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QUESTION:I have Virginia house built in 1936 that has gas floor furnace heat.
We have lived there for 10 years. Over the past 4 years, the furnace
has been getting harder and harder to get to operate in the fall. The
control unit under the house in line with the furnace was replaced
several years before we moved in 10 years ago.
1996: Crawl under house and light pilot. Come in house and turn up
thermostat, but furnace doesn't fire up. Thinking maybe there is some
sort of oxidation inside the mercury vial, I short the terminal on the
outside of the mercury vial and the furnace kicks on. Let the furnace
run for a few minutes, and the whole system worked good the rest of
the winter.
1997: Pretty much the same thing as in 1996.
1998: Pretty much the same thing as in 1996.
1999: After doing the same thing as previous years, still couldn't
get furnace to ignite. Replaced the thermostat on the wall with a new
one. Still no luck. Went under the house and shorted across the
thermostat lugs on the control unit with a wire, and the furnace
ignited. Thinking that the terminals or the thermostat wires were
oxidized, I undid the thermostat wires and scraped them along with the
lug faces and screwed it all back together. System worked well from
there (might have had to short across thermostat lugs one more time
before I got it running...can't remember exactly).
2000: This year I had to repeat everything that I did in 1999, but
there is nothing I can do to get the system to work with the
thermostat on the wall. The only way I can get the furnace to fire up
is to short across the thermostat wire lug connections on the control
unit under the house. If the thermostat is on up in the house, then
when I short across the lugs under the house, then it will stay lit
when I remove the short. However, once I shut the furnace off via the
thermostat in the house, then I cannot get it to re-ignite via the
thermostat.
Where could the problem be. Any suggestions? I would love to be able
to fix this myself and save on a big repair bill. I have multimeter,
so if I need to check voltages or anything I can do that, but just
don't know what to look for. By the way the there are three lugs on
the control unit. The thermostat wires go to the outer two lugs and
the thermopile wires go to the center lug and an outer lug.
ANSWER: Thermopile means the gas valve is actually powered
by the tiny voltage generated from the flame on the
thermoplie unit. IIRC, it is about 350mv but there
may have been several standards.
More than likely, the thermoplie is weak or the pilot
flame is not striking it in such a way to maximize the output voltage.
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