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Old 07-03-2020, 10:06 AM   #1
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Powering fan w/o G wire


I recently converted to a Nest and didn’t have the magical “C” wire. Moved the G over, at the furnace jumped from Y to G and all works just fine. NOW I’d like to leave my fan on all the time (not just when he A/C is on). Shouldn’t I be able to disconnect the jumper in the furnace going from Y to G and jump it from G to C? This didn’t work. Thanks!
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Old 07-03-2020, 11:05 AM   #2
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Re: Powering fan w/o G wire


That would be R to G.

With the existing jumper in place - Y to G, if you add another and power up g, y will be energized all the time which wouldn't be good.

What you need to do is see if your control board can operate the fan in cooling mode without power being applied to G. Older ones can't.

Remove the existing jumper and see if the fan comes on at the right speed when there's a call for cooling.

If it works, you can mount a single pole switch at the furnace connect to R and G and run continuous fan at will. Leave Y and G without jumper.

I warn you, if you don't have a ecm blower motor, continuous fan operation can really increase the electric bill and reduce dehumidification.

You're looking at adding 7 to 12 kwh per day to electric use, minus normal blower run time for cooling/heating. Most regular furnace blowers are 300 to 500w running in continuous mode.
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I am not in the business of any trade I give advice on. I have non-professional hvac experience + good knowledge of theory. Attempt repairs at your own risk. Never jump out safeties - especially pressure switches - on a furnace for testing with fuel supply on; use a meter. Do not troubleshoot with live line voltage present unless there's no alternative.
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Old 07-03-2020, 11:33 AM   #3
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Re: Powering fan w/o G wire


I’d ditch the nest before it screws up and causes you problems.
Terrible device with terrible support and a terrible reputation.
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Old 07-03-2020, 01:05 PM   #4
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Re: Powering fan w/o G wire


They also sell 'bridges' that get wired at the air handler. It will then provide a 'C' wire to thermostat.

Feeling lucky? Many thermostat locations are wired with a harness of multicolored wires. Any extra unused wires are usually curled around the outer casing of harness. If this was so with your Nest install, you may have extra wires going back to air handler. That's win #1, no need to run a 'C' wire.

Next look at schematic of air handler for a 'C' wire, likely going to compressor somewhere else. You can tap off that terminal (add to it for the always on 24V you're looking for, move along). Often another harness of wires is near air handler but does not go to thermostat. It goes to compressor. Trace those wires to air handler board looking for a 'C' terminal.

So, if win #1 was that there are unused wires between thermostat and air handler, win #2 is being able to trace a candidate wire attached to 'C' terminal of air handler, tapping off that terminal with a wire that goes to 'C' terminal on Nest. Any available extra wire will do but best to label each end as 'C'.

Last edited by diyorpay; 07-03-2020 at 01:09 PM.
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