Wall heater thermostat question...

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,
--
Please, no "Go Google this" replies. I wouldn't
ask a question here if I hadn't done that already.
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
DaveC wrote:

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Dave, http://www.freepoolheat.com/gassystem.html#millivolt
Jabs

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
DaveC wrote:

As you guessed, the voltage (250, 500, or 750 millivolts, MOL) comes from the thermopile heated by the pilot flame. Most systems are 750 mv. If you have a voltmeter, measure the output of your thermopile.
Use a thermostat designed for millivolt systems (usually the specs will state). Many battery-operated programmable thermostats will work, too.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

This is Turtle.
You have a break in the wires to or from the heater to thermostat or a bad thermostat. Your going to have to trouble shoot it out.
TURTLE
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.