A stove is part of a system which includes the chim- ney, the operator, the fuel and the home. Each part of the system affects how well the stove operates. When there is a good match between all the parts, the system works well.
Wood stove or insert operation depends on natural (unforced) draft. Natural draft occurs when the smoke is hotter (and therefore lighter) than the outdoor air at the top of the chimney. The larger the temperature difference, the stronger the draft. As the smoke rises from the chimney it provides suction or ‘draw’ that pulls air into the stove for combustion. A slow, lazy fire with the stove’s air inlets fully open indicates a weak draft. A brisk fire, supported only by air entering the stove through the normal inlets, indicates a good draft. The stove’s air inlets are passive; they regulate how much air can enter the stove, but depend entirely on the draft of the chimney.
Depending on the features of your installation - (i.e. metal or masonry chimney installed, inside or outside the house, matched to the stove’s outlet or oversized) - your system may warm up quickly or it may take a while to warm up and operate well. With an ‘airtight’ stove or insert, one which restricts the amount of air getting into the firebox, the chimney must keep the smoke warm all the way to the outdoors. Some chimneys do this better than others. Here is a list of features and their effects.
Masonry Chimney
Masonry is a traditional material for chimneys, but it can perform poorly when it serves an ‘airtight’ stove. Masonry is a very effective ‘heat sink’ - it absorbs a lot of heat. It can cool the smoke enough to diminish draft. The bigger the chimney, the longer it takes to warm up. It’s often very difficult to warm up an outdoor masonry chimney, especially an oversized one, and keep it warm enough to maintain an adequate draft.
Metal Chimney
Most factory-made metal chimneys have a layer of insulation around the inner flue. This insulation keeps the smoke warm. The insulation is less dense than ma- sonry, so a steel chimney warms up more quickly than a masonry chimney. Metal doesn’t have the good looks of masonry, but it often performs much better.
Indoor / Outdoor location
Because the chimney must keep the smoke warm, it is best to locate it inside the house. The relatively warm house then surrounds the chimney keeping it warm. This also means that heat from the chimney walls will transfer to the house and not be wasted outdoors. An indoor chimney will not lose its heat to the outdoors, so
it takes less heat from the stove to heat it up and keep it warm.
Flue Sizing
The interior size of a chimney for an ‘airtight’ stove should match the size of the stove’s flue outlet. When a chimney serves an airtight stove, “more” is not “better”; in fact, it can be a disadvantage. Exhaust gases move more slowly through larger chimneys and can lose more heat to the chimney walls. This weakens the draft strength. If an oversized flue is also outside the house, the heat it absorbs gets transferred to the outdoor air and the flue is further cooled.
It’s common for a masonry flue, especially one serving a fireplace, to be oversized for the stove. It can take quite a while to warm up such a flue, and the results can be disappointing. The best solution to an oversized flue is an insulated steel chimney liner, the same diam- eter as the stove or insert’s flue outlet; the liner keeps the exhaust warm, and the result is a stronger draft. An uninsulated liner is a second choice - the liner keeps the exhaust path restricted to its original size, but the air around the liner must still be heated. This makes the warm-up process take longer.
Pipe & Chimney Layout
Every turn the exhaust must take as it travels to the chimney top will slow it down. The ideal pipe and chim- ney layout is straight up from the stove, and into a verti- cal chimney. If you are starting from scratch, use this layout if possible. If the stovepipe must turn to enter a chimney, locate the thimble about midway between the stove top and the ceiling. This achieves several goals: it allows the exhaust gases to speed up before turning, it leaves some pipe in the room for heat transfer, and gives you long-term flexibility to install a future stove without relocating the thimble.
There should be no more than 8 feet (2.4m) of single- wall stove pipe between the stove and a chimney; lon- ger runs can cool the exhaust gases enough to cause draft and creosote problems. Use double-wall stove pipe for long runs.
Single venting
Each ‘airtight’ stove requires its own chimney. If an air- tight stove is vented to a flue that also serves an open fireplace, it is easier for the chimney draft to pull air in through those channels than it is to pull air through the stove, and performance suffers. Imagine a vacuum cleaner with a hole in the hose to see the effect here. In some cases the other appliance can even cause a negative draft through the airtight, and result in a dan- gerous draft reversal.
