The short answer
Set it to AUTO for most Northern Virginia homes. ON runs the blower around the clock, heating or not. On an older standard blower motor that’s about 500 watts, around 12 kilowatt hours a day by our math, and in summer it puts moisture back into the house.
The part most people don’t know
The fan switch doesn’t change how much heat or cooling you get. It only decides whether the blower stops when the furnace or AC stops. On AUTO, the blower runs only while the system is heating or cooling. On ON, it never stops.
That blower can be a big electric load. In a field study of 31 Wisconsin homes, standard furnace blower motors averaged about 500 watts, and the newer variable-speed motors (ECMs) about 100. Five of the 31 homes ran the fan nonstop all year.
Run the numbers on a standard motor: 500 watts for 24 hours is about 12 kilowatt hours a day, about 360 over a 30-day bill. That’s the fan alone, before any heating or cooling.
The $67 Heating Check
We check your furnace, heat pump or boiler before winter. Through December 31. See the offer
Even the efficient motors give some of it back
An ACEEE paper on those efficient motors found that a 400 watt difference adds up to nearly 3,000 kilowatt hours over 7,400 hours of fan-only running. The catch it found: once owners had a quiet, cheap-to-run fan, contractors told them to leave it ON, which ate into the savings.
Since July 3, 2019, new furnaces have had to meet a federal fan efficiency rule. DOE projected that furnace fans bought from 2019 through 2048 would save 3.99 quadrillion Btu. A furnace older than that may well have the older, hungrier motor. If yours is getting up there, see old furnace efficiency.
In summer, ON puts the water back
An air conditioner dries the air by condensing water on its cold coil, which drips into a drain. On AUTO, the blower stops when the AC stops and that water drains away. On ON, air keeps blowing across the wet coil and some of that water evaporates back into the house, per the Florida Solar Energy Center.
In an earlier FSEC test of a 10-minute cycle, running the fan ON cut the moisture the AC removed by 44 percent compared with AUTO. That’s how a house at 75 still feels sticky. More in how to reduce humidity in your house.
Why the vents blow cool air on ON
In winter, with the fan ON, the blower keeps pushing air between heating cycles. That air runs through ducts in a cool basement or attic and comes out at room temperature or colder, so it feels like a draft. Plenty of people read that as a broken furnace. Switch to AUTO and the cool drafts stop. If it’s still running nonstop on AUTO, see furnace running constantly, and our error codes and warning lights guide covers what a blinking furnace light means.
When ON or circulate makes sense
ON isn’t always wrong. With a variable-speed blower, or a thermostat with a circulate mode that runs the fan part of each hour, a little extra fan time can even out a house with a warm upstairs. Just know what it costs, and that in summer it costs dryness too. If one room runs cold, the fix is usually airflow to that room: cold room in the house. For the rest of the thermostat, see the best thermostat setting for winter and thermostat not working.
What we check
Our heating check is $67 per system, through December 31: furnace, heat pump or boiler. On a furnace, for $229 (regularly $467) we also pull the blower wheel and clean it blade by blade, fit a new filter, set your dampers for winter and tune the furnace. Ask the tech what kind of blower motor you have, so you know what ON costs at your house.
Book it
Book the heating check, call (703) 997-9757, or book online from goswiftpro.com/cookiesoffer and a box of warm cookies comes with the visit. Rated 5.0 stars across 1,138 reviews on Google, Angi, Yelp, the BBB and Facebook, Virginia Class A license 2705192276.
Questions people ask
Should my thermostat fan be set to ON or AUTO?
AUTO for most homes. The blower runs only while the system heats or cools, which uses less electricity and lets the AC dry the air in summer.
How much electricity does running the furnace fan all the time use?
On an older standard motor, about 500 watts, which is about 12 kilowatt hours a day or 360 over a 30-day bill. A variable-speed motor used about 100 watts in the same field study.
Should I use AUTO in the summer?
Yes. With the fan ON, water the AC pulled out of the air evaporates back off the wet coil between cycles, so the house feels damp at the same temperature.
Does setting the fan to ON help a cold room?
A little, by mixing the air, but it rarely fixes it. A cold room usually comes down to ducts, air leaks or where the room sits in the house, and the fix is airflow to that room.
