Furnace not heating

Thermostat: set to Heat, set point above room temperature, fresh batteries if it uses them. A thermostat that shows a blank screen or a low-battery icon is a common cause of no heat.

Power: the furnace has a light-switch-style disconnect on or near the cabinet and a breaker in the panel. Both should be on. A furnace with a condensate pump may also stop if the pump's safety switch has tripped because the pump is full.

Blower door: the door switch cuts power to the control when the blower compartment door is not fully seated. Reseat it, do not tape or jumper it.

Filter: a filter that is heavily loaded can trip the limit switch on high fire and make the furnace appear dead between attempts. Note its condition; change it if you have a spare of the right size.

Code: modern furnaces show a status code on a display or as LED flashes through a window in the door. Write it down exactly, including any minor digit, before doing anything else.

A no-heat call almost always comes from the control refusing to run because a safety condition was not met. The sequence is the same on most furnaces: the inducer starts, the pressure switch must close to prove draft, the ignitor heats, the gas valve opens, the flame sensor must prove flame within a few seconds. Fail at any step and the control retries a set number of times, then locks out.

The code identifies which step failed. A pressure-switch code points at draft: venting, intake, condensate or the inducer. An ignition-lockout code points at gas supply, ignitor or flame sensing. A limit code points at airflow and overheating. None of these tells you which part has failed; that takes testing.

Gas smell, a carbon-monoxide alarm, soot, scorching, or a furnace that is roaring, popping or backfiring is a leave-the-house situation. Call FortisBC at 1-800-663-9911 or 911 first. A furnace that is simply not running is a breakdown, not a danger, and the priority is heat rather than evacuation.