Should I Run My Furnace/Air Handler Fan in “Auto” or “On” Mode?

Response by Laura Titensor, Customer Experience Advisor

The honest answer? It depends.

But before we get into recommendations, let’s walk through what each setting actually does—and how it affects your comfort, energy use, and air quality.

Fan “Auto” Mode

When your thermostat is set to “Fan Auto,” the blower only runs when your system is actively heating or cooling your home.

In other words, once your home reaches the desired temperature, the fan shuts off—and air stops circulating until the next heating or cooling cycle begins.

Fan “On” Mode

When your thermostat is set to “Fan On,” the blower runs continuously, circulating air throughout your home all the time—whether your system is heating, cooling, or neither.

Benefits of Running the Fan in “On” Mode

There are two main advantages to running your fan continuously:

1. More Even Comfort Throughout Your Home

Continuous air circulation helps balance temperatures between rooms and levels.

For example, in a multi-level home:

• Cooler air from downstairs gets pushed upward

• Warmer air from upstairs gets redistributed to the lower levels

The result? Fewer hot and cold spots—and a more consistently comfortable home.

2. Continuous Air Cleaning

Whole-home air purification systems can only clean the air that passes through them.

When your fan is off, air isn’t moving—and it isn’t being cleaned.

If you have a system like a NovusAer, running the fan continuously means:

• Air is constantly being filtered

• Clean air is continuously distributed throughout your home

This is how you get the cleanest air possible, all day long—not just during heating or cooling cycles.

Downsides of “On” Mode

Like most things, there are a couple of trade-offs to consider:

1. Increased Energy Use

Running the fan all the time does use more electricity.

However, there are a couple of important things to keep in mind:

• Most modern systems run the fan at low speed when not actively heating or cooling

• Low-speed operation uses significantly less energy

• Continuous operation reduces frequent start/stop cycles, which is less energy efficient and harder on the motor over time

2. Air Movement (Drafts)

Continuous circulation means you’ll feel more air movement in your home.

Some people love this—it makes the air feel fresh and less “stale.”Others may find it slightly drafty.

The good news:

• In “On” mode (outside of heating/cooling), the fan runs at a lower speed, so airflow is gentle—not overpowering

Our Recommendation

If your priorities are comfort and clean air, especially if you have a whole-home air purification system like a NovusAer, we recommend using “Fan On” mode to keep air moving and continuously purified.

If your priority is minimizing energy use and reducing air movement, then “Fan Auto” mode is the better choice.

Bottom Line

• Want the cleanest, most consistent air? → Use “On”

• Want to save a bit on energy and keep things still? → Use “Auto”