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    barrymel's Avatar
    barrymel Posts: 2, Reputation: 1
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    #1

    Jan 25, 2009, 08:12 AM
    Furnace won't start when thermostat in Auto mode
    Combination electric heat/a.c. unit works fine in "on" position. Fan switch works fine. Won't work in auto. White-Rodgers mercury style thermostat. Do I just need to replace thermostat?
    KC13's Avatar
    KC13 Posts: 2,556, Reputation: 99
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    #2

    Jan 25, 2009, 08:30 AM
    When you say "auto", are you referring to the fan selector switch, or the mode selector switch? Do you have a voltmeter? If so, check for 24v from "G" to "C" at indoor unit with system set to heat/cool, fan set to auto. If other parts of system run but fan does not AND you do not measure 24v G-C, the fan circuit is not being powered. Check again with fan switch set to "on".
    barrymel's Avatar
    barrymel Posts: 2, Reputation: 1
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    #3

    Jan 25, 2009, 08:49 AM
    On the base of the thermostat, there are 2 choices for the fan: Auto and ON. I have the Fan set to Auto, and the system has 3 choices: Cool, Off, and Heat. I have that set to Heat.
    When I take the cover off the thermostat, and have the Fan set to On, I can hear the furnace switch on and off as the mercury bulb bounces back and forth. But if I max out the dial to 80, and leave the fan set to Auto, nothing happens. Bad thermostat? I hope so.
    KC13's Avatar
    KC13 Posts: 2,556, Reputation: 99
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    #4

    Jan 25, 2009, 09:00 AM
    Probably, but even if it isn't you should get rid of that mercury in favor of an electronic/digital type. Spend the few extra $$ for a good brand (Honeywell), avoid the cheapies (Hunter, Maple Chase). If you need help with the installation/wiring, you'll find it here as well. Good luck, we're all counting on you... :D
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