Should Your AC Fan Be On or Auto? What the Humidity Research Shows

What actually happens when the fan stays on

During a cooling run, water condenses on the indoor coil and drips into the drain pan. With the fan on Auto, the fan stops with the compressor and most of that water drains away. With the fan on On, air keeps flowing over the wet coil and pan.

Researchers at the Florida Solar Energy Center measured this on two systems (Khattar, Swami and Ramanan, FSEC-PF-118-87, published in ASHRAE Transactions):

The authors also found that the re-evaporation rate grew as the number of cycles increased, and they concluded that cycling the compressor and fan together gives better dehumidification. For extra air movement they suggested ceiling or paddle fans instead. A later ASHRAE Journal article by FSEC researchers confirmed that a coil's moisture removal degrades at part load, especially when the supply fan runs continuously.

A real-home example: building scientist Allison Bailes switched his own condo's thermostat from Auto to On for five days. Relative humidity averaged about 70% with the fan On, compared with about 58 to 59% on Auto.

Why 70% humidity matters

The EPA advises keeping indoor relative humidity below 60 percent, ideally between 30 and 50 percent, to limit mold. The Building America Solution Center gives the same 60% ceiling for homes in warm-humid climates. A setting that pushes a home from the high 50s to around 70 moves it the wrong way.

On vs Auto vs Circulate

The duct point comes from Bailes' list of reasons: running the fan constantly increases total duct leakage, and unbalanced leakage can also pull more outside air through the house. If your ducts run through a hot attic, that is hot, humid air.

When fan On can make sense

• Dry climates. ACCA's technical expert put it as a location-specific answer: in arid climates there is little downside, while in humid "wet coil" climates, fan On generally raises indoor humidity.
• Heating season. The coil is not wet, so there is nothing to re-evaporate.
• A room that is always hotter or colder. Mixing air can even things out. Try Circulate first.
• Short-term filtration, for example during wildfire smoke, if humidity stays in range.

Test it in your own home

1. Buy a humidity meter. The EPA notes they cost about $10 to $50 at hardware stores.
2. Run the fan on Auto for three or four days in normal summer weather. Write down the humidity morning and evening.
3. Switch to On (or Circulate) for the same number of days at the same temperature setting.
4. Compare. If humidity rises toward or above 60% on the second setting, go back to Auto.

FSEC's own guidance for Florida homes is to run the supply fan in Auto mode with no fan overrun after the compressor stops (FSEC/UCF). "Fan overrun" is a delay some systems use to keep the blower going after cooling ends; if your house runs humid even on Auto, ask a technician whether that delay is set and whether your system is oversized.