If your thermostat is set to "auto," the furnace fan should turn off when heat stops being generated, then on again when the heat kicks back on. If your thermostat is set to "auto," and the furnace fan is running continuously, then something is wrong. Here's a look at some problems that can lead to a continually running furnace fan — and what your HVAC contractor can do about them.
Tripped Electrical Switches
Many furnaces have an electrical switch that controls the flow of electricity to the blower. When the switch is on, power flows to the blower and it blows out air. When the switch is off, power is cut to the blower, and it stops blowing air. If the switch breaks and is left in the "on" position, then your fan won't turn off as expected. A furnace repair contractor can locate the broken switch and replace it. This is usually a small and inexpensive part.
The problem may not be with the furnace but with the thermostat itself. The switch that controls the "auto" and "on" settings may not be working. You may think you're switching the thermostat to "auto," when really it is stuck in the "on" mode. Your HVAC contractor can easily test the thermostat to see whether this is the issue you're dealing with. If the thermostat is to blame, the easiest option is usually to just replace the whole thermostat with a new one. This could be a good opportunity to upgrade to a smart or WiFi thermostat if you don't have one already.
Broken Control Board Contacts
Newer furnaces have a control board that controls what electrical signals are sent from the burner to the blower, and vice versa. Sometimes, the contacts on the board may become melted or otherwise damaged, which may make the fan unresponsive to signals from the burner mechanism. If your HVAC contractor sees that your control board has broken or melted contacts, they will often recommend having the control board replaced. This is often covered under warranty in new furnaces. Fixing individual contacts is very intricate work and is not often worth the time.
If your furnace is continually blowing out the air even though your thermostat is set to "auto," this is something to look into. Talk to your furnace contractor ASAP. They can come out, diagnose the problem, and give you a quote for the repairs.
For more information, contact a local heating repair contractor.