A chimney generally needs a new liner when the flue tile is cracked or has gaps, when there's no liner at all, or when the flue is the wrong size for what vents into it, such as a gas water heater left alone after a furnace upgrade or a new stove or insert. A liner isn't routine maintenance, though, and it isn't something to agree to over the phone. The decision should rest on camera footage you've seen yourself, because nobody can judge the inside of a flue from the roof or the basement.
This is general information. The City's Department of Licenses and Inspections (L&I), or your township or borough in the suburbs, has the final word on code and permits. If the chimney hasn't been looked at this year, start with our chimney inspection guide. This article picks up where the report says "reline."
What a liner does
The liner is the inner wall of the flue. It keeps heat, exhaust and moisture away from the brick, mortar and framing, and it sets the size of the passage, which affects draft and condensation.
For gas appliances, the Philadelphia Residential Code (the 2021 edition, in effect since January 1, 2026) sets the bar in plain terms:
- Chimneys are to be lined in accordance with NFPA 211, the national chimney standard.
- When an appliance is permanently disconnected from an existing chimney, or a new one is connected, the liner has to be continuous and free of cracks, gaps and other damage that would let out exhaust, moisture or creosote, and the chimney is to be resized as necessary to control condensation and give the appliance proper draft.
- If an inspection finds a chimney unsafe for its use, it has to be repaired, rebuilt, lined, relined or replaced.
When a chimney needs a liner
Cracked, gapped or missing tile
A 2005 trade paper on relining best practices, published by the manufacturer DuraVent, lists why clay liners fail: the thermal shock of a chimney fire, acids in flue gases, freeze-thaw cycles and general aging. The usual signs are cracks and broken pieces that have shifted.
The paper also says no average life expectancy can be given for clay tile, because some lasts decades and some deteriorates in a few years. So "your chimney is 80 years old" is not a diagnosis. Age is a reason to look, and the footage is the reason to reline.
An orphaned water heater
This one follows a furnace or boiler replacement. The new high-efficiency unit vents through the wall, and the gas water heater is left alone in a large masonry flue. An ASHI Reporter article explains the chain of events: the old furnace kept the tile warm, the water heater alone can't, and once the tile falls below the dew point of the exhaust, moisture condenses on the tile and mortar. Then the wet masonry freezes and thaws. The article says outside chimneys and cold climates make it worse, and that 85% efficient furnaces and boilers can cause the same problem.
The signs ASHI lists:
- loose or missing mortar, or spalling or missing brick, above the roofline;
- backdrafting at the water heater (exhaust spilling back into the room);
- condensation on the metal flue pipe;
- damage at the water heater itself.
You may not need a liner here. A Contractor magazine column gives the alternative: a direct-vent water heater, which leaves the chimney out of the system. If the tank is near the end of its life anyway, price both options. Our water heater guide covers the choices and the permit. Replacing a furnace now? Our furnace and boiler guide explains why the water heater's venting belongs in that quote.
A new stove, insert or other appliance
Ideally the flue area matches the appliance. The relining paper's example: the common 8-by-12-inch clay liner has more than twice the area of a 6-inch flue, the most common size for a modern wood stove. It says relining is common when an insert goes into an existing fireplace.
Bare brick, or wood too close to the chimney
If the camera shows no liner at all, the lining rule above applies to gas appliances. The same goes for hidden combustibles: the paper names framing or "nailers" built into the masonry as a hazard, and calls a zero-clearance liner a good solution. The Philadelphia code also allows a masonry chimney without the required clearance if it's relined with a system listed for reduced clearances. In a rowhome, where the chimney can sit in or near a party wall, ask how clearances were checked.
The Chimney Safety Institute of America (CSIA) recommends a Level 2 inspection if you're planning to line or reline. That's the one with a video scan. It should cover every flue in the chimney and the connected appliances, and record the flue's inside dimensions, height and offsets.
The relining paper is frank that an installer who inspects a chimney and then sells the liner is in an apparent conflict of interest. That's a reason to ask for evidence, not an accusation:
- Get the video or photos showing the defect, and where in the flue it is.
- Ask what problem the liner solves: damaged tile, wrong size, condensation or clearance.
- Ask what has to be fixed first. The paper says new liners should only go in chimneys that are structurally sound and fully cleaned, with a loose crown or eroded mortar repaired before relining.
- Ask whether the outside of the chimney is sound. Relining suits a chimney whose brick shell is good and whose liner has failed. It won't save a failing shell.
- On a big job, take the footage to a second company for another opinion.
Liner types, and which fuel each is for
Whatever the material, the basic test is the listing. The code's main options are clay lining that meets ASTM C315 and listed lining systems that comply with UL 1777, and the relining paper says the label states which fuels a liner suits. Ask to see it.
- Aluminum (flexible). Gas only. DuraVent's aluminum liner, for example, is for natural gas or propane Category I appliances, meaning standard draft-hood furnaces and water heaters, and those listed for Type B vent. ASHI's suggested fix for an orphaned water heater is a thin metal liner, often aluminum. Not for oil or wood.
- Stainless steel (flexible or rigid). Used across fuels, with the listing deciding which ones. The paper says flex is the most common type and is needed where the chimney has offsets, and that there's no evidence either form lasts longer. CSIA says liners for gas and oil don't need insulation, but those serving solid fuel do.
- Poured-in-place. A cement-like mix is pumped around a form inside the flue. It generally costs more than stainless and takes specialized equipment. The paper says it's sometimes favored for heritage buildings because it allows any style of cap.
A condensing furnace or boiler isn't vented this way. The code sends Category II, III and IV appliances to the manufacturer's instructions, which for most of these units means out through the wall.
Permits in Philadelphia and the suburbs
We couldn't find an L&I page that addresses relining by name, so treat this as a question to settle in writing. L&I's mechanical permit page says a permit is needed to install devices regulated by the Mechanical and Fuel Gas Codes, including venting and gas equipment, but not for ordinary repairs or maintenance. Replacing appliances in one- or two-family homes can qualify for an EZ permit without plans, and a licensed contractor must do the work. The City's EZ standard for chimney and vent extensions covers extensions, not liners, and requires prior Historical Commission approval for historically designated buildings. If yours is, check a new cap or taller termination first; see our historic district guide.
The suburbs vary. Falls Township in Bucks County announced in November 2025 that relining needs a mechanical permit, and that the final inspection requires an NFPA 211 Level 2 certification in addition to the inspector's approval. Your township may differ, so call its code office.
If money is tight
Philadelphia's Heater Hotline, run by the Energy Coordinating Agency at 215-568-7190, lists chimney and flue among its free repairs. It's for owner-occupied single-family homes with income up to 150% of the Federal Poverty Level, and it doesn't work on water heaters. The page doesn't say whether a liner qualifies, so ask. Our home repair programs guide lists other options.
What a fair quote should list
A typical price isn't much use here, because the job varies so much. These are the items that move it, so get each in writing:
- Which flue is being lined and which appliances it serves.
- Liner material, UL 1777 listing and diameter, plus how the size was worked out for your appliance.
- Insulation, if any, and why.
- Cleaning before the liner goes in.
- Masonry repairs: crown, mortar, flashing.
- Removing old tile, if needed. The paper calls this difficult and potentially dangerous, so ask whether it's included.
- Top plate, cap, connector and the basement cleanout. The code calls for a cleanout with a tight-fitting, noncombustible cover.
- How they'll reach the top of the chimney, including flat roof access and any offsets in the flue.
- The permit, who pulls it, and who schedules the inspection.
- A written report and final footage, plus the warranty terms.
When to call a pro
Relining isn't a weekend project, because the sizing and the listing both have to be right. Book an inspection, and hold off on fires, if:
- you've found tile pieces or a lot of debris at the basement cleanout;
- the furnace or boiler was replaced and nobody checked the water heater's venting;
- you see rust or moisture on the flue pipe, or stains and flaking brick around the chimney;
- you're adding a stove or insert, or buying a house with an older chimney.
If a carbon monoxide alarm goes off, get everyone out and call 911 first. Have the flue and appliances checked before using them again, and see our smoke and CO alarm guide. If you smell gas, leave and call PGW's emergency line at 215-235-1212 or 911.
When you're ready, you can post your chimney project on HomeKept and get responses from local pros. Say what the chimney serves, how many stories the house has, whether there's a roof hatch, and attach any inspection report or footage. It's free for homeowners, you can compare up to three estimates, and the service area is Philadelphia, Bucks and Montgomery counties. A person reviews every pro profile, and license and insurance badges appear only after documents are checked. That isn't a background check, so also check the contractor yourself, including the Attorney General's home improvement contractor search.
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