I am looking to replace the mercury thermostat with a Lux Kono thermostat simply to have the ability to run the fan only and circulate the air. I believe this thermostat does that periodically. I'm in the San Francisco bay area and the call for heat is shorter than the need to circulate the air.

The furnace has a panel contact switch so it doesn't run if the panel is off. It has a Johnson Control G779 Universal Intermittent Pilot Control unit. The wiring diagrams are intelligible but it's not connecting for me because I can't trace the physical wires for the thermostat. I can't seem to understand where the thermostat wires are going and the disconnect switch transformer has two red wires bunched up into the wall. I want to run a new 18/8 thermostat wire from the furnace. I've been looking at replacing the furnace and water heater but can't afford it for a couple years.

I'd greatly appreciate any assistance.

Furnace in Closet


wiring lower

Mercury thermostat


disconnect switch

ICM and blower rate module

  • Can we have some close-up shots of the wiring inside the furnace please? – ThreePhaseEel Dec 28 '18 at 12:44
  • Thermostat wiring is pretty standard, and if you're thinking of replacing it, I expect it's not one of the new variable-rate models. Also a shot of the wiring diagram would be helpful. – Harper Dec 28 '18 at 20:25

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