What Auto and On actually mean
On Auto, the blower only runs when the furnace or heat pump is heating. The heat comes on, the fan pushes warm air through the house, and the fan shuts off a short time after the heat stops.
On On, the blower runs all the time, whether the system is heating or not. Between heating cycles, it keeps moving air through the house, and that air is not being heated.
The trade-offs in winter
Auto is the better choice for most homes in winter. Air coming out of the vents between cycles on On can feel cool, especially from ducts in a cold attic or crawlspace, and the blower running all day uses more electricity.
On does have a place. It mixes the air, so it can even out a room that is always colder or warmer than the rest of the house, and the air passes through your filter more often. If you run On, check your filter more often too, since it will load up faster.
If your thermostat says Circulate, that setting runs the fan part of each hour even when the heat is off. It is a middle ground between Auto and On.
Try a one-day test
Look at your thermostat today and note which setting it is on. Then try the other one for a full day and pay attention to comfort, how often you hear the blower and whether any rooms change.
If the blower keeps running on Auto even after the heat shuts off, or it never comes on at all, that is not a setting problem. Something in the system needs a look. Call me at (470) 999-7395 or book a $79 diagnostic online and I will check it out.
