Confirm the problem
Note whether the blower runs, whether air is cold or absent, whether the thermostat display is active and whether the problem affects the whole home.
Check simple external items
Confirm the thermostat mode and setpoint, furnace service switch, breaker, filter and open registers. Look outside for snow, leaves or ice blocking approved intake and exhaust terminations, but do not dismantle them.
Read, do not bypass
Many furnaces display a diagnostic code through a sight glass or status light. Photograph the code and model number for the technician. Never tape down a door switch or repeatedly reset a unit that keeps locking out.
Protect the home
Use only approved portable heaters with clearance from combustibles and direct wall connections. Protect plumbing from freezing and arrange service promptly during severe cold.
- What changed immediately before the problem?
- Does the issue affect airflow, heat production or both?
- Is there an error code, odour, leak or repeated shutdown?