A continuously running blower can signal hidden safety hazards that need immediate attention.

A blower that never stops often means the system cannot achieve the required pressure or temperature balance, which can lead to overheating, incomplete combustion, or backdrafting of combustion gases into living spaces. Start by turning off power and visually inspecting all safety switches including the door interlock, pressure switches, and flame rollout switches for proper seating and cleanliness. Use a noncontact thermometer to compare inlet and outlet temperatures and watch for abnormal heat buildup that indicates airflow restriction from clogged filters or blocked vents. Check for signs of combustion spillage such as soot, rust, or a yellow flame and, if present, stop using the appliance and ventilate the area before further diagnostics. Verify control board outputs by observing whether the blower runs when thermostatic or time commands are absent; a stuck relay or failed control often forces continuous operation and requires component replacement. When in doubt with electrical or combustion components, shut off the unit and call a licensed technician because improper attempts can create carbon monoxide risk or electrical fire hazards.

Common mistakes include assuming the thermostat is the cause which wastes time chasing calibration when the real issue is a stuck fan relay or bad control board; the fix is to isolate and test the control relay and replace it if it does not release. Another frequent error is neglecting to inspect venting and filters, leading to overheating that masks combustion problems; clean or replace filters and clear ducts before further testing. People also ignore intermittent safety switch failures that only trigger under certain conditions; exercise each switch manually during a safe shutdown and replace any switch that does not operate reliably.

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