topic Re: Fan running when furnace is off in Nest Thermostats

Fan running when furnace is off

absaroka6 — Sat, 01 Jan 2022 21:49:27 GMT

The fan keeps activating even when the thermostat isn't asking for heat.

Thought it was a problem with the furnace, since the fan schedule is disabled, and there's no fan schedule set up on the thermostat, but when I pull the thermostat off the wall, the fan stops immediately. When I put it back, the fan starts up again, so it's clear the thermostat is activating the fan. But I've checked every setting in the Nest, and no fan schedule is set.

When the thermostat calls for heat, all works fine, but why is the fan running so much when no heat is needed?

Edit: I had such confidence in the Nest thermostat that I waited for a long time before removing it and attaching a Honeywell. The Honeywell operates the furnace correctly, so I now know it was the Nest malfunctioning.

Re: Fan running when furnace is off

sporfrog — Sat, 01 Jan 2022 22:48:53 GMT

It's possible there is a wire incorrectly connected to the nest thermostat. There is also a setting for some Nest thermostats that allows the Nest to control when to run the fan rather than the HVAC equipment. Often the thermostat will call for heat (or cooling) and the HVAC system will start and run the fan as needed. In other cases the thermostat calls for the fan to run at the same time it calls for heating and cooling.

A photo of the old thermostat (or working thermostat) wiring might help solve this.

Re: Fan running when furnace is off

absaroka6 — Sun, 02 Jan 2022 01:22:48 GMT

No, that's not it. I've used this Nest thermostat for about a year and a half, with A/C and heat, and never had a problem. Happy to attach a photo, though. This is how it was wired since I installed it - roughly July 2020, until today. Until last night, I had never had a problem, and after switching back to the Honeywell, all is again back to normal. Edit - I also verified about 50 times, that the Nest's fan settings were all disabled and off. Reset it several times, including a factory reset.

Re: Fan running when furnace is off

sporfrog — Tue, 04 Jan 2022 01:27:28 GMT

That wiring looks correct assuming the colors are used in a typical fashion. Presumably they are connected to the same letter terminals on the Honeywell. Any chance that blue wire is connected to C (common) at the HVAC equipment? Low likelihood it will make a difference for your reported problem, but Nest thermostats generally work better with a common.

You might also consider a manual software update just in case it is bogus code causing the problem.

Re: Fan running when furnace is off

absaroka6 — Tue, 04 Jan 2022 04:49:13 GMT

Thanks for the follow-up. I agree the symptoms don't necessarily indicate that the lack of a C wire was the problem, but I did some research , and it does seem likely the thermostat just wasn't getting enough power. There seem to be a lot of examples of users having Nest failures after long periods of successful use, when there is no C wire present. I was able to connect the C wire at both ends and at least for the moment, all seems to be working properly.

Given my experience, I would recommend everyone to add a C wire, if at all possible, regardless of whether the setup process or Google says it's necessary. It's just too much to risk, particularly if you live in a very cold climate where you could have a costly freeze if this little device fails.

Re: Fan running when furnace is off

Elec — Tue, 04 Jan 2022 18:14:01 GMT

You seem to be quite knowledgeable about these Nest Therms. I'm getting there but still these things really suck. What would cause heat to turn off at 65 when there are no settings telling it to? Have reset both heater and therm, gone over settings and cleared schedules. There's nothing indicating why it could..but

Re: Fan running when furnace is off

absaroka6 — Wed, 05 Jan 2022 21:39:02 GMT

Just to close this topic, I wanted to let people know that if you have a randomly running fan or constantly running fan, the solution is likely to add a C wire. Adding the C wire appears to have solved my problem. The Nest Thermostat E now has a reliably adequate power source and no longer needs to cycle my fan, or anything else. I recommend anyone who has pulsing or constantly running furnace or fan, to install a C wire. In fact, I recommend a C wire before these problems start, regardless of what the setup process recommends. It's a big relief to me as the temperature plunges here in Wyoming, and I won't have to worry about thermostat failure. Stay warm!

Re: Fan running when furnace is off

Lalaine — Thu, 06 Jan 2022 09:45:47 GMT

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Re: Fan running when furnace is off

Jacads — Thu, 20 Jan 2022 23:30:04 GMT

I am having the current problem with my natural gas heater in my garage. Worked fine on the factory cheapy thermostat but I find the exhaust fan on the heater not the main fan cycling on and off constantly. currently just have wires to the R and W1 perhaps I need a c wire there?

Re: Fan running when furnace is off

absaroka6 — Fri, 21 Jan 2022 02:19:14 GMT

I think the problem is exactly what you suspect - it is having trouble keeping itself charged, and you need a C wire. It's unfortunate. I think these should never be installed without a C wire, even though the manufacturer says it's okay.

Re: Fan running when furnace is off

Jacads — Sun, 23 Jan 2022 01:24:05 GMT

You were correct needed a C wire so I bought the google adapter off Amazon and works perfectly

Re: Fan running when furnace is off

Ubach — Fri, 04 Feb 2022 16:09:53 GMT

My fan was running ALL the time with G wire connected. ( I also have C wire).

Disconnect G wire and the fan will turn on only when furnace is running,

Re: Fan running when furnace is off

sporfrog — Sat, 05 Feb 2022 16:07:44 GMT

The G wire typical purpose is so that that Thermostat can turn on just the fan without heating or cooling. For example say you want to circulate air in the home you can tell the thermostat to turn on the fan - if there is a G wire. It's good that disconnecting solved the problem, but there is a downside. Glad it's working for you.

Re: Fan running when furnace is off

firehorse4 — Wed, 09 Feb 2022 03:42:59 GMT

Thanks for the information on the fan randomly running. I have the same issue and read your solution post. I checked my C terminal and it is wired to my thermostat and my furnace already.

I removed the G wire from the furnace terminal and the fan doesn’t run now but the fan still randomly shows it is running in the Nest thermostat display and my phone app. Obviously it is now isolated to the thermostat.

Any help is very much appreciated

Re: Fan running when furnace is off

Bjmred — Sun, 06 Mar 2022 00:16:37 GMT

I have a c wire and fan still runs constantly

Re: Fan running when furnace is off

Eman288 — Mon, 07 Mar 2022 01:41:25 GMT

Hello. Had a similar issue with my nest. My central AC fan kept cycling on and off even when set off. Added a c wire and purchased a new thermostat. Seemed to work till about a month ago. Any thoughts?

Re: Fan running when furnace is off

Vineet — Tue, 15 Mar 2022 23:53:27 GMT

Does nest have an official fix here? I too am having the the same problem. I have the 4 wire config (no C wire). Basically was working perfectly and after about a year and a half, my fan tuned on despite all settings off and runs all the time unless I remove the thermostat from the wall plate. Does disconnecting the G wire solve this (assumption being we dont need to use the fan - just need the heating and cooling systmes)? And it is that safe to have the G wire unconnected just sitting there?

Re: Fan running when furnace is off

absaroka6 — Wed, 16 Mar 2022 00:13:36 GMT

You're situation sounds just like mine. If you only have 4 wires, I think you could put the G wire in the C terminal at your thermostat and do the same at the furnace. (Do this with the power off). You would then effectively have a C wire, but you would lose the ability to run you fan on its own. That isn't a big loss, really. The fan would still run when your heat or AC is on, but you would then have a C wire, and power to the thermostat. I think this would solve your problem.

Re: Fan running when furnace is off

Vineet — Wed, 16 Mar 2022 01:10:08 GMT

Thanks for your response absaroka. Switching the G wire to C wire on the thermostat seems easy enough. Im a bit more nervous about the steps on the HVAC side. Is it as simple as just switching G to C or is there also an additional "jumping cable step" as well? Also any video recos on this would very appreciated.

Re: Fan running when furnace is off

Bjmred — Wed, 16 Mar 2022 01:13:46 GMT

Disconnecting G wire did not work for me. Subsequently, I installed/uninstalled 3 different thermostats. I suspected it was the fan relay switch. It was. I called an HVAC guy. Inexpensive part but expensive labor.