Homeowners think of the chimney as the brick; the brick thinks of itself as decoration. The component doing the actual work, containing fire’s exhaust for two stories through the middle of your house, is the liner, and its four jobs explain every liner service we offer.
Job 1: heat containment
Flue gases run hundreds of degrees, with excursions far higher. The liner keeps that heat in the channel and away from the masonry’s weak points, and critically, away from the framing behind it. The pyrolysis mechanism, wood slowly baked toward a dropping ignition point, is what happens where this job goes undone.
Job 2: gas-tightness
Combustion exhaust carries carbon monoxide, and the liner is the sealed pipe that escorts it past your bedrooms and out. Every crack or gap is a potential CO detour, invisible, odorless, and cumulative, which is why liner integrity is a containment question, not a performance one.
Job 3: masonry protection
Flue gases are chemically hostile, acidic condensates and creosote compounds that eat mortar joints from the inside. Bare masonry exposed to them (the pre-1940s construction norm, and the state of any flue whose tile has failed) erodes joint by joint, opening the gaps that break jobs 1 and 2.
Job 4: draft sizing
Draft is a function of flue diameter, height, and temperature, and appliances specify the channel they need. An oversized flue (classic when a fireplace becomes an insert without relining) runs cool and lazy: weak draft, heavy condensation, accelerated creosote. Right-sizing the liner is what makes the appliance perform to its spec sheet.