Slightly OT - Furnace problem

Jerry Foster · 2006-12-16T06:04:05+00:00

I have a floor furnace (made of METAL) that uses a millivolt-type control(thermostat) system. After working flawlessly for years, the furnace failedto come on one cold morning earlier in the week. I thought the pilot hadgone out. But today, after a bit of a struggle getting to it, I found thepilot was burning away merrily. So, I took the thermostat off the wall andshorted the two wires together. The furnace lit off. Off to the hardwarestore for a new thermostat (carefully verifying that it was for a millivoltsystem...). 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 litoff.When I was checking the old thermostat, I measured 240 mv with the contactsopen and about 80 mv with them closed. The new thermostat reads about 90 mvwith them closed and the contact resistance measures 1.5 ohms (measurementsmade with an H/P DVM).So, the question is, did something go wrong with the furnace control systemso that it won't tolerate the contact resistance or did I simply get a badnew thermostat?Jerry

Dec 16, 2006 20 Replies

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Leon Fisk 19 years ago

Hi Jerry,

Did you actually get a look at the thermocouple? If so did you take a second and brush it off? If it has been in use for awhile they have a tendency to develop a build up of white crud on them which can affect their output. If you haven't done it yet I would blow out the pilot, wait for it to cool off a bit, brush off the thermocouple good and also check/clean the hook-up for it as someone else mentioned. If nothing else it shouldn't make things any worse :)

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