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Old 07-14-2010, 08:12 AM   #1
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Furnace Exhaust Fan Always On


Hi there ... I have a gas furnace that has a power exhaust fan on it. This is in a house ~18 years old that I have just bought. I installed a new thermostat since I bought the house ... it now seems that the power exhaust is on all the time ... even though the furnace isn't on ...

Anyone have an idea of why this may be ?? The new thermostat is a Venstar 1900.

Kevin

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Old 07-14-2010, 11:45 AM   #2
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Furnace Exhaust Fan Always On


Look to see if the new stat is a power stealing thermostat. As in it doesn't use batteries, or have a common terminal connection. if so, its the stat.

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Old 07-14-2010, 11:57 AM   #3
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Furnace Exhaust Fan Always On


HI Beenthere .. yeah it is definitely a power stealing stat. I had to connect a spare wire in the bundle to "C" on the furnace. The "C" terminal already had 2 other wires on it ...

Can anything be done about this?
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Old 07-14-2010, 02:11 PM   #4
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Furnace Exhaust Fan Always On


Power stealing thermostats bleed voltage to operate. Furnaces with inducers often do what yours did. Because its just enough current to initiated the purge sequence. But not enough to operate the ignitor relay and begin the safety sequence.
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Old 07-14-2010, 04:06 PM   #5
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Furnace Exhaust Fan Always On


so nothing for a DIYer to fix I guess eh?
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Old 07-14-2010, 04:18 PM   #6
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Furnace Exhaust Fan Always On


You can use a resister across W And C terminal at the furnace, some need to go on to the W and G terminal. Should be able to stop it.

A battery, or a thermostat that uses common from the furnace is a better thermostat.

Those power stealing thermostats use to come with a resistor, for this reason.
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Old 07-14-2010, 04:23 PM   #7
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Furnace Exhaust Fan Always On


Any value resistor? The C terminal is different than a common?
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Old 07-14-2010, 04:35 PM   #8
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Furnace Exhaust Fan Always On


C and Common are the same thing.

1,000 ohm, 1 watt.

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