How it works

A wood-burning fireplace consumes roughly 100 to 300 cubic feet of air per minute when fully fired. A 40,000-BTU gas log consumes about 40 CFM. Atmospheric gas water heaters and furnaces add to the total. In an older leaky home, this air seeps in through gaps around windows, doors, and band joists; in a new tight home, it may not be available, requiring a dedicated combustion air supply duct from outside.

Code-compliant combustion air supplies use a passive duct sized to the appliance load, typically 6 inches in diameter for most residential fireplaces, terminating at the firebox or in the same room. Sealed-combustion direct-vent appliances bypass the issue entirely by drawing combustion air through their own dedicated coaxial intake. Sweeps verify combustion air provisions during CSIA Level 2 inspections of any wood-burning or atmospheric gas appliance.

DFW context

DFW homes built before 1995 typically have sufficient natural infiltration to satisfy combustion air requirements without a dedicated duct, though tightening from window replacements and air sealing can change that calculus. New construction since 2015 nearly always requires either a dedicated combustion air duct or selection of sealed-combustion appliances. TSE evaluates combustion air adequacy on every wood-burning appliance and recommends supply ducts where necessary.

Related terms

• [Backdraft](/glossary/backdraft/)
• [Negative pressure](/glossary/negative-pressure/)
• [Direct vent](/glossary/direct-vent/)
• [Carbon monoxide](/glossary/carbon-monoxide/)

Sources

• NFPA 54 (National Fuel Gas Code), Chapter 9
• IFGC 2021, Section 304
• IRC 2021, Section M1701