Furnace running but not heating: what to check before an emergency call
A safe, non-invasive checklist for a furnace that runs but produces little or no heat, plus the details an HVAC technician needs.
Updated September 28, 2026Start with controls and airflow
Without opening combustion equipment or bypassing safety controls, confirm the thermostat is calling for heat, the normal furnace switch is on, the breaker has not tripped and the filter is not obviously blocked. Record any error code before cycling power.
Stop troubleshooting if carbon monoxide is involved
Ontario describes carbon monoxide as invisible and odourless and instructs occupants to leave immediately and call emergency services from outside if a CO alarm sounds. A no-heat problem and a CO alarm are not the same service call: life safety takes priority.
Give the technician a useful symptom description
Say whether the blower runs, whether the system starts and stops repeatedly, whether some rooms are warm, what thermostat temperature is shown, and whether there are unusual sounds or smells. That information helps triage parts, tools and urgency.