What size liner does a gas insert need?

Match the vent collar, run it full-length

A gas insert vents through a liner sized to its own vent collar, not to your masonry flue. On a sealed direct-vent gas insert, that's usually a co-linear pair of flexible aluminum or stainless liners — one to pull combustion air in, one to push exhaust out — commonly around 3 inches each. A B-vent or natural-draft gas insert instead uses a single larger pipe, often about 4 inches. Either way, the liner runs the full height of the chimney, and the manual sets the exact size and configuration. Guessing the diameter risks a unit that won't light or won't vent.

Why gas liners run smaller than wood

Gas burns far cleaner than wood, with no creosote and lower vent volumes, so gas insert liners are typically smaller in diameter than the 6-inch liner a wood insert needs. That's also why you can't reuse or guess — a wood liner and a gas liner aren't interchangeable, and a direct-vent unit's two-pipe system is completely different from a single-flue setup. Getting the two confused is a safety issue, not a fit preference.

The sealed, safe default

We favor direct-vent gas inserts for exactly this reason: the sealed combustion pulls its air from outside and keeps the room's air untouched, which is why they're safe in tight, well-insulated homes. Units like the Napoleon Oakville X4 or Kozy Heat Chaska 25L drop into an existing firebox and turn it into efficient, power-optional heat.

And because this is a gas appliance, the connection is licensed work — we run and pressure-test the gas line, size the liner, pull the permit, and meet the inspector. That single accountable chain — appliance, liner, gas line, permit — is our lane and no one else's. Call (828) 545-0525 for a free assessment.