Short answer: For most Ontario homes, leave the furnace fan on AUTO, especially in summer, and switch to ON or a thermostat’s Circulate setting only for a reason: rooms on different floors that never feel the same, allergy season with a good filter, or smoky days. Running the fan around the clock costs little with a modern variable-speed (ECM) blower and a lot with an older single-speed (PSC) blower, and in cooling season it can put moisture back into the house.
Last reviewed: September 2026
What ON, AUTO and Circulate actually do
AUTO: the blower runs only while the furnace or air conditioner is heating or cooling. ON: the blower runs all the time, moving air through the filter and ducts between cycles without heating or cooling it. Circulate (on many newer thermostats) runs the fan for part of every hour as a middle ground.
What running the fan all the time costs
The blower motor decides the cost. According to the U.S. Department of Energy’s Building America Solution Center, a single-speed PSC blower motor typically draws 500 to 550 watts, while a variable-speed ECM motor uses about 80 watts in continuous fan mode.
| Blower motor | Typical draw on continuous fan | Energy if it runs 24 hours a day for 30 days |
|---|---|---|
| Older single-speed (PSC) | 500 to 550 W | About 360 to 400 kWh |
| Variable-speed (ECM) | About 80 W | About 58 kWh |
To estimate your own cost, multiply the kWh by the all-in price per kWh on your Ontario electricity bill. The furnace manual or rating plate usually says whether the blower is variable-speed (ECM); a technician can confirm it at the next maintenance visit.
Summer: why AUTO usually wins
When the air conditioner runs, its cold indoor coil pulls moisture out of the air and the water drains away. If the fan keeps blowing after the compressor stops, some of the water still on the coil evaporates back into your home. In a humid Southern Ontario summer that can leave rooms feeling clammy even at the right temperature, so AUTO is the better default from June to September. If the house still feels sticky on AUTO, work through why an air conditioner stops removing humidity.
Winter: when ON helps and when it does not
ON evens out temperatures between floors, which helps in two-storey homes and finished basements that run cold. The trade-offs: between heating cycles the fan moves unheated air, which some people feel as a draft near the vents, and with a PSC motor the electricity adds up over a long winter. A Circulate setting is often the best compromise.
When running the fan is worth it
- Uneven temperatures: upstairs and downstairs differ by a few degrees for most of the day.
- Allergies or pets: more air passes through the filter, which only helps if the filter is a good one and is changed on time. See how to improve indoor air quality and home air treatment options.
- Smoky or high-pollen days: continuous circulation through a higher-rated filter can lower indoor particles, as long as the system is designed for that filter.
- An HRV that uses the furnace blower to spread fresh air through the house: check how your HRV is set up before changing the fan setting.
What to watch if you keep the fan on
- Check the filter every month; more air means it loads faster.
- Listen for new noises. A blower that squeals, rattles or hums is worth a look before it fails; see furnace repair.
- Keep supply and return vents clear so the blower is not working against blocked airflow.
- Book maintenance once a year: how often a furnace needs service and AC maintenance in Mississauga.
Not sure which blower motor your system has? Aire One Peel technicians can check the motor type, the filter and the airflow during a regular maintenance visit. Call 905-564-8545 or contact us.
Frequently asked questions
Does running the furnace fan all the time use a lot of electricity?
It depends on the blower motor. An older single-speed PSC motor typically draws 500 to 550 watts, which is about 360 to 400 kWh a month if it never stops. A variable-speed ECM motor uses about 80 watts on continuous fan, or about 58 kWh a month (U.S. Department of Energy, Building America Solution Center). Multiply the kWh by the all-in price per kWh on your electricity bill to estimate the cost.
Does running the HVAC fan reduce humidity?
Not in summer. With the fan on ON, moisture left on the cold air conditioner coil evaporates back into the house between cooling cycles, so indoor humidity can rise. AUTO stops the fan with the compressor and lets that water drain away, so it is the better setting for humidity control.
Is it bad for the furnace to run the fan all the time?
Running the fan continuously adds hours to the blower motor and loads the filter faster, but blowers are built to run for long periods. The bigger risks are a clogged filter and new noises that go unchecked, so check the filter monthly and keep up with yearly maintenance.
What does the Circulate setting on a thermostat do?
Circulate runs the fan for part of every hour even when there is no call for heating or cooling. It mixes the air between floors and pushes more air through the filter, at a fraction of the running time of ON. How much of each hour it runs depends on the thermostat model.
How often should I change the filter if the fan runs all the time?
Check it every month and change it when it looks grey or clogged. Because far more air passes through it than on AUTO, expect to replace it sooner than the interval printed on the package.