Energy
A Dirty Furnace Filter Doesn't Just Waste Energy — It Can Crack a Heat Exchanger
Ignoring a furnace filter long enough restricts airflow so much that the resulting overheating can crack the heat exchanger — a repair that dwarfs the cost of filters.
A furnace's heat exchanger is designed to shed heat into a steady stream of air pulled across it by the blower. A clogged filter throttles that airflow, so less air moves across the exchanger per minute even though the burner keeps firing. The metal runs hotter than its design tolerance, and repeated heat-cool cycling under that stress is a known contributor to hairline cracks forming over time.
A cracked heat exchanger is also a carbon monoxide risk, since it's the barrier between combustion gases and the air circulated through the house — which is why most furnace safety inspections check it specifically. Changing a filter every one to three months, depending on type and household, is cheap insurance against a repair that can run well into four figures.
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