DaveC wrote:
Have a 50-year-old (?) natural gas wall heater in my apartment. Thermostat is
connected to gas valve in heater by 2 wires. No evidence of transformer or
low voltage (ie, 30 vac) on the wires.

Symptom is that the 'stat won't open the valve. If I short the terminals for
just a second, the valve opens, and shuts at the appropriate temperature. (I
since have read that it's not a good idea to short those terminals together;
it could damage the valve, or some other component.)

What replacement thermostat should I be looking for? All I see are multi-wire
types that require a transformer voltage. Will any mercury-bulb type work?
(That's what the old one was.)

I'm also curious where the voltage comes from to operate the valve. Is this
generated by the thermocouple / sensor next to the pilot flame?

Suggestions welcome.

Thanks,

Did you *actually* put an AC voltmeter across the 2 wires with the
thermostat contacts open??
If you *did* and got no reading, the system may use a thermopile
(which generates a tiny voltage) for power source. In that event,
you can NOT use any common thermostat due to the resistance of the
anticipator heater.

Best to have the landlord's permission before doing any work on this.

Jim