Lining Your Woodstove Flue

Q: We're about to buy a new wood-burning stove, and our local dealer refuses to install it unless we buy a stainless steel liner that goes all the way out the top of the chimney. Do we really need to do this? If so, is it something we can do ourselves?

A: There are four situations relating to your existing chimney that would cause your code authority to require a stainless liner for a wood stove installation:

1) The existing chimney is unlined. In the past, some masonry chimneys were built without any liner at all. Without a liner, wood exhaust can leak through the mortar joints into the house. Further, liquid creosote will eventually dissolve the binder in the mortar and soak into the joints, until the chimney itself could catch fire!

2) The existing chimney is lined, but the liner is cracked or damaged. Masonry chimney liners are made of clay or pumice, which can crack from chimney fires, settling, or other causes. Cracks in a pumice or clay flue liner will let the smoke and liquid creosote through, leading to the same problems described above.

3) The existing chimney is lined, but the liner is too large. Oversized flues cause numerous problems, like smoke spillage into the house when they're cold, stove overfiring when they finally heat up, and the kind of excessive creosote formation that leads to chimney fires. Code prohibits venting any woodburner into a chimney flue with a cross-sectional area (CSA) that is more than three times the CSA of the flue collar on the stove. Thus, if your stove has a 6" flue collar (28 sq.in. CSA), the chimney flue must have a CSA less than 84" (3 x 28). In other words, if the chimney flue is larger than 10" round (78 sq.in. CSA) or 9" x 9" (81 sq.in. CSA), you must install a 6" stainless liner. This rule gets tougher if one or more of the chimney surfaces is on an outside wall, so it is exposed to outdoor temperatures for its entire length: in those cases, the CSA of the flue can't be larger than two times the CSA of the flue collar, so you must reline if the existing flue is larger than 8" round or 7" x 7".

4) The existing chimney is air-cooled, like the manufactured metal chimney used for zero-clearance wood fireplaces. Cooling the exhaust from an airtight woodstove leads to balky updraft, smoke spillage and excessive creosote formation. Code requires installation of an insulated  liner to eliminate those problems.

Our Forever Flex liner kit for wood stove installations consists of a rain cap, a top plate, a length of flexible stainless steel pipe, a tee with bottom cap and removable snout, and a tube of hi-temp silicone sealant to affix the top plate to the chimney. For pricing, click here.

Installing a chimney liner doesn't really require much mechanical aptitude, and many of our online customers choose to do it themselves. Here's how:

Rain Cap	To install our wood stove chimney liner kit, you'll need: *work gloves (the edges of the stainless parts are SHARP) *a cutting torch, offset grinder or sawzall to enlarge your damper opening for fireplace hearth installations *a caulking gun to apply the silicone sealant *sheet metal shears to trim off the excess stainless flex *a standard screwdriver to tighten the draw bands on the tee, tee cap, top plate and rain cap
Top Plate	First, attach the tee cap to the bottom of the tee.
Flex Pipe	Remove the tee snout by loosening the draw band with a screwdriver and leave it below, carrying all other liner parts up onto the roof. Attach the tee to the bottom of the flex pipe by tightening the draw band, and lower this assembly down the chimney. Have your partner down below line up the tee with the thimble opening in the chimney and re-attach the snout by tightening the draw band.
Tee w/ Removable Snout	Place the top plate assembly over the protruding flex pipe and affix it to the chimney top or flue tile with high-temp silicone sealant. Tighten the top plate's draw band around the flex pipe. Cut the excess flex pipe off at the top with sheet metal shears. Fit the rain cap over the stand-up collar on the top plate and tighten the cap's draw band.
Tee Cap	Down below, mortar the tee snout into the thimble opening and attach your stove pipe (note: it will be necessary to crimp either the tee snout or the stove pipe to make this fitting).

Note: Each Code Authority chooses the standards that regulate appliance and chimney installation and usage in their jurisdiction, and may modify code specs as desired. The above-referenced specification is from the Washington Association of Building Officials Woodstove Installation Code Book and NFPA 211, and may not reflect the code requirements in your area. Contact your Code Authority for local regulations.

Manufacturers who submit an appliance to a recognized laboratory for safety testing may receive a specific venting listing for that appliance which may be accepted by your Code Authority in lieu of the standard code requirements. These listings are unique to each model, and can be found in the installation manual.