From a pure energy-efficiency standpoint, the ranking is:
AUTO → CIRC → ON
Here is the rationale, clearly differentiated.
1. AUTO — Most Efficient
- Fan runs only when heating or cooling is actively operating
- Lowest electrical consumption
- Prevents unnecessary duct losses
- Best humidity control during cooling
- Least wear on the blower motor
This is the default and most efficient choice for the majority of homes and systems.
2. CIRC (Circulate) — Balanced / Moderately Efficient
- Fan runs intermittently when heating or cooling is not running
- Designed to improve air mixing without continuous operation
- Uses more energy than AUTO, but significantly less than ON
- Can improve temperature balance in multi-story homes
Efficiency depends on:
- How many minutes per hour the fan runs
- Blower motor type (ECM vs PSC)
For homes with comfort imbalance or filtration needs, CIRC is often the best compromise.
3. ON — Least Efficient
- Fan runs continuously
- Highest electrical usage
- Can increase indoor humidity in cooling mode
- Accelerates blower wear
- Pulls air through ducts even when not conditioning
Generally not recommended for efficiency, especially with older single-speed systems.
Equipment-Specific Notes
- Variable-speed (ECM) blowers: Reduce the penalty of CIRC and ON, but AUTO is still most efficient
- Older PSC motors: ON can materially increase energy use
- Cooling season: AUTO strongly preferred to avoid moisture re-evaporation
- Heating season: AUTO still most efficient; benefits of ON are minimal
Bottom Line
- Maximum efficiency: AUTO
- Best efficiency/comfort balance: CIRC
- Use ON only deliberately, not as a default