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We've got two Nest Learning Thermostats (3rd Gen) running two separate heating and cooling units in an office. They both work fine for heating and cooling, with one exception: one unit will not turn on the fan only. The fan comes on as normal for both AC (probably, too cold to test) and heat, but if set to run Fan Only nothing happens.

I'm confused because it seems like the unit is able to run the fan if heating just fine. Both thermostats are wired up exactly the same. Not the initially recommended configuration but one Nest support recommended during the initial setup that has proven effective.

I'm not 100% sure that both heating/cooling units are the same. They were wired up the same with identical old thermostats before we installed the Nests, but the hardware is up on the roof where I've not been yet. Never tested Fan Only before switching to Nest, so troubleshooting scenarios are unclean! Wondering if anyone has any ideas to isolate the issue. The only thing I can think of to start is to try switching the Nests but I don't guess that will help as the controller seems an unlikely source of the problem.

Wiring Diagram In Nest

Old Thermostat Wiring

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To troubleshoot this, you can remove the Nest and use a jumper wire to touch the red and green wires together. The fan should come on. If the fan does not come on, then the system is not wired correctly on the other end.

You'll have to take a look at the air handler side of the wires to see what happens to the green wire. If the wire was broken or if they were trying to get around a limitation of some other thermostat, the could have jumped the fan wire to the heat or cooling to get it to work at the expense of not being able to run with the fan only.

The fact that the white wire doesn't go to the W terminal on the Nest already proves that something is out of the ordinary. It might not be broken, but there might be a combination of equipment that requires some odd workaround. That or the person that tried to get it to work last time didn't know as much as they should have.

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  • Thanks for this tip, that confirmed that the unit is not wired up and the thermostat is not the issue. Red and Green connected=nothing! Guess a trip to the roof is up next. – jeffhot Jan 13 at 19:03
  • Is the roof where the air handler/blower is at? I know that the outside AC unit is commonly installed on the roof, but the air handler and furnace are usually inside. – JPhi1618 Jan 13 at 19:06
  • There is a small crawl space between the room with most of the duct work, air return, etc. that I haven't accessed yet which may contain some of the HVAC components. Will have to see. – jeffhot Jan 14 at 17:57
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I have had the same problem for over a month. A/C and heat works fine but when I want to schedule for fan only then thermostat shows fan running but nothing happen. I googled the solutions and tried almost everything. Then below is the option that worked for me .

jump the Red & green wire see if fan turns on

a- If fan turns ON then it’s thermostat base have problems (return it) b- if nothing happen then it’s wiring problem.

Check the green cable on HVAC it might be rusty /loose or broken.

I would suggest trim the wire on both ends (thermostat end and HVAC) and reconnect it.

Mine work after reconnecting the green wire to HVAC. Thanks

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