What the liner does
The same DOE guide lists four jobs a liner does: it makes the flue tighter, which helps draft; it protects the brick and mortar from acidic condensation; it keeps chimney-fire heat away from the house framing; and it can shrink a flue that's too large for the appliance. That last one matters more than it sounds. Modern, more efficient furnaces and water heaters send out cooler exhaust, so condensation forms more easily in an oversized, unlined chimney and slowly eats the masonry from the inside.
The three common liner types
- Clay tile: the usual liner in chimneys built with one. It works for all fuels when maintained, but a chimney fire can crack the tiles, broken pieces can block the flue, and clay can't always handle the condensation from newer gas appliances.
- Metal (usually stainless steel): the common choice for relining an existing chimney. Stainless suits wood, gas or oil, and the guide notes these systems usually need high-temperature insulation to perform safely.
- Cast-in-place: a cement-like mix poured inside the chimney to form a seamless, insulated passage. The guide says it can also shore up an aging chimney and suits all fuels.
When an appliance change triggers a liner
This is where most homeowners first hear the word. The DOE guide summarizes the National Fuel Gas Code this way: if a gas appliance replaces an oil or wood appliance in a chimney, the flue has to be lined; if a gas appliance replaces another gas appliance, it has to be lined unless the new one is similar in type, input rating and efficiency. It adds that swapping one natural-draft water heater for another usually doesn't require a liner. The guide dates from 2007 and quotes the 2002 code edition, so ask your contractor which edition your municipality enforces.
The classic case is the orphaned water heater. Replace a chimney-vented furnace with a high-efficiency furnace that vents out the side wall, and the water heater is left alone in a flue sized for both. The Building America Solution Center says that chimney may now need a liner to meet current code, and that even a new 80% furnace paired with a natural-draft water heater sometimes does. Our guide on replacing an oil boiler or furnace covers the questions to ask, and our backdrafting check shows what to watch afterward.
Wood inserts have their own rules. EPA says a certified installer should make sure the liner in a masonry chimney is installed correctly, and a factory-built fireplace can only take an insert designed and tested for that unit (EPA wood-burning appliance types).
What to ask before you approve a liner
- What exactly is wrong: no liner, a damaged liner, a blockage, or a flue that's too big? A blockage on its own calls for cleaning, not automatically a new liner.
- Which appliances and fuels will use this flue when the job is done?
- What liner material and size, and is insulation included?
- Can I see photos of the defect, including any camera inspection of the flue?
- Is a permit needed, and who arranges the inspection? In Philadelphia, see our guide to furnace and HVAC replacement permits.
For a wood-burning fireplace or stove, EPA points to the Chimney Safety Institute of America (CSIA) and the National Fireplace Institute to find certified professionals (EPA wood-burning FAQ). If a gas or oil furnace, boiler or water heater uses the flue, bring in an HVAC contractor too, since sizing depends on the appliance. If water is also getting in at the top, our guide on cracked chimney crowns explains that separate repair.
What you can safely check yourself
From normal standing spots, look for rust or loose vent pipes where they enter the chimney, soot stains, or smoke coming into the room. Don't go on the roof, take venting apart, or light a test fire. Smelling smoke when you burn wood means something may be wrong, and EPA recommends an annual check by a certified sweep either way. The U.S. Fire Administration likewise says to have chimneys cleaned and inspected every year by a professional.
Carbon monoxide has no smell, so no smoke doesn't mean no problem. If a CO alarm sounds, CPSC says to go outside right away, call 911 or the fire department, and stay out until responders say it's safe. Don't hunt for the source.