The Stainless Chimney Liner, Specified: 304 vs 316 and the Sizing Rules

A chimney liner is a corrosion problem wearing a fire-safety hat, which is why the metal grade matters more than the brand on the box.

304 vs 316: pick by fuel

• 304 stainless handles wood-burning appliances venting dry, hot flue gas. It’s the budget-correct choice for open fireplaces and many wood stoves run hot.
• 316 (usually 316L/316Ti) adds molybdenum, which resists the acidic, wetter condensate of gas, oil, pellet, and any high-efficiency appliance — and of wood burned low-and-slow. Condensate in a modern flue is mildly acidic enough to pit 304 over years; 316 shrugs it.

If the appliance ever runs cool exhaust — condensing anything — 316 is the answer. The price gap is small against relining twice.

Sizing: match the appliance, not the chimney

The liner matches the appliance outlet, and the old rule holds: same diameter as the flue collar, with the liner’s cross-sectional area between the collar’s area and roughly twice it (wood stoves particularly hate oversized flues — draft slows, smoke cools, creosote condenses). A 6-inch stove collar wants a 6-inch liner even inside a huge old masonry flue; the gap gets insulated, not used.

Insulation isn’t optional decor: a half-inch ceramic wrap or poured insulation keeps flue gas hot to the cap, which is what maintains draft and keeps creosote and condensate from forming mid-chimney.

Flex vs rigid

Flexible liner (smooth-wall or corrugated) exists because real chimneys bend and old flue tiles offset; it installs from the top in an afternoon. Rigid liner flows better — the smooth bore gains measurable draft — and suits straight shots. Corrugated flex costs roughly 20% in draft equivalence, so upsizing insulation, not diameter, is how flex installs stay honest.

Whatever the metal, a liner is only as good as its cap, top plate, and bottom connection — most “liner failures” are water entries at the ends, not the stainless giving up.