How it works

When natural gas burns, the combustion products include water vapor, carbon dioxide, and trace sulfur and nitrogen oxides. As the flue cools, especially during shoulder-season cycling on gas inserts, those gases condense into mildly acidic water that pools in the liner. Chromium alone is not sufficient to resist this acid; molybdenum is what shields the alloy from pitting and stress corrosion at the condensate dew point.

For chimney liner applications, 316 is supplied as either a flexible double-wall corrugated tube or as rigid sectional pipe. The flexible variant is favored for retrofits into existing masonry chimneys with minor offsets, while rigid 316 is used in new Class A chimney runs serving high-efficiency gas appliances. Sweeps verify alloy markings on the liner during installation to confirm 316 specification when condensate exposure is expected.

DFW context

DFW’s high concentration of gas-log retrofits and direct-vent gas insert installations makes 316 stainless the dominant relining alloy for the region. The local climate’s humidity also worsens condensate accumulation in shoulder seasons. TSE specifies 316 stainless for every gas appliance reline, and the installation includes a properly insulated wrap and a sealed top plate to keep moisture out of the chase or masonry surround.

Related terms

• [Stainless 304 alloy](/glossary/stainless-304-alloy/)
• [UL 1777 listed liner](/glossary/ul-1777-listed-liner/)
• [Direct vent](/glossary/direct-vent/)
• [B-vent](/glossary/b-vent/)

Sources

• ASTM A240 stainless steel standard
• UL 1777 Standard for Chimney Liners
• NFPA 211 (2024), Section 10.6