The Chimney SweepOnline Fireplace, Woodstove, Gas Stove and Barbecue Shop
Q: We just bought a wood-burner, and the guy at the fireplace shop says they aren't allowed to install it unless they also put in a stainless steel liner kit all the way to the top of my chimney. Why would that be? What's in a liner kit anyway? And why stainless steel?
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, liquid creosote will eventually dissolve the binder in the mortar joints, destroying the structural integrity of the structure. Even worse, when the mortar joints are saturated with creosote, 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 terra cotta 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 3x that size (84)". 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 2x 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. Installing an insulated liner helps eliminate those problems.
Note: Woodstove liners must be made of stainless steel, because up inside the chimney, the corrosive wood creosote would rapidly turn most other metals into swiss cheese.
Here are three typical "reline jobs" with a list of the parts required
For liner kit pricing, click here.
3/13/07: Do I need to reline?
Q: We must replace our old catalytic Scandia 310C wood stove with 8" flue due to a cracked back panel. We use it to heat our 20x20' downstairs playroom, and provide supplementary heat to the rest of house. We burn maybe 2 cords a season: poplar, some oak.
In 15 yrs, we've never had any problems with it or the exterior, two-story 8" x 13"terracotta lined chimney. The chimney and stovepipe are always "very clean," never much creosote, hardly any ash, lining and joints intact. Now we're told that newer stoves "require" lining the chimney with smaller diameter flue pipe. I understand that the warranties of some models require such, as does our county codes department. Are there any stoves we can get that will not require refluing the chimney? I want to meet all relevant codes; but wonder if, given the excellent cleanliness record with our old stove, the newer "requirements" may be a bit overstated, as in CYA, as in "idiotproof"? Are the newer stoves truly drawing so much less air than the old Scandia, which, due to age and design, is in no way truly "airtight"? Any advice would be appreciated, especially on possibilities of slightly less efficient stoves that might vent into a larger flue. Thanks so much!
The flue liner requirement results from a National Fire Prevention Association flue sizing code spec that dates back many years. Briefly stated, NFPA-211 specifies that inerior chimneys (chimneys that are contained within the heated house until they exit the roof) cannot have a flue opening measuring more than three times the size of the exhaust opening on the stove, and chimneys which run up the outside of the house cannot have a flue opening measuring more than twice the size of the exhaust opening on the stove.
The main reason for the code sizing spec is draft: if a column of wood exhaust has to expand to several times its size to fill an oversized chimney flue, the flow rate is reduced and the exhaust cools in the flue, resulting in smoke back-puffing and excessive creosote formation. The problem is worse in chimneys on outside walls, as cold outdoor temperatures cool the structure, making it harder to maintain good flue gas temperatures within. This issue has intensified in recent years, as today's super-efficient woodstoves radiate more of the heat from the fire into the house, so they have cooler exhaust temperatures than the older models like your Scandia.
An 8 x 13 terra cotta flue liner with rounded corners has a cross-sectional area (CSA) of 100 sq.in., so if the chimney runs up the outside of the house, no woodstove with a flue collar CSA of less than 50 sq.in. can be vented into it unless a properly-sized liner is installed. An 8" woodstove flue collar has a CSA of 50.75 sq.in., so your Scandia installation was juuuust barely up to code. Given the relatively hot flue gas temperatures the Scandias produced, it's not surprising you had no drafting or creosote problems.
Most of today's woodstoves have 6" flue collars (28 sq.in. CSA), so code prohibits venting them into an exterior chimney with a CSA larger than 56 sq.in. This means if you vent a stove with a 6" flue collar into your chimney, code requires that you also install a 6" stainless steel liner.
If you don't want to install a liner, you'll need to find a replacement stove with an 8" flue collar, such as the Hearthstone Equinox.
A cautionary note: even if you choose a new stove with an 8" exhaust collar, your experience might be less than ideal. First, as mentioned above, a new stove might not produce sufficient exhaust temperatures to get a good draft going in your oversize flue. Second, today's stoves with 8" flue collars are SIZE LARGE, and may produce too much heat for you to bear in your 20 x 20 playroom.
4/24/12: Can I do a partial reline?
Q: I want to convert my conventional fireplace to a wood burning insert. I have a standard brick masonry chimney which is about 22 feet tall. I am getting conflicting advice with regards to lining the chimney. One source is telling me I only have to line from the insert to about 6 feet up and another source is advising me to line the whole thing. I need to be cost conscious and want to know how much liner to order. Any help would be appreciated.