Can I Get My Furnace Fan to Function Independently of Heating/Cooling?
August 8, 2014 12:35 AM   Subscribe

We have this furnace. In the summer it would be nice if we could run only the fan (We don't have central air), but according to the manual, Fan Only Mode requires the R and G thermostat contacts be wired. As you can see, only the RC/RH and W contacts are wired. Would simply swapping the orange wire from RC/RH to RH/G create the circuit needed for sweet, sweet fan action, or will it be necessary to run a new wire from G to the control board in the furnace, therefore requiring professional help?

And why the heck would the RC/RH be wired together in the first place if there isn't a cooling unit?
posted by Alvy Ampersand to Home & Garden (4 answers total) 2 users marked this as a favorite
Here's a nice chart explaining the various terminals in the thermostat.

You'll need to run a new wire from the thermostat to the furnace. Moving the orange wire to RH/G won't do anything.
posted by jon1270 at 3:14 AM on August 8, 2014 [1 favorite]

Best answer: To clarify, the black wire that's connected to the RH is supplying the power that the thermostat can use to tell the furnace what to do. When the thermostat asks for heat, it basically flips a switch that connects the RH terminal to the W terminal, which is in turn connected to the W terminal in the furnace via the other black wire. When the W terminal in the furnace is energized, it fires up the burners and starts blowing hot air.

Inside the furnace there's also a G terminal. If that terminal is connected to signal voltage then only the fan turns on, but currently that terminal isn't connected to anything.

I said you'd need to run a new wire, but that's not strictly accurate. You *could* move the wire that's currently connected to the W terminal (in the thermostat) to the G terminal, and also make the same change inside the furnace. That way the thermostat would be able to call for the fan to come on, but wouldn't be able to call for heat so you'd have to switch it back when the weather gets chilly. I'm not recommending this, just saying that it could be done.

And why the heck would the RC/RH be wired together in the first place if there isn't a cooling unit?

The orange wire was probably put there by the thermostat manufacturer. It's not doing anything for you right now, but if you did have central air then the jumper would allow the thermostat to use the same transformer for both heating and cooling signals, and removing the jumper would facilitate using separate transformers for heating and cooling.
posted by jon1270 at 3:31 AM on August 8, 2014

The fans in cooling units are pretty gentle, I don't think you'll be very happy with just the fan on your unit. Also, it's a pretty energy inefficient way of moving air. It's indirect. Put your hand in front of the vents, it's not really all that cooling without the actual coolant included.

You'll feel much cooler with box fans or ceiling fans, where WAY more air is moved and the air stream is much stronger.
posted by Ruthless Bunny at 5:53 AM on August 8, 2014

Response by poster: We have a ceiling fan & window A/C, so it's not really so bad. Running the furnace fan is more about just getting some air circulation going, plus it makes me nervous to have the furnace itself sitting completely inert for 3+ months at a time.

Thanks for the responses!
posted by Alvy Ampersand at 7:55 AM on August 8, 2014

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