Variable Speed Fan Issues

I installed the Nest thermostat this past Thursday. So far I think it's a good product. However I am running into issues with the variable speed fan on our high efficiency furnace.

When the fan setting on the thermostat is set to "ON" the fan runs on a low speed and when the heat cuts in the fan increases speed like it should.

But in the evening the fan will pick up speed for no apparent reason just as if the heat has cut in, yet the thermostat display is coloured black and the temp is at what the thermostat is set to. It will run on high for a bit spool down for a bit and then speed up again.

This only seems to happen in the evening and appears to be okay throughout the day and overnight. It has me baffled.

Nest support figure it's an incompatibility issue with the fan and suggest I disconnect the fan wire, so it won't run when set to "ON".

We never had this problem with our low end Honeywell programmable thermostat and we like to have the fan running all the time.

Has anyone else run into this issue?

Thank you for setting up this forum. They are a great tool for information sharing.

Hi Phil.

I am a homeowner, not an installer or heating expert. I've had the Nest for about a week and can offer a few observations in case you have not reached Nest support or gotten answers to your questions about fan speeds. Hope it helps.

1) I did not know it previous to installing the Nest, but my house has a Trol-A-Temp system which is designed to bring in fresh air via a controlled damper to the house for 15 minute intervals (no heating or cooling involved) several times a day. This does seem to override whtever Nest would like to do at any time it is set to activate. The control panel is mounted near the ceiling and I had never noticed it until I had a fall service visit to check over the unit and the service person pointed it out and showed me how to program it. If you have this type of device, it could account for your fan speeding up seemingly while not calling for heat. Question: when you observed the fan at high and the Nest black/at target temp, was there any heat coming out of the heating duct?

2) I can confirm that when I set my Nest fan control to ON, it runs at slow speed, and the fan speeds up when heating, or when my fresh air system calls for a cycle of outside air.

3) After letting the thermostat learn our temp settings and schedules, I have begun playing with manual manipulation by disabling the auto-away, auto-sched and early on settings. This will compromise teh energy benefits of the Nest temporarily, but I am interested in trying a few things and have found this to be a way to "take control", tuning these features on individualloy for a period of days to separate behaviors. Now, since this is a learning thermostat, I know it may take a few days for any one change to normalize and it's just experimenting. My point is, you could disable one of these at a time and see if the behavior you are seeing goes away with any one of them.

Question: at teh time when the high fan/no heat needed behavior occurred, were you approaching another temp setting timeframe? Meaning, were you approaching a night setting of, say, 65? I wonder if the early on setting would anticipate the desired lower temp and start circulating air (hopefully not firing up air conditioning) to try to reach the lower target at teh specified time. This does not seem probable, as Nest designeders would be unlikely to waste energy cooling a heated house sooner during winter, just a thought.

Scott

Hey Scott;

When the fan randomly cuts into high it's just blowing cool air.

Unfortunately the problem isn't being caused by the conditions you had noted. The furnace fan has never reacted this way in the past with our other programmable thermostat and we used that one for about 5 yrs or more.

Hopefully the suggested incompatibility by Nest support will be rectified or I can figure out what may be possibly triggering it.

I appreciate your response.

Thanks

Phil