The Combustion Appliance Zone (CAZ) Hazard
In homes with naturally drafting gas furnaces or water heaters, exhaust gases rise up the metal chimney through natural buoyancy. However, if return ductwork leaks inside the furnace closet or a choked filter creates severe negative pressure, the blower can suck exhaust gases—including lethal carbon monoxide (CO)—backward down the flue into the living space.
Combustion Safety Protocol
Never operate a gas furnace with the blower door off or unsealed return plenums. If you smell combustion fumes or notice carbon monoxide alarms chirping, immediately power down the furnace and conduct a depressurization draft test.
About ClariSelect Editorial & Research Standards:
Sizing measurements, static pressure curves, and filtration ratings referenced across our educational guides are compiled from published ANSI/ASHRAE Standard 52.2 engineering data, AHAM AC-1 verified ratings, and manufacturer technical documentation. ClariSelect provides objective editorial research and analysis with zero sponsored placements.