Is it bad if the fan runs while the system is off?

Not if a fan setting is calling for it. Running the fan only moves air; it does not heat or cool. Google notes that long fan use can raise your electric bill and may mean replacing the air filter sooner, so turn off the setting if you did not mean to use it.

Why does my ecobee fan run when the System Mode is Off?

ecobee says both the fan minimum runtime and a Fan Hold run regardless of the System Mode. Set the minimum runtime to zero, or cancel the Fan Hold, to stop it. Holds are hidden while the mode is Off, so switch to another mode first to see them.

Why did my Nest fan stop when I turned the thermostat off?

If the fan was running on a fan schedule, that is expected. Google says a fan schedule cannot start while the thermostat is set to Off. Only a fan you started by hand with the fan timer keeps running in Off.

The fan keeps running after the heat shuts off. Is that the thermostat?

It can be. ecobee has Heat and Cool Dissipation Time settings that run the fan after a cycle, and the Honeywell Home T6 Pro Smart has installer options that keep the fan on for up to 15 minutes after a call ends. If neither is set and the fan never stops, the cause is past the thermostat.

Fan keeps running when the system is off

Usually a fan setting is still active. Off does not change the fan mode, so a fan left on On keeps running. An ecobee fan minimum runtime or Fan Hold and a Nest fan timer started by hand also run with the system off. If no fan setting is active, the cause is beyond the thermostat.

Why would the fan run with the system set to Off?

Because Off and the fan are two different controls. The system mode decides whether the thermostat calls for heating or cooling. The fan mode decides when the blower runs. On a Honeywell Home T6 Pro they even have separate buttons, Mode and Fan, and the VisionPRO TH8000 has separate SYSTEM and FAN keys. Switching the system to Off leaves the fan mode where it was.

So the first thing to look at is the fan setting on the screen. If it says On, the fan is doing what it was told. Honeywell's T6 Pro manual describes On as "Fan is always on." Sensi says that with the fan On, "your blower fan will run continuously all day." Set the fan to Auto and it should stop once no heating or cooling is running.

The Off mode page covers everything else that Off leaves running.

Which fan settings keep running in Off?

Beyond plain On, several part-time and timed fan features carry on with the system off. Each brand handles this its own way:

Fan features that can run while the system is set to Off
Thermostat	Runs with the system Off	Does not run in Off
ecobee	Fan minimum runtime; Fan Hold (Fan Timer on Premium, Enhanced and Lite)	A Comfort Setting with the fan on On only runs it in Heat, Cool, Auto or Aux
Google Nest Thermostat, Thermostat E, Learning Thermostat (3rd and 4th gen)	A fan timer you started by hand	The fan schedule
Honeywell Home VisionPRO TH8000	Not stated for Off; On or Circ stays active until you choose Auto	Not stated
Honeywell Home T6 Pro	Not stated for Off; the fan is set separately	Not stated
Emerson Sensi	Not stated for Off; On runs the blower continuously	Not stated

From the ecobee, Google Nest Help, Resideo and Copeland pages listed under Sources. Not stated means the pages we checked do not say.

ecobee is the brand most likely to surprise you. Its fan minimum runtime, set in minutes per hour, runs the fan "regardless of the selected System Mode." So does a Fan Hold. To stop it, set the minimum runtime to zero or cancel the hold. ecobee adds that holds stay active while the System Mode is Off but cannot be seen, so you need to switch to another mode to check for one. The Circulate page explains how these part-time settings work.

On a Nest, it depends how the fan was started. A fan schedule will not start while the thermostat is Off. A fan you turn on by hand with the fan timer will keep running in Off until its timer ends or you press Stop. Google's Off mode page also notes that a Nest in Off still heats or cools to hold Safety Temperatures, and Google's fan page says forced-air systems run the fan whenever they are heating or cooling.

Does the fan keep running for a while after a cycle?

Some thermostats are set up to do this on purpose. If you switched to Off just as a cycle ended, the fan may keep going for a few minutes:

ecobee. "Heat Dissipation Time" and "Cool Dissipation Time," in the Thresholds menu, keep the fan running after a heating or cooling cycle to move the remaining warm or cool air.
Honeywell Home T6 Pro Smart. Installer options 390 (cooling) and 391 (heating) keep the fan on after the call ends. Both are off from the factory and go up to 15 minutes.

ecobee also has a setting called "Fan control in Heat Mode." When it is set to HVAC, the ecobee hands control of the fan to the furnace during heating cycles. In that case the furnace, not the thermostat, decides when the fan starts and stops during heating.

Anything longer than these timers, with no fan setting active, is not explained by the thermostat.

What should you check on the thermostat?

The fan runs with the system set to Off

Does the fan setting on the screen or in the app say On?

Yes
That is why. Set the fan to Auto and it stops when no heating or cooling is running.
No
Is a part-time fan, fan timer, Fan Hold or fan schedule active?
- Yes
  Some of these run with the system off. Turn it off, cancel the hold or let the timer end.
- No
  Did the fan start right after a heating or cooling cycle, and stop within about 15 minutes?
  - Yes
    A fan run after the cycle is probably set. Check ecobee Dissipation Time, or ask the installer about Honeywell options 390 and 391.
  - No
    The thermostat is not asking for the fan. The cause is beyond the thermostat, so call a licensed HVAC technician, and check the signs at the bottom of this page.

If the fan stopped when you expected it to keep running, that has its own explanation: on a Nest, a fan schedule does not start in Off. And if the issue is the system rather than the fan turning on by itself, see thermostat switches between heat and cool and Auto mode.

When is a running fan not a thermostat problem?

When the thermostat shows the fan on Auto, no part-time fan, timer or hold is active, the system is Off, and the blower runs on regardless. At that point the thermostat is not calling for the fan, so the cause is past the thermostat, and that work belongs with a licensed HVAC technician. Leave the thermostat set to Off with the fan on Auto so the technician sees the same behavior you do.

For brand-specific fan menus, see the hubs for Honeywell Home, ecobee, Google Nest and Emerson Sensi.

Frequently asked questions

Is it bad if the fan runs while the system is off?: Not if a fan setting is calling for it. Running the fan only moves air; it does not heat or cool. Google notes that long fan use can raise your electric bill and may mean replacing the air filter sooner, so turn off the setting if you did not mean to use it.
Why does my ecobee fan run when the System Mode is Off?: ecobee says both the fan minimum runtime and a Fan Hold run regardless of the System Mode. Set the minimum runtime to zero, or cancel the Fan Hold, to stop it. Holds are hidden while the mode is Off, so switch to another mode first to see them.
Why did my Nest fan stop when I turned the thermostat off?: If the fan was running on a fan schedule, that is expected. Google says a fan schedule cannot start while the thermostat is set to Off. Only a fan you started by hand with the fan timer keeps running in Off.
The fan keeps running after the heat shuts off. Is that the thermostat?: It can be. ecobee has Heat and Cool Dissipation Time settings that run the fan after a cycle, and the Honeywell Home T6 Pro Smart has installer options that keep the fan on for up to 15 minutes after a call ends. If neither is set and the fan never stops, the cause is past the thermostat.