What is back-puffing? Interestingly enough some know or have a vague idea, others are frankly clueless. I have even had some Home Inspector reports handed to me, which are speaking to back puffing and are TOTALLY WRONG. Another reason why Home Inspectors SHOULD NOT perform inspections for Real Estate transactions. Another discussion topic however, related to why they do not perform a Level 2 inspection as mandated in NFPA 211 or why they do not verify no obstructions in flue again mandated by NFPA 211.
But back to the item of discussion, back-puffing. Both back-puffing and build up of creosote within the stove are indications that there is a larger production of smoke than the chimney draft is able to handle. It occurs when smoke becomes dense in the firebox and ignites. The reason for back-puffing is most often due to mechanical and/or operational reasons. These can be related to installation problems or operational problems, so I will list a few to review. Modern, efficient stoves extract most of the heat from the smoke, which can make it more of a problem to rise up the chimney and does require a bit more understanding of proper operation than did the older less efficient cousins.
Cause 1: Cool chimney or pipe system
This is one of the most common causes, and is less likely to occur with factory built chimney or an insulated liner inside the masonry chimney. It also is less likely to occur if chimney is inside house versus on outside wall. The reason is to maintain a good draft, the chimney and stove pipe need to be and to remain warm. If you have long runs of single wall flue pipe, enough heat can be radiated into the room that there just isn’t enough heat left to warm the chimney enough. The cool chimney will absorb more heat from the smoke, taking it to the point it simply does ot rise up and out of the chimney. Essentially as smoke cools it looses it lighter than air property and simply hangs in the air. Furthermore, cool smoke condenses on the flue pipes and chimney creating a layer of creosote.
Cause 2: Masonry construction
Masonry chimneys are difficult to warm as there is a huge mass involved, however once warm they will also remain warm for longer periods. If the masonry chimney is exterior to the home and exposed to winter temperatures, this even becomes worse, as there is a much greater degree of heating required. This may mean you need to run with the stove damper wide open and even the door open a bit for a longer period to get the chimney warmed and working properly. A better solution is to have an insulated liner installed.
Cause 3: Outside location
An outdoor chimney is much more difficult to warm, and will loose the heat to the outdoor environment much faster.can transfer its heat to the great outdoors readily. Also, the higher the chimney the more problematic this can become as the flue gases will cool as they go past more and more chimney wall. Some recommend an external chase around the chimney, while better than nothing is not the ideal solution. First I have seen far too many chimney chases which were not installed properly and in fact themselves added to the fire hazard or risk. The best solution is actually an insulated steel liner or insert, which will make the chimney behave much like a factory built chimney. There may be places in the south where external chimney chases are great, but in the Midland and Penetang area, frankly they are not the wisest solution.
So far we have covered 3 items, and they all sound similar! But realistically these are the most common reasons for the problem. Lets continue.
Cause 4: Oversized flue
The flue pipe should ideally be the same size all the way up. Venting a stove to a larger flue creates some issues related to physics, hot gases loose heat when they are allowed to expand, and in a larger flue there is a lot of room for expansion. Now sometimes with more than one appliance sharing a single flue, you have a larger flue size. One of the reasons I am a strong supporter of one appliance per flue. The worst case of an oversized flue is when somebody vents a stove or insert into the fireplace chimney, which is always much larger than the stove/insert flue size. When the fireplace chimney is on an exterior wall, things get even worse. Sometimes a stove vented into a fireplace chimney on an exterior wall will never heat the chimney enough to operate properly and will always be a problem with smoke and back-drafting or puffing. As in any other masonry chimney, the best solution is to have a steel liner or insert take the flue gases to the top of the chimney and an insulated liner is even better.
Cause 5: Leaky chimney or pipe system
The chimney, flue pipe and stove must be one continuous and preferably air-tight system. Air-tight to keep smoke in is a great idea, but also makes the system work better. A leak in a chimney causes loss of draft, air will be sucked in through the holes and reduce the upward suction or draft you want and need. It a proper system the joints between sections of stove pipe will be secured with screws, and the joint between the flue collar and the stove pipe will also be tight. I have smoke pencils and infrared cameras to find leaks in the system.
Cause 6: Pipe layout
The design or layout of the flue pipes should be as straight as possible and as vertical as practical. Fewer elbows are better and a 45 degree elbow offers less resistance than does a 90 degree elbow. In fact two 45 degree elbows offer less resistance than a single 90 degree elbow! Long horizontal runs of pipe should be avoided if at all possible as they are detrimental to having a strong and solid chimney draft.
Cause 7: Blocked passages in the stove
Over the years some stoves were simply designed poorly. Vermont Castings was one that comes to mind, having heat-exchange passages in the rear where smoke must pass before leaving through the chimney. Smoke traveling through these spaces slows down in velocity and in doing so is also given the opportunity to cool in temperature, condense and leave creosote deposits. These can then lead to partially blocked passages, which restrict the smoke flow, increasing the chances of back-puffing. The area behind the firebrick must be hand cleaned with a wire brush. On the Vermont Castings Defiant and Vigilant stoves, you can reach into the the smoke entry port at the bottom right end of the firebox and through the flue opening to clean the upper end of the passage. On the Vermont Castings Resolute stoves the upper left and right inner panels must be removed to release the refractory brick, which allows cleaning behind it. The Defiant and Vigilant are both stoves where the air inlet tubes can actually become blocked with ash buildup, in these stoves close the stove doors and open the damper and use the vacuum to push the air in through the primary air inlet on the rear of the stove – this should clear the air ports inside the firebox (ensure there is a blanket over the openings of the firebox to catch any dust and ash in the blown air). Look at the owners manual for your wood stove, it should give instructions on maintenance and proper cleaning of all smoke passages to ensure proper operation.
A good Chimney Sweep will clean not only the chimney but also the wood stove.
Cause 8: Insufficient combustion air from house.
The homes today are much better insulated and sealed than in the past. The wood stove does not have any mechanical draft assistance to pull the air in and allow it to overcome this tightness or to overpower other appliances which are pulling air out (dryer, range hood, bathroom fan, etc) These appliances with fans will always grab the air first, and if the wood stove is in the basement it is already at a disadvantage placed on it by physics. If your stove burns better with window nearby slightly open, that is a sign that it is lacking combustion air supply. The solution is often to provide a direct source of combustion air from outside to the firebox, or to place a proper HRV or central ventilator into the home.
Cause 9: Overdry firewood
Realistically the ideal drying time for firewood is generally around 12-18 months. As wood drys there is actually less highly volatile gas remaining in it, however what does remain if overly dry can get out much faster than from greener wood. What this means is that with wood that is overly dry, there is a period in the early burn cycle where there is a lot volatile gas in the smoke than if green wood were being burned. If the wood is over 2 years old, use smaller loads and adjust the air supply to maintain desired hear or mix with greener wood to diminish the amount of volatile gases in the smoke and to help absorb some of the heat produced. Also keep in mind that smaller pieces have more surface area for the same amount of wood, the volatile gas is released from the surface, therefore smaller pieces are able to release more gases into the smoke. It helps to mix larger pieces into the load and not to use only small wood in the fire. Often times in early fall, you may be burning some of last year wood, which has had the extra summer to dry and back-puffing can be more common than it is in the spring.
Cause 10: Seasonal changes
Back-puffing is impacted by chimney draft, which is impacted by temperature. In the fall and spring, when smaller fires are built as it is not as cold, the draft may not be as strong. The colder outdoor air in winter and hotter fire will both help to improve the chimney draft. During spring and fall you may need to use smaller loads of wood and let more air in, with some stoves you may need to leave the damper open for the total burn.