Fan running when furnace is off

[absaroka6]

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.

[sporfrog]

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.

[absaroka6]

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.

 

[sporfrog]

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.

[absaroka6]

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.

[Elec]

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 

[absaroka6 · Accepted solution]

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!

[Lalaine]

Show the topic 

[Bjmred]

I have a c wire and fan still runs constantly

[Cmicth13]

I have the same issue. I have a C wire. Bought the nest yesterday and the fan ran all night even though the target temp was hit or exceeded. Replaced it with the old thermostat for the time being. 

[Eman288]

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?  

[Radioteeth]

C wire isn't necessarily the solution. My thermostat has a C wire -- Fan keeps running. I but I bought a new unit  just to make sure. Same issue. It turns off when I pull the unit or use basically any other thermostat. Pulling the G wire doesn't work, not does urning off the fan schedule and disabling airwave. 

[shenson]

I have a C wire and also when i remove thermostat from base the fan continues to run. any more suggestions 

[shenson]

also my thermostat is an E model 6 wire connection

[EdwardT]

Hi folks,

Thanks for letting us know. We'd like to know more about this. Please fill out this form so we could investigate further.

Thanks,

Edward

[DaveBaumann]

I added the C wire and my Nest learning thermostat now works properly as it has for the past 4 years, (blower no longer runs continuously unless I enable it to do so, and I am now able to disable it through the Nest control options).

Very odd that this was required after all these years. Was there a recent software update that caused  this ? Or, as someone surmised, is the battery no longer recharging thus requiring a C wired ?

[Jeff]

Hey DaveBaumann,

I'm happy to see that things are working well once you got the C-wire connected to your Nest Thermostat. It's possible that over the years that the battery power was just getting by. Either way, it's always best to use the C-wire if possible. 

For others in the thread still seeing issues, can you confirm if you've installed the C-wire on your setup?

Thanks,

Jeff

[JimKov]

I do have. C wire connection but the fan keeps running even though the heat is off. Any other suggestions?

[EdwardT]

Hi JimKov,

Thanks for letting us know. To isolate the issue, could you try removing the display of your Nest thermostat and check if the fan would still run?

Thanks,

Edward

[JimKov]

Fan stopped when I removed display then started again after replacing it

[JimKov]

My second thermostat on my other furnace is not causing the fan to run. However I am getting a low battery warning. Both are wired the same. 

[Jacads]

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?

[absaroka6]

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.

[Jacads]

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

[Ubach]

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,

[sporfrog]

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.

[JimKov]

with wiring similar to yours, I was able to get my fan to stop by going to the settings - schedule- fan (rather than temperature) and turn the fan off .

[Jeff]

Thanks for the input, JimKov. I appreciate you letting us know your process for doing that. Hopefully others can pick up on that and try it out too.

Thanks,

Jeff

[Markjosephp]

Hi everyone,

Thanks for the assistance, @Jeff.

I wanted to follow up and I know it has been a while but I wanted to ensure that everything is covered here.
 

Please let us know if you have any other questions or concerns as I will be locking this in 24 hours.

Best regards,

Mark

[firehorse4]

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

[Vineet]

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?

[absaroka6]

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.

[Eman288]

Did you try that?  Put the g in the c spot?  I’m assuming in the unit the g is still connected correctly just at the thermostat you plug the g into the common?

[Jacads]

Yes they (google) make a small device I bought off Amazon for I think about $20 that attached to furnace and you keep the same wiring works great.

[Vineet]

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.

[Bjmred]

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. 

[Vineet]

Thanks BjMred, Absoroka - In my case, Ive replaced the wall plate and tried a new nest thermostat and the problem persisted. Would that indicate its a fan relay switch issue to you? I'm just not sure since a brand new nest also had the same "fan on" issue so not sure if its a power issue. 

[Bjmred]

All 4 of the thermostats were new. 2 Nest, 1 smart Honeywell and a very basic non smart one. Fan ran with all of them. You will know for sure its not the thermostats or the wiring at the thermostat. If you turn off the power at circuit breaker to your HVAC system. Uninstall the thermostat. Turn power back on and listen for the fan to run ( may take a few minutes).If it comes on, you know the problem is at the hvac unit itself. Some HVAC systems have a reset button for the fan relay. If so, push it. (Again power off} Mine did not. Sometimes fan relay switches get stuck. You can tap on the switch itself and you may be able to knock it to being unstuck. May need to replace it. 

[Vineet]

My fan does not turn back on if the nest thermostat is disconnected so Im thinking it has something to do with the nest thermostat and the power issue. 

[Bjmred]

Or you might have a bad wire? Have you reinstalled your old thermostat to see if it works properly?