The Thermostat Fan Setting Dilemma
Every thermostat has a switch for fan operation: AUTO (runs only when heating or cooling is active) and ON (runs the indoor blower continuously 24/7). While leaving the fan ON maximizes air filtration, it can ruin indoor humidity control during hot summer months.
Summer Moisture Warning
When the air conditioning compressor shuts off, approximately 1 to 2 pounds of liquid water remains clinging to the evaporator coil. If the fan continues running on 'ON,' that moisture evaporates back into your home within 15 minutes, driving indoor relative humidity back above 65%.
Seasonal Rules of Thumb
- Summer Cooling: Set to AUTO. Protect dehumidification by letting moisture drain down the condensate pipe when the compressor cycles off.
- Winter Heating: Set to ON (or Circulation Mode). Continuous air movement evens out hot and cold spots across multi-story homes and provides continuous MERV filtration.
- Use Intermittent Circulation: Modern smart thermostats offer a 'Circulate' mode, running the fan for 15–20 minutes every hour to balance air without constant power draw.
About ClariSelect Editorial & Research Standards:
Sizing measurements, static pressure curves, and filtration ratings referenced across our educational guides are compiled from published ANSI/ASHRAE Standard 52.2 engineering data, AHAM AC-1 verified ratings, and manufacturer technical documentation. ClariSelect provides objective editorial research and analysis with zero sponsored placements.