2-wire Gravity furnace fires up when wires directly connected, but not with t-stat

sf_mike

- I live in San Francisco and have a 1940's gravity furnace, 2-wire setup.
- Any t-stat I connect doesn't consistently fire up the furnace (most recently got a new Honeywell Round)
- However, when I touch the 2 wires directly together to close the circuit it always fires up.
- Have triple-checked the wiring on the t-stats, in fact the first one we inherited when we moved in and the furnaced fired up just fine, then stopped consistently doing so about a year ago.
- Don't know much about furnaces, is there voltage that comes out of the furnace and perhaps is it not strong enough once I connect a t-stat to cause this problem?

Appreciate any expertise and insights!

ratio

A picture of the gas valve might help. Have you tried a plain old heating-only snap stat (this or something like it)? The Honeywell Round is IIRC a power-stealing stat (no batteries, no common wire) & may have trouble with the millivolt systems common in those furnaces.

JUGHNE

Do you have a 120 volt supply to your furnace?
You may not and that's fine.
You may what is called a powerpile system, if so the pilot flame produces enough electrical power to run the gas valve.
That power generated is fairly weak and requires very good electrical connections.
A power pile thermostat is a special animal.

The first one may have lost its ability to close its switch and provide a good connection, which tying the wires together will provide.

Picture of the gas valve would help.

sf_mike

Advice from @ratio solved it -- thanks!
Installed an all mechanical heat-only Honeywell t-stat designed for millivolt systems, problem solved. Appreciate the help, kids and wife are happy campers now!

ratio

Glad everyone's happy'd up!