I have a floor furnace (made of METAL) that uses a millivolt-type control (thermostat) system. After working flawlessly for years, the furnace failed to come on one cold morning earlier in the week. I thought the pilot had gone out. But today, after a bit of a struggle getting to it, I found the pilot was burning away merrily. So, I took the thermostat off the wall and shorted the two wires together. The furnace lit off. Off to the hardware store for a new thermostat (carefully verifying that it was for a millivolt system...). I installed it and it worked fine. Well, the day got colder, so I tried it again, figuring I might need it by morning. Nothing. Nada. Zip... I found a jumper and shorted the two terminals. The furnace lit off.
When I was checking the old thermostat, I measured 240 mv with the contacts open and about 80 mv with them closed. The new thermostat reads about 90 mv with them closed and the contact resistance measures 1.5 ohms (measurements made with an H/P DVM).
So, the question is, did something go wrong with the furnace control system so that it won't tolerate the contact resistance or did I simply get a bad new thermostat?
Jerry