What is Negative Pressure? | TSE Glossary

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Negative pressure is the condition in which indoor air pressure falls below the surrounding outdoor pressure, causing outside air to be drawn into the home through any available opening, including the chimney. It is the underlying physical condition behind most chimney backdraft incidents and a measurable parameter that HVAC technicians and chimney sweeps track during diagnostic visits. Persistent negative pressure makes wood-burning and atmospheric gas appliances unsafe.

Indoor pressure relative to outdoors is set by the balance of air entering and leaving the home. Exhaust fans, clothes dryers, central HVAC return leaks, and natural stack effect all remove air. Combustion appliances also remove air. When removal exceeds the rate at which makeup air infiltrates, indoor pressure drops below outdoor, and any opening becomes a potential inlet, including chimneys with combustion equipment running.

Sweeps measure pressure differential with a manometer placed across exterior windows, doors, or appliance flues, reading in pascals. Negative pressure of 5 pascals or more across a chimney significantly impairs draft, and 10 pascals or more can fully reverse flow. Remediation requires identifying the dominant exhaust source and either reducing its draw or providing intentional makeup air.

DFW’s tight new construction since the 2015 IECC code adoption produces homes with very low natural air leakage rates, making them prone to negative pressure when exhaust fans operate. Homes with combined wood-burning fireplaces, large range hoods, and high-efficiency furnaces frequently fail pressure testing on cold-day worst-case scenarios. TSE provides combustion safety testing as a standalone service for owners experiencing recurring backdraft or smoke spillage.

• [Backdraft](/glossary/backdraft/)
• [Combustion air](/glossary/combustion-air/)
• [Carbon monoxide](/glossary/carbon-monoxide/)

• ASHRAE 62.2 ventilation standard
• NFPA 211 (2024), Section 12
• CSIA Reference Manual

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