Set it to Auto for the summer. With the fan on Auto, the blower only runs while the system is actively cooling. With it on On, the blower runs 24/7, and in cooling season that works against you in two specific ways.
At a glance: Auto vs On vs Circulate
| Setting | Fan runs | Cost/month est. | Humidity control | Best for |
|---|---|---|---|---|
| Auto | Only during cooling cycle | Low — fan uses 200–500W only when needed | Best — coil drains between cycles | Most homes |
| On | Constantly | +$15–50/mo extra | Worse — re-evaporates coil moisture | HEPA filtration, dry climates only |
| Circulate | Brief runs every hour (10–20 min) | Low-medium | Neutral | Even temps without humidity penalty |
The table above covers 90% of decisions. The sections below explain the mechanics behind each column.
Why Auto wins in cooling season
When your AC runs, water condenses on the cold evaporator coil. That’s how it pulls humidity out of the air. The compressor cycles off, but the coil is still wet for a few minutes.
- On Auto: the blower stops with the compressor. The condensate drips down the coil and out the condensate drain. The moisture leaves the house.
- On On: the blower keeps pushing air across that wet coil and re-evaporates the water back into your rooms. You feel it as clammy, sticky air even though the thermostat reads the right temperature.
So in a humid climate, “On” can make your house feel worse while reading the same number on the wall.
The second cost is energy. A continuously running blower draws roughly 300 to 700W (less on a modern ECM/variable-speed motor). Running it around the clock instead of 30 to 40% of the time adds up on a summer bill, and it loads your filter faster.
The humidity mechanism, exactly
Here is the step-by-step of what happens in your air handler during a cooling cycle:
- The compressor starts, refrigerant flows through the evaporator coil, and the coil surface drops to around 40–55°F.
- Warm indoor air passes over the coil. Moisture in that air condenses on the cold metal surface.
- The compressor cycles off when the thermostat setpoint is reached. The coil is now coated in liquid water.
- That water is supposed to drip into the condensate pan and drain out of the house through the condensate line.
In Auto mode, the blower shuts off with the compressor. The condensate has 5–10 minutes to drain before the next cycle. It leaves the house as liquid water.
In On mode, the blower keeps running after the compressor stops. Air is still moving across the wet coil. That air picks up the moisture via evaporation and carries it back into your living space. Your AC just pulled that water out of the air, and the fan just put it back. The result: indoor relative humidity climbs, and the house feels clammy at the same thermostat reading.
This is why people sometimes complain their AC is running fine but the house still feels humid. If the fan is set to On, that is often the direct cause.
The Circulate setting: a real middle ground
Many smart thermostats have a third option between Auto and On. The naming varies by brand, but the function is the same: the blower runs for a set number of minutes per hour regardless of cooling demand.
Nest: In the app, go to Settings > Equipment > Fan, or tap the fan icon on the main screen. You can set a minimum fan runtime per hour (e.g., 15 minutes per hour). Google’s support calls this a “Fan Schedule.”
ecobee: Go to Main Menu > Fan > Fan Mode, then select “Auto” or “On.” For a timed middle ground, go to Settings > Installation > Fan Control > Fan Minimum On Time. Set it to 10–20 minutes per hour. ecobee’s SmartRecovery and Comfort settings may also trigger the fan independently.
Honeywell T6 Pro: Press the Fan button on the thermostat face. Toggle between Auto, On, and Circulate. Circulate runs the fan approximately 35% of the time, unpredictably distributed, which avoids the full-time humidity problem.
Honeywell T9 / T10 Pro: Open the app, tap the thermostat card, then Fan. Select Circulate. The default is 10 minutes per hour; you can adjust this in the app settings.
Circulate gives you 10–20 minutes per hour of airflow. That is enough to move air between rooms, catch particulates on the filter, and even out temperature stratification. The coil still has most of the hour to drain. You get most of the benefit of “On” without the humidity penalty.
The cost math: how much does continuous fan actually add?
A standard PSC (permanent split capacitor) blower motor draws 300–500W. A newer ECM (electronically commutated motor) draws 80–200W at lower speeds.
If cooling cycles run about 8 hours per day in peak summer, the blower runs 8 hours on Auto. On On mode, it runs 24 hours.
16 extra hours/day × 300W = 4.8 kWh/day 4.8 kWh × 30 days = 144 kWh/month 144 kWh × $0.13/kWh (US average) = about $19/month extra just for the fan
With a PSC motor at 500W, that climbs to about $31/month. With an ECM at 150W, it drops to about $10/month. These are the numbers behind the “+$15–50/mo” range in the table above. Actual cost depends on your motor, your climate, and your rate.
That number may not sound large, but it is in addition to your AC compressor cost, and it is entirely avoidable by switching to Auto.
When “On” is actually appropriate
“On” is not always wrong. There are three situations where it makes sense:
Whole-house HEPA filtration. If your air handler has a high-efficiency filter (MERV 13+) or a dedicated whole-house air purifier, the filter only works when air is moving through it. If someone in the household has severe allergies or asthma, continuous filtration may matter more than the humidity or cost tradeoff. In that case, pair “On” with a whole-house dehumidifier to offset the moisture problem.
Significant temperature stratification in a dry climate. If upstairs is 10°F warmer than the thermostat location and you are in Phoenix or Albuquerque, the humidity argument disappears. Re-evaporating a small amount of coil moisture into already-dry air is not a problem. Continuous airflow to even out temperatures is a reasonable trade.
When you already have a whole-house dehumidifier. If a whole-house dehumidifier is handling moisture independently, the main objection to “On” is gone. The dehumidifier will pull back what the fan re-evaporates.
If none of those apply, Auto or Circulate is the right call.
How to change the setting on your thermostat
- Nest: Tap the settings gear > Equipment > Fan, or use the fan icon on the home screen. Select Auto, On, or set a minimum fan runtime per hour.
- ecobee: Main Menu > Fan > Fan Mode. Toggle between Auto and On. For minimum runtime, go to Settings > Installation > Fan Control.
- Honeywell T6 Pro: Press the Fan button on the thermostat face. Cycle through Auto / On / Circulate.
- Honeywell T9: Open the Honeywell Home app > thermostat card > Fan. Select your mode.
The setting applies immediately. No system restart needed.
FAQ
Should the fan be on Auto or On in summer? Auto. In cooling season, running the fan continuously causes the blower to re-evaporate moisture from the evaporator coil back into your house. Auto stops the blower when the cooling cycle ends, so condensate drains properly and your AC actually reduces humidity.
Does running the fan on “On” cool the house faster? No. The fan alone does not cool anything. Cooling comes from the compressor and refrigerant cycle. Running the fan between cycles just moves room-temperature air around and re-evaporates coil moisture. It does not speed up the cooling process and may make the house feel warmer by raising humidity.
Why does my house feel humid even though the AC is running? Check your fan setting first. If the fan is set to On, it is likely re-evaporating condensate back into the air between cooling cycles. Switch to Auto and give the system an hour to stabilize. If the house still feels humid on Auto, the issue may be an oversized AC unit (short cycles, coil does not get cold enough to condense much) or a refrigerant problem. See AC running but not cooling enough for next steps.
What is the “Circulate” fan setting on a thermostat? Circulate (or a minimum fan runtime setting) runs the blower for a set number of minutes each hour, even when there is no cooling demand. It gives you air movement and filtration without running the fan all day. Most smart thermostats let you set 10–30 minutes per hour. It is the best default if you want more than just Auto but do not want the humidity and cost hit of full “On.”
A note on winter
The same logic flips slightly in heating season. There is no condensate to re-evaporate, so the humidity argument disappears. That is a separate decision; see fan on or auto in winter for the heating-season tradeoffs.
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Related: Best Thermostat Settings for Summer · AC running but not cooling enough? · Is your house humid, not hot?
