askmehelpdesk.com/heating-air-conditioning/mears-controls-313584.html


You might also have to search Google for a wiring diagram for that thermostat or post a picture of the one you have.

I also need help wiring a Mears M21 SP thermostat.

THX
Ken

See the post above your's