How it works

Chimneys rely on the buoyancy of hot gases relative to cold outside air to maintain upward draft. When indoor pressure drops below outdoor pressure, a competing pressure differential pulls air down the chimney, opposing draft. Common causes include kitchen and bath exhaust fans, central return-air leaks, clothes dryers, and tight new construction. Wind washing over the chimney top can also momentarily reverse flow.

Sweeps test for backdraft potential during CSIA Level 2 inspections by closing all exterior doors, running every exhaust fan in the home, and observing whether smoke from a punk stick at the firebox or appliance flue draws into the chimney or back into the room. Persistent backdraft requires correction of the source, typically by adding combustion air supply, sealing return ducts, or installing makeup air systems.

DFW context

DFW tract homes built since 2010 are increasingly tight, with low air change rates that amplify backdraft risk when high-volume exhaust fans operate. The combination of a wood-burning fireplace and a 1200-CFM commercial-style range hood is a common DFW recipe for backdrafting. TSE diagnoses these scenarios with smoke testing and pressure measurement, and remediation often involves installing a passive or motorized makeup air system tied to the range hood.

Related terms

• [Negative pressure](/glossary/negative-pressure/)
• [Combustion air](/glossary/combustion-air/)
• [Carbon monoxide](/glossary/carbon-monoxide/)
• [Smoke shelf](/glossary/smoke-shelf/)

Sources

• NFPA 211 (2024), Section 12.6
• ASHRAE 62.2 ventilation standard
• CSIA Reference Manual