I am drawing this up. I need one bit for information before I can draft this.
REASON: Some heating valves are "fail safe", that is, they fail open. So, in the case of a wiring failure (cable going to the valve is damaged), the valve fails OPEN, thus in heating as water is circulating through it's respective radiator.
I need to know if your heating system starts when the switchleg of the thermostat is energized OR de-energized?
You will need an extension cord for this. Please measure your two (2) wires for 120v-to-ground. Measure to ground in the extension cord, NOT the other conductor in the thermostat. The ground (in the extension cord) is the center lower U-shaped female port at the female end of the extension cord. You don't have a ground at the thermostat....hence, you've the extension cord so as to have a ground reference.
Tell me what happens to BOTH conductors when it's:
- Not heating....Is the black live? Is the white live? Both live?
- When heating...Is the black live? Is the white live? Both live?
You should use a meter for this, not an inductive no-contact "TICK" tester ("Google" it). REASON: "TICK" testers operate on the presence of induction, and they can produce horrible false-positives for results. I can use them, but a homeowner can get VERY confused by them for this application.