How it works

When a homeowner installs a direct-vent gas insert into an existing masonry fireplace, the original masonry flue is too large for the appliance. Co-linear vent retrofits two purpose-sized pipes into that flue, using the masonry chimney as a chase. The exhaust pipe carries hot flue gases up and out a top plate; the air pipe draws cool combustion air down to the appliance from a separate inlet on the top plate.

Co-linear systems require careful sizing per the appliance manufacturer’s specifications, with maximum vertical run, allowable offsets, and termination options spelled out in the install manual. Sweeps verify both pipes for full length, confirm separation at termination, and check for crushed sections during annual service. Damaged or kinked pipes degrade combustion efficiency and can spill CO into the firebox enclosure.

DFW context

Co-linear vent kits are TSE’s standard solution for converting a wood-burning masonry fireplace to a direct-vent gas insert in DFW historic homes. The original tile-lined chimney becomes a chase for two flexible aluminum pipes, preserving the brick exterior while modernizing the appliance. The conversion eliminates wood-burning maintenance and improves heating efficiency while keeping the architectural character of older Highland Park, Lakewood, and East Dallas hearths.

Related terms

• [Co-axial vent](/glossary/co-axial-vent/)
• [Direct vent](/glossary/direct-vent/)
• [Insert](/glossary/insert/)
• [Direct-vent gas fireplace](/glossary/direct-vent-gas-fireplace/)

Sources

• ANSI Z21.88 direct-vent gas appliance standard
• NFPA 54, Chapter 12
• Manufacturer installation manuals (e.g., Heat & Glo, Mendota)