Start here: set the thermostat fan to “auto,” not “on.” That alone fixes many cases (on “on,” the blower runs between heating cycles and pushes cool air). Confirm it’s set to “heat” above room temp.
A dirty air filter
A clogged filter chokes airflow, the furnace overheats, and a safety switch shuts the burners while the fan runs. Replace it (see how often to change your furnace filter).
Overheating / limit switch
Poor airflow trips a limit switch that cuts the burners but leaves the fan on. Clear the airflow issue; a repeatedly tripping limit switch needs a tech.
Ignition / pilot (gas)
Burners not lighting means no heat. If you ever smell gas, leave and call the gas utility from outside, then call us.
Other causes
Failing flame sensor, clogged condensate line (high-efficiency), thermostat wiring β diagnosis calls.
When to call R&T Yoder
Fan on “auto,” thermostat right, filter clean, still cold? Call. 50,000+ jobs. Book furnace repair or a seasonal HVAC tune-up. (614) 206-5401.
FAQ
- Why blowing cold air? Usually fan on “On” vs “Auto,” or a dirty filter causing overheat shutdown; also ignition or a tripped safety switch.
- Cold-warm-cold cycling? Overheating/short cycling from restricted airflow (dirty filter). Replace filter; inspect if it continues.
- Can a dirty filter cause it? Yes.
- Fan on or Auto? Auto for normal heating.
- When to call? Fan Auto + thermostat right + clean filter but still cold, or it keeps shutting off, or any gas smell (leave first).