I have a propane forced air furnace with an old style manual thermostat. When the fan is on the AUTO setting, the fan will come on anytime the furnace kicks on. But, I run a woodstove sometimes and want to circulate air around so I've tried turning the fan to the ON position which should cause the fan to run continuously. The ON position does nothing for me. Any ideas? Is it wiring or the fan relay? I know this is all really vague but would appreciate any suggestions on what I can try.
Furnace fan works on AUTO but not on ON
- Thread starter waldreps
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Jadnashua
Retired Defense Industry Engineer xxx
Most, but not all, furnaces have a control to force the fan to run continuously, BUT, they may not have run the wire to make it work to your thermostat. The simplest thing to do is to open up your manual and see if your furnace has a 'G' terminal. That's green, not ground, on the control panel. If it does, you'd need a cable running from the furnace to your thermostat with that wire connected. If you're lucky, there's an extra wire in your cable and your furnace has that option, and all it will take is hooking that up. For minimal control, all you need for a thermostat are two wires: R and W. When the thermostat calls for heat, it connects the red wire to the white wire. If you have a green wire, it would connect that red (power) to the green, activating the fan. The thermostat is just a fancy switch, but your furnace must have the necessary control interface to accept it.
In the absence of a green wire to force the fan on, when in Auto mode, the furnace itself turns the fan on when required.
In the absence of a green wire to force the fan on, when in Auto mode, the furnace itself turns the fan on when required.
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It takes a large delta-T for moving heat around with an air handler has any efficiency, which is why furnaces typically run with 65F incoming air and 125F (or higher) output air, a 60F delta. And the amount of heat you can move from a 75F room with a wood stove to a 60F remote room (a 15F delta) is really pathetic at the flows that are designed to heat he space with a 60F delta.
And with register exit air temps in the sub-95F range it makes the humans in the cooler remote rooms less comfortable due to the wind chill effects, even if it's raising the temp in the room a few degrees.
Unless the air handler is a super-efficient ECM drive type, you'll be better off using small electric space heaters in the rooms when occupied, and letting them coast along when not occupied, rather than running 500-1000 watts of air handler 24/7 to raise the temp in the remote rooms another 5F (if you're lucky.)
And with register exit air temps in the sub-95F range it makes the humans in the cooler remote rooms less comfortable due to the wind chill effects, even if it's raising the temp in the room a few degrees.
Unless the air handler is a super-efficient ECM drive type, you'll be better off using small electric space heaters in the rooms when occupied, and letting them coast along when not occupied, rather than running 500-1000 watts of air handler 24/7 to raise the temp in the remote rooms another 5F (if you're lucky.)
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