Chimney liner installation and relining in Greater Seattle

Under the residential code as adopted in Seattle, a masonry chimney has to be lined, and the lining material has to suit the type of appliance connected to it, in accordance with that appliance's listing and the manufacturer's instructions. The mechanical code adds that the liner must be continuous and free of cracks, gaps, perforations or other deterioration that would let combustion gases, moisture or creosote escape. A failed liner is a compliance problem, not only a maintenance one.

• Cracked, spalled or collapsed clay tiles. Tiles that have cracked through, lost large chunks, or dropped out of alignment no longer form a continuous passage. This is what a camera scan is looking for.
• A chimney fire. CSIA describes masonry chimney fires as burning at around 2,000 degrees F, hot enough to melt mortar, crack tiles and collapse liners. Its guidance is that the remedy ranges from replacing flue tiles, to a new liner system, to rebuilding the chimney.
• A new or different appliance. When an appliance is connected to an existing chimney, or permanently disconnected from one, the code requires that chimney to be brought into compliance for size, flue condition, cleanout and clearances. Fitting an insert, or swapping a furnace for a higher-efficiency model, routinely forces the question.
• An unlined or partially lined chimney. Older stacks were sometimes built with no lining, or lined only part way up. Both turn up on houses that predate modern lining requirements.
• Too little clearance to combustible framing. Where the masonry does not hold the required airspace to wood framing, the code's own exception is a lining system listed and labeled for chimneys in contact with combustibles — which is a relining job.
• A flue that is the wrong size for what is attached. The mechanical code requires an existing chimney to be resized as necessary to control flue gas condensation and to give the appliance the draft it needs.

The code recognizes a short, closed list of flue linings for residential appliances: clay flue lining complying with ASTM C315, a listed and labeled lining system complying with UL 1777, a factory-built chimney unit listed for installation inside a masonry chimney, or another approved material able to resist corrosion, erosion, softening and cracking from flue gases and condensate at temperatures up to 1,800 degrees F. Anything sold as a liner outside those categories is not a code-compliant liner.

Clay tile

The fired-clay sections stacked inside most older masonry chimneys. Code installs them to ASTM C1283: laid in medium-duty, water-insoluble refractory mortar, joints tight and smooth on the inside, supported on all sides, and separated from the surrounding masonry by an airspace or insulation. Replacing tile in an existing chimney means breaking the old tile out from the top down, which is why a stainless system is more often the practical answer to a cracked flue.

Stainless steel

A flexible or rigid metal liner, sized to the appliance, run down the existing flue and connected at the appliance and at the top. It is the common answer for relining masonry and for making an old flue serve a stove, insert or new heating appliance. The alloy is not interchangeable — it is chosen by fuel.

Cast-in-place

A cementitious lining formed inside the existing chimney around a temporary form, leaving a seamless new flue surface against the old masonry. CSIA's inspection procedure judges a cast-in-place liner on thickness and on the absence of large surface irregularities, so both are fair questions to ask about a product.

Factory-built chimney unit

A listed chimney assembly installed within the masonry shell. A factory-built metal chimney can itself be relined, but the manufacturer sets the limit — DuraVent, for example, requires the liner's inside diameter to be at least 2 inches smaller than the inside diameter of the existing chimney.

"Listed and labeled" is not marketing language — it is the code's own test, and the listing is the document that defines what the product may be used for, at what clearance, and with which fuels. Two standard numbers come up constantly on liner quotes, and they cover different products.

UL 1777

The safety standard for chimney liners field-installed into new or existing masonry chimneys for residential appliances, where the maximum continuous flue-gas outlet temperature does not exceed 1,000 degrees F. Its scope explicitly excludes Category II, III and IV gas-burning appliances and systems that produce corrosive acid condensation or positive chimney pressure — so a high-efficiency condensing appliance is a different venting question entirely.

UL 103

The standard for factory-built chimneys venting gas, liquid and solid-fuel residential and building heating appliances, again at a maximum continuous flue-gas temperature of 1,000 degrees F. The standard lets a manufacturer choose between two limited-duration high-temperature test options, 1,700 degrees F or 2,100 degrees F. Product literature often shortens the higher rating to "HT", so ask which listing the specific product you are being quoted actually carries.

Notice of usage

Where a flue is relined with a material outside the code's general list, the code requires a permanent label near where the connector enters the chimney, stating the type or category of appliance and the fuel that flue may serve, and that other appliances must not be connected. Gas-only linings are the common case — it matters if a future owner wants to burn wood in that flue.

A flue is a heat engine as much as a pipe. Flue gases carry water vapor and stay a gas only while they stay above their dew point — DuraVent's instructions put that at roughly 128 degrees F as a manufacturer approximation. A flue with far more cross-section than the appliance needs cools the gases too fast, they condense on the liner wall, and moisture runs back down instead of leaving the roof.

That condensate is not clean water. Manufacturer documentation notes that ordinary indoor air carries chlorides given off by carpeting, paints, thinners and detergents, and that when combustion air is drawn from inside the house those contaminants can combine with flue gas condensate to form aggressive chemicals, hydrochloric acid among them, that attack connectors and liners. Condensation control is corrosion control.

• The floor. A chimney flue may never be smaller in area than the connector coming off the appliance. That is the hard minimum in the residential code.
• The ceiling, for solid fuel. Liner manufacturers cap the flue for a solid-fuel appliance at three times the cross-sectional area of the appliance outlet collar, and never smaller than the collar itself. The manuals note the formula shifts with chimney height.
• Fireplaces are sized off the opening. Code sizes a masonry fireplace flue as a fraction of the fireplace opening: one-twelfth for round flues, one-tenth for square flues and rectangular flues under a 2-to-1 aspect ratio, and one-eighth at 2-to-1 or steeper.
• High-efficiency appliances are the sensitive case. Lower flue gas temperatures leave less margin above dew point, so correct sizing matters more, not less, as appliances get more efficient.

Insulating a stainless liner does two jobs at once. It keeps flue gases warmer, which holds them above dew point and reduces condensation and creosote deposition, and it is usually what the liner's listing depends on for its clearance-to-combustibles rating. It is not an add-on invented at the quote stage.

• Solid fuel. CSIA's position is that liners serving solid fuel-burning appliances need to be insulated, describing the target as a zero/zero installation — zero clearance to the chimney interior and zero to the exterior. The insulation may be blanket, expanded mica, or a masonry insulating mix.
• Gas and oil. CSIA states these liners generally do not require insulation to meet manufacturers' installation and warranty requirements, because flue gas temperatures and heat transfer are lower. Manufacturers still call insulating good practice there, to stabilize draft and minimize condensation.
• The exception, and its condition. Manuals allow insulation to be omitted where the existing clay tile liner is in good condition and the chimney meets existing codes, and require the blanket over the full length in unlined, partially lined or cracked-tile chimneys. Establishing which of those you have is exactly what an inspection is for.
• How much, and of what. Listed systems specify it precisely: several manufacturers require, for wood-fired service at their zero-clearance listing, roughly half an inch of 8-pound-density ceramic blanket or about an inch of a poured insulating mix. The figures are product-specific — the installed product's own listing governs.

NFPA 211, the national standard for chimneys, fireplaces, vents and solid fuel-burning appliances, names relining as one of the circumstances calling for a Level 2 inspection — alongside adding, removing or swapping in a dissimilar appliance, sale or transfer of the property, and any fire, weather or seismic event likely to have damaged the chimney. A Level 2 requires the internal surfaces of the flue liners to be examined with image-scanning equipment, so the camera scan is part of the standard, not an upsell.

• What is actually wrong with the existing liner. Cracks, missing sections, failed joints and displaced tile show on the scan, and their location and extent decide whether the answer is tile replacement, a listed liner system, or something larger.
• Whether the material and the size suit what is connected. A Level 2 requires evaluation of the flue lining material and the flue sizing against the type and input rating of the connected appliances — the step that catches an oversized flue before you pay for the wrong liner.
• Clearance to combustibles. A Level 2 verifies clearance from the chimney to combustible material everywhere it can be reached, which determines whether a listed reduced-clearance lining system is needed.
• Whether the chimney is sound enough to line. The scan and the accessible runs in attic, crawl space and basement show whether you are relining a good chimney or postponing a rebuild. They also produce the measurements the quote depends on: flue height, interior dimensions, offsets, and the appliance outlet collar.

Liner manufacturers write prerequisites into their installation instructions, and meeting them is a condition of the listing and of the warranty. This is why a relining scope normally includes work that looks, on paper, like a different service.

1. 1

   The flue is swept clean

   Instructions require all creosote — including hard tar glaze — plus soot, dirt and debris to be removed before installation, and call thorough cleaning a warranty prerequisite. A flue too dirty to inspect has to be swept before the liner can even be assessed.

2. 2

   The chimney is confirmed structurally sound

   The same manuals require inspection for cracked, loose or missing brick, stone and mortar joints, and state directly that a chimney which is not structurally sound should not be relined. Where that is the finding, masonry repair or a rebuild comes first and is quoted on its own.

3. 3

   The obstruction is dealt with, not worked around

   Where damaged tile, an offset or a narrowed section will not pass the diameter the appliance requires, the tile is broken out or the route is resolved. Forcing an undersized liner to fit is how appliances end up under-drafting.

4. 4

   The top of the chimney is right

   The liner terminates through a top plate with a storm collar and rain cap, and the crown detail matters: code requires a caulked bond break around flue liners, and CSIA notes NFPA 211 calls for a bond break where the tile passes through the crown so the flue can expand vertically.

5. 5

   The scope is written down before anything is ordered

   Material, alloy, diameter, listing, insulation type and terminations belong in writing, with a price, before the order is placed. Nothing starts until you approve it.