Fan "On" vs "Auto"
Auto runs the blower fan only while your system is heating or cooling. On keeps it running all the time, including between cycles. Sensi's manuals say to leave the fan on Auto so the thermostat controls it. Pick On if you want constant air movement, but Google warns that long fan use can raise your electric bill.
What is the difference between fan On and Auto?
The fan setting decides when the blower runs. It does not change whether your home is heated or cooled.
On Auto, the fan follows the system. Honeywell's T6 Pro and VisionPRO guides word it the same way: "Auto: Fan runs only when the heating or cooling system is on." When a cycle ends, the fan stops with it. Sensi says that on Auto the blower comes on only when the system needs it.
With the fan set to On, it never stops. Honeywell's line is "On: Fan is always on." Sensi says the blower runs continuously all day. Between heating or cooling cycles, the fan keeps moving air through the ducts.
| Auto | On | |
|---|---|---|
| When the blower runs | Only during heating or cooling | All the time |
| Between cycles | Off | Still running |
| Manufacturer guidance | Sensi manuals: leave the fan on Auto | Google: may help even out temperatures between rooms |
| Running cost | Fan runtime limited to the heating and cooling runtime | Google: can use a lot of energy and raise your bill |
| Air filter | Not stated | Google: may need replacing sooner |
When should you use Auto?
For most homes, most of the time. Both Sensi manuals give the same advice: leave the fan mode on Auto to allow the thermostat to control the fan as necessary. It also keeps things easy to read: when the room is quiet, nothing is running, and when you hear air, the system is heating or cooling.
Some thermostats can keep the fan going for a short time after each cycle without you switching to On. The Honeywell T6 Pro Smart has installer options for this: 390 for cooling and 391 for heating. Both are off from the factory and can be set to 30, 60 or 90 seconds or 2 to 15 minutes. For the cooling version, Honeywell warns that this "may reintroduce humidity into the living space."
When should you use On?
When you want air moving constantly, for example because some rooms run warmer or cooler than others. Google says longer fan usage may help even out the temperatures between rooms.
Keep in mind how long On lasts. On a Honeywell VisionPRO, the manual says a choice of On or Circ stays active until you press FAN again and select Auto. On an ecobee, a Comfort Setting with the fan set to On runs it constantly for the whole Comfort Setting. ecobee also has "Fan Hold" (called "Fan Timer" on the Smart Thermostat Premium, Enhanced and Lite), reached through Main Menu > Fan > On. Google Nest has a manual fan timer as well.
Is there a setting between On and Auto?
Most brands offer one. It runs the fan for part of every hour, not all of it. The details differ by brand and even by model:
| Brand and model | Setting | How much the fan runs |
|---|---|---|
| Honeywell Home T6 Pro Programmable | "Circ" | Randomly, about 33% of the time |
| Honeywell Home VisionPRO TH8000 | "Circ" | Randomly, about 35% of the time, not counting heating or cooling runtime |
| Honeywell Home T5, T5+, T6 Pro Wi-Fi | "Circulate" | 30% of the time every hour |
| Honeywell Home T4 Pro | None (Auto and On only) | Not applicable |
| ecobee | "Fan minimum runtime" | Minutes per hour you choose; over 5 minutes is spread evenly across the hour |
| Google Nest | Fan schedule | You choose 15, 30, 45 or 60 minutes of operation |
| Emerson Sensi | "Circulating Fan" | 10% to 100% in 5% steps (10% to 95% at a Sensi Lite thermostat); off by default |
Sensi gives a worked example: at 50%, the fan runs 30 minutes every hour, and any heating or cooling runtime in that hour counts toward it. ecobee's minimum runtime works on the same idea and runs the fan regardless of the selected System Mode. On an ecobee in Auto, the fan simply follows the Fan Minimum Run Time setting. Our page on the fan Circulate setting covers these options in more depth.
Where to set the ecobee minimum runtime depends on the model. On the Smart Thermostat Premium and Enhanced it is Main Menu > General > System > Fan. On the ecobee3, ecobee3 lite, ecobee4 and SmartThermostat with voice control it is Main Menu > Fan > Auto, and on the Lite and Essential, Main Menu > Fan Mode > Auto. In the app it is Menu > System > Fan. On a Nest, the fan schedule is in the Google Home app under Settings > Schedules > Fan. On the Nest Learning Thermostat (3rd gen) and Nest Thermostat E, that schedule cannot change from day to day.
Which one costs more to run?
On, because the fan runs for more hours. How much more depends on how long your system would otherwise sit idle each day, and on the fan itself. Google is the only manufacturer here that addresses cost: it says the fan can use a lot of energy and increase your electric bill, and that extended fan use increases system runtime, which may require replacing the air filter sooner. Honeywell, ecobee and Sensi do not publish a figure, and we do not estimate one.
A part-time setting sits in between: the fan moves air every hour but runs for fewer hours than it would on On.
Why might the fan run when the system is set to Off?
Not every fan feature stops when you turn heating and cooling off. On a Nest, Google says the fan schedule does not run while the thermostat is Off, but a fan timer you start by hand does. On an ecobee, the fan minimum runtime runs no matter which System Mode is selected. And a fan left on On keeps running until you change it. If your fan seems to run on its own, see fan runs when the system is off.
Sensi notes a related mix-up in its Touch 2 manual: with the fan on, it may have sounded like the system was heating or cooling, but only the fan was running.
Does every thermostat let you choose On?
Not always. Google says a Nest thermostat needs a G wire to run the fan by itself. If there is none, the fan only runs automatically when the system is heating or cooling. Whether yours has one depends on how the system was wired when it was installed.
Frequently asked questions
- Is it bad to leave the thermostat fan on all the time?
- Of the manufacturers covered here, only Google comments on it. Google says longer fan use can help even out temperatures between rooms, but the fan can use a lot of energy, and the extra runtime may mean replacing the air filter sooner. Honeywell, ecobee and Sensi describe On without giving a cost.
- Does setting the fan to On cool the house?
- No. On only moves air through the system; the heating or cooling still runs only when the thermostat calls for it. Sensi points out that with the fan on, it may sound as if the system is heating or cooling when only the fan is running.
- Why does my Nest fan only run when the heat or air conditioner is on?
- Google says a Nest thermostat needs a G wire connected to run the fan by itself. Without one, the fan will only run automatically when the system is heating or cooling. Whether you have that wire depends on how the system was installed.
- Is Circulate the same as fan On?
- No. Circulate runs the fan for part of each hour, while On runs it nonstop. On Honeywell Home it depends on the model: about 33% on the T6 Pro, about 35% on the VisionPRO TH8000 and 30% on the T5, T5+ and T6 Pro Wi-Fi. The T4 Pro has no Circulate mode at all, only Auto and On.