Why a furnace blows cold air
Common causes include a thermostat fan setting on On instead of Auto, a dirty filter that triggers overheating shutdowns, a failed igniter or flame sensor, a gas supply or control problem, or a blower that runs before burners light.
Furnace igniter replacement
Modern furnaces use hot-surface igniters that wear out over time. A failing igniter can cause the furnace to click, glow weakly or never light the burners. Replacement is a common repair, but it involves gas appliance components and should be done by a qualified technician.
Safety warning signs
If you smell gas, hear hissing near gas lines, see soot or scorching, or a carbon monoxide alarm sounds, leave the building and contact your gas utility or emergency services from a safe place. Never bypass safety controls.
Safe checks before calling
- Thermostat on Heat, fan on Auto, correct temperature
- Furnace power switch on and breaker not tripped
- Clean or replace the air filter
- Registers open and unblocked