Back Puffing

Back-puffing and creosote build-up in a stove indicate that the fire produces smoke faster than the chimney draft pulls it out of the stove. Back-puffing happens when the smoke becomes dense enough to ignite in the firebox. Since there are mechanical and operational causes for the condition, there are also mechanical and operational remedies. Since installations and burning habits vary widely, you should look over the complete list below, start with the easiest remedies and go as far as you need to minimize the condition. Keep in mind that the reason smoke rises is because it is hotter and lighter than air. If it cools off before it gets to the top of the chimney, there's less reason for it to rise.
Modern, airtight stoves extract most of the heat potential from wood right within the firebox. This sends less heat up the chimney, which is good for efficiency, but does require the user to do more to manage the draft than older leakier, less efficient stoves did.

Cause 1: Cool chimney or pipe system

To maintain a good draft, a chimney and stove pipe need to stay warm. Long runs of single-wall stovepipe can radiate enough heat to cool the smoke to the point where it doesn't contribute much heat to the chimney. A cool chimney absorbs heat from the smoke; as the smoke cools, it loses its bouyancy because it is no longer much lighter than air. Check the following list of chimney features to see the effects they have on performance.

Cause 2: Masonry construction

Because of its great mass, masonry absorbs a lot of heat. It takes longer to warm up than an insulated steel chimney. You may need to run the stove or insert open for a longer time than you want, to warm the chimney all the way to the top. This is a very common remedy.

Cause 3: Outside location

An outdoor chimney can transfer its heat to the great outdoors readily. And the higher you go on the chimney, the cooler that area is. This also means that the chimney will take extra time to warm up. If your chimey is outside the house, you can get better performance by building an insulated chase around it; otherwise you'll probably need to send more heat up the chimney than you want, in order to warm the flue enough to make it draw well. This may be part of the start-up process, or you may need to run the stove open a couple of times a day during continuous burning.

Cause 4: Oversized flue

Hot gases lose heat through expansion; this is an issue when the stove is vented to a flue that is bigger than it needs. A 6" diameter flue is 28 square inches in cross-section; an 8" flue is 50 square inches. So venting a stove with a 6" collar into an 8" round flue is a 75% increase in the volume of the smoke, which is enough to start the cooling process. The worst case for oversized flues is when a stove or insert is vented through a fireplace chimney, which is always at least twice as big as the flue for a stove. If the chimney is outside the house, this makes the problem more severe. It can take a very long time to warm up an oversized outdoor fireplace chimney, and even with continuous burning you may need to open up the stove a couple of times a day to warm the chimney.

The mechanical approach that will help to maximize draft strength is to re-line the chimney with a liner the same diameter as the stove or insert's flue collar. The physical restriction of the flue gasses will prevent heat loss through expansion. Chimney liners also keep a chimney cleaner, which is a safety advantage.

It may be possible to build an insulated chase around an outdoor steel chimney. This will help keep it warmer, which improves safety as well as performance.

Cause 5: Leaky chimney or pipe system

To maintain a good draft, a chimney or stove pipe must be solidly connected to the stove. Leaks in the chimney or pipe will have the same effect as a leak in a vacuum cleaner hose; the draft will pull air through the path of least resistance. You want the stove to be the only path. This means that the stove should not share its flue with any other appliance. If the stove shares a flue with a furnace, for example, it might be easier for the stack draft to pull air in through the furnace, especially if the furnace has a barometric damper. Every bit of air that gets pulled in through the furnace will be that much less air that gets pulled through the stove. If a masonry chimney has a cleanout door, it should be tightly sealed to prevent leakage.

Joints between sections of stove pipe must be secured with screws, and the joint between the flue collar and the stove pipe should also be snug. You can test for sizeable leaks with a small hot fire, with the stove's inner damper open; go around the pipe joints with a candle or stick of incense, and look for spots where the force of the draft pulls the smoke in.

Cause 6: Pipe layout

The layout of pipe should be as straight and as vertical as possible. Wherever possible the stove shoudl vent vertically as far as you can manage it before it turns, to give the flow of smoke a chnace to get started. Each elbow adds resistance to the flow of smoke. A pair of 45-degree elbows adds less resistance than a 90-degree elbow. Long horizontal runs of pipe are harmful to draft strength, and should be avoided.

Cause 7: Blocked passages in the stove

Stoves like the Vermont Castings' Defiant, Vigilant and Resolute have heat-exchange passages behind the fireback; smoke travels through these before it leaves the stove. Since the smoke must slow down a lot to get through these passages, it has a chance to condense out some soot or creosote, which can then partly block the flow and increase the chances of back-puffing. Hand-clean the areas behind the fireback with a wire brush, then remove the sweepings. On Defiant and Vigilant, you can reach through the smoke entry port at the bottom right end of the firebox, and you can reach through the flue opening (and then remove the smokeshelf) to clean the upper end of the passage. On Resolute, you must remove the left and right inner panels to release the fireback to clean behind it.

On Defiant and Vigilant stoves, ashes can build up deeply enough to clog the air inlet tubes. Close all the stove doors, open the damper, and use a vacuum running in reverse to push air in through the primary air inlet on the back of the stove. This should clear the air ports on the tube inside the firebox.

The owner's manual of your woodstove will give instructions for thorough cleaning of smoke passages to ensure good flow of flue gases through the stove, whether the damper is open or closed. Creosote, ash and fly ash can build up in baffle areas and interfere with free flow of smoke.

Cause 8: Insufficient air supply through the house.

A well-insulated or tightly-sealed house can suffocate a stove. A natural-draft appliance like a stove doesn't have the power to overcome this tightness, or to overpower other appliances like clothes dryers, range hoods, etc. Even with enough chimney draft, the most powerful appliance will get the available air first. You can test to isolate this as a possible cause of draft problems by running the stove with the nearest door or window cracked open slightly. If there's a noticeable difference in stove performance that's your sign that the stove should have a source of outdoor air for combustion hooked directly to the stove without mixing it with indoor air.

Cause 9: Overdry firewood

Dry firewood split and stacked 12-18 months is usually the best thing to burn; the drier it is, the less highly volatile gas it has. But the remaining volatiles in overly dry wood can gas out much faster than it will gas out from greener wood - so there is a period in the early part of a burn cycle with dry wood when there's more volatile gas in the smoke than there would be with green wood. If your wood is extra dry (more than 2 years old) you can change to smaller loads (and adjust the air supply accordingly to maintain the desired heat output) and/or mix in greener wood to absorb some of the heat and diminish the density of the volatiles in the smoke. Also a load of smaller pieces of wood has more total surface area than a load of larger pieces. Since volatile gas is driven off the surface of the wood, there's more driven off from loads of small peices than from loads of large pieces. If you've got small pieces of firewood, mix in larger pieces to help diminish the production of volatile gas. Also in fall, people may be burning some of last winter's leftover firewood, which has had all summer to get extra dry, so back-puffing is more common in fall than in spring.

Cause 10: Seasonal changes

Back-puffing is a function of chimney draft, which is weaker in spring and fall because people build cooler fires then, and because the outdoor air isn't as cool as in winter. The diminished difference in temperature and weight between smoke and outdoor air in spring and fall weakens chimney draft. Colder outdoor air in winter improves draft on indoor chimneys and can harm it on outdoor chimneys (because the colder outdoor air absorbs lots of the heat which the chimney gets from the smoke). In spring and fall, you may need to use smaller loads of wood and compensate by letting in more air; and in some installations you may need to leave the damper open all the time.

Back-puffing is a common occurrence and is manageable or even avoidable when you understand the causes and remedies.

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