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I have a gas furnace freedom 80 ,heating only with two wire thermostat with mercury switch, Anyway a couple of days ago I increased the themostat setting to put the furnace on,it had been off for a few days before that as the weather was quite warm, as soon as I moved the thermostat up it began smoking and a thin wire inside ,kind of like a preset was burnt and a 5 amp fuse blew on the furnace.
model no. AUD100C936H4
Any idea why this happened .
The furnace has a 'White -Rogers'integrated furnace control circuit board model 50A50-473
 

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You likely have a short in your thermostat wiring to ground. When the heat cut in, the high amperage flow to ground burnt out the heat anticipator wire in the tstat and then blew the fuse. Shorts are difficult to find so you may want to call a pro, need a new tstat also.
 
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