Author – Todd Woofenden
If your stove seems to work okay once you get it going, but smokes when you are done using it, as the fire is going out, here are some possibilities. (Note: If your stove smokes when you are deliberately operating the stove at a low-burn, see the section, “Constant or Erratic Smoking.”)
Chimneys built on the outside of the house are notorious for smoking problems when the fire is low. While chimneys built up the middle of the house have the house to keep them warm, exterior chimneys cool quickly, since they are exposed to the cold weather. So as the fire dies and less heat is emitted to keep the flue warm, the draft tends to weaken, creating the potential for smoke spillage.
But usually when a stove smokes at the tail end of the fire, a cold, exterior chimney is a contributing factor, but not the only factor. Since it is generally impractical to build a new chimney inside the house (where it should have been built in the first place), first consider the other possible factors. If you can eliminate some other problems, the reduced draft that the cold chimney causes might no longer be critical.
Pay particular attention to the sections on chimney height, over-sized flue, and depressurized house.
Inadequate Chimney Height
The rule for chimney height, and the problems caused by inadequate height, are identical for stoves and fireplaces. For an explanation, see the section on inadequate chimney height in the article on fireplace problems.
Oversized Flue: For wood stove venting systems, bigger is not better. An excessively large chimney flue requires more heat to keep warm, and cools more quickly than a properly sized flue. Lower flue gas temperatures in large flues cause reduced draft, increased creosote buildup (since the smoke tends to cool and condense in the flue), and the potential for smoking problems, especially when the fire is not burning high.
It is fairly easy to determine the minimum and maximum chimney flue sizes for a wood stove:
Minimum Flue Size
Unless specified otherwise in the manufacturer’s installation instructions for your stove, the flue must be at least as large as the flue collar on the stove. (Remember, the flue collar is the opening on the stove that the stove pipe connects to.) For example: if the flue collar diameter is 6″, the cross-sectional area of the chimney flue must be at least equivalent to a 6″ diameter circle.
Note: Many flue liners are square or rectangular rather than round. While many venting experts regard round liners as superior to square or rectangular ones (due to peculiarities of air currents in vertical passageways, expansion factors, and chimney cleaning considerations), the critical factor in determining minimum flue size is the cross-sectional area of the flue, not the shape. In short, the flue size can’t be smaller than the stove collar size.
Maximum Flue Size
Current codes and standards generally recommend that the chimney flue be no larger than three times the cross-sectional area of the stove’s flue collar. Anything over three times larger will very likely result in serious venting problems, not to mention potentially disastrous chimney fires due to increased creosote buildup in the flue.
So, What Size Chimney Flue is Best
Ideally, the chimney flue should be the same size as the stove’s flue collar. Whether or not the stove can tolerate a larger flue (and how much larger) depends on the type of stove and how you use it.
Note: In any event, do not exceed the maximum flue size specified by applicable codes in your area.
Old, Non-Airtight Stoves
Old stoves such as Franklin stoves, which offer little control to the user over burn rate, tend to vent a great deal of heat into the chimney. Therefore, a somewhat oversized chimney may not be a problem..
Airtight stoves, which allow the user great control over combustion air, often vent lots of smoke into the chimney, often at relatively low temperatures. Since smoke moves more slowly and cools more quickly in these large flues, an over-sized flue can be a real problem, even if it falls within the less than three times rule. It is best to have a flue close to exactly the same size as the stove’s flue collar. (And, of course, it is best to learn to operate the stove in the correct temperature range. See the article, “Tips for Woodstove Users,” for details on proper stove operation.)
New, EPA-Certified Stoves
The new generation of EPA-certified stoves tends to vent much less smoke and much less heat into the chimney. The reduced smoke output tends to ease the problem of creosote buildup, but the reduced heat vented to the flue means less heat to keep the flue warm, and therefore less draft in a large flue.
The new stove models range in draft sensitivity from very draft sensitive (for which even a slightly oversized flue is a problem) to not at all draft sensitive (for which an oversized flue isn’t such a problem). As a general rule, the catalytic models are a bit more draft sensitive than the non-cats, but there are many exceptions.
If you have a new, EPA-certified stove and you think your chimney flue might be too large, talk to your local stove shop to get some feedback on the track record of your particular stove model before you make a decision on changing the venting system.
The solution to smoking problems caused by an over-sized chimney flue is to re-line the flue using the correct liner size. See article on “Types of Liners.” Generally, re-lining of existing chimneys is done with stainless steel or cast-in-place systems.
Inadequate available air in the house can create smoking problems, especially as the fire burns low, reducing the heat vented into the chimney. This type of problem and the solutions are the same for fireplaces or wood stoves. For a full discussion, see the article on Depressurized House.
Improper Air Space Around the Liner
Terra cotta liner systems incorporate an air space between the brickwork and the liner. Problems associated with an improper air space are the same to fireplaces or wood stoves. For a full discussion, see the article on Improper Air Space.
~ Todd Woofenden