1. Thermostat set to "heat" and "auto"
If the fan is set to "on", it blows air even when the furnace isn't heating. That air feels cold. Switch it to "auto".
2. Thermostat batteries
Weak batteries can cause strange behavior. Replace them if your thermostat uses them.
3. A dirty filter
A clogged filter can make the furnace overheat and shut the burners off while the fan keeps running. Replace it.
4. The furnace switch and breaker
There's usually a light-switch-style power switch near the furnace. Make sure it's on and the breaker hasn't tripped.
5. Gas supply (or propane level)
Are other gas appliances working? For propane homes, check the tank level.
6. The condensate drain (high-efficiency furnaces)
High-efficiency furnaces drain water through a small hose. If it's clogged or frozen, a safety switch can stop the burners.
Still cold?
Common causes then include a failed igniter, a dirty flame sensor or a safety switch doing its job. These are quick fixes for a technician. Furnace repair starts with a free diagnosis. Call (319) 404-4031.
By John Greenley, owner of Greenley Heating & Cooling, Janesville, Iowa.