Vermont Casting 2550 Flue Sizing, Pipe & Chimney Layout, Single venting, Creosote, Fuel

Encore Woodburning Stove

Flue Sizing

The inside size of a chimney for an ‘airtight’ stove should match the size of the stove’s flue outlet. When a chimney serves an airtight, more is not better; in fact, it can be a disadvantage. Hot gases lose heat through expansion; if we vent a stove with a six-inch flue collar (28 square inch area) into a 10 x 10” flue, the gases expand to over three times their original size. This cools the gases, which weakens draft strength. If an over- sized flue is also outside the house, the heat it absorbs gets transferred to the outdoor air and the flue usually stays cool.

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 smoke warm, and the result is a stronger draft. An uninsulated liner is a second choice - the liner keeps the smoke restricted to its original size, but the smoke still must warm up the air around the liner. This makes the warm-up process take longer.

Pipe & Chimney Layout

Every turn the smoke 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, to a completely straight chimney. If you’re starting from scratch, use this layout if possible. If the stovepipe must elbow to enter a chimney, locate the thimble about midway between the stove top and the ceiling. This achieves several goals: it lets the smoke speed up before it must turn, it leaves some pipe in the room for heat transfer, and it gives you long-term flexibility for installing a different stove without relocating the thimble.

There should be no more than 8’ (2.4m) of single-wall stove pipe between the stove and a chimney; longer runs can cool the smoke enough to cause draft and creosote problems. Use double-wall stove pipe for long runs.

Single venting

Each ‘airtight’ stove requires its own flue. If an airtight stove is vented to a flue that also serves an open fire- place, or a leakier stove, it’s easier for the chimney draft to pull air in through those channels than it is to pull air through the airtight, and performance suffers. Imagine a vacuum cleaner with a hole in the hose to see the ef- fect here. In some cases the other appliance can even cause a negative draft through the airtight, and result in a dangerous draft reversal.

Creosote

Creosote is a by-product of slow wood-burning. It’s an organic tar that can condense in the flue if it’s dense in the smoke, and slow-moving, and cools off to less than 290°F. Condensed creosote is volatile, and can generate chimney fires if it gets hot enough. All the features that affect chimney draft also affect creosote condensation - so use whatever combination of installa- tion features and operational steps will encourage good draft and minimize creosote production.

Because letting the smoke cool off and slow down is one of the keys to creosote production, it makes sense to line a chimney to match the stove’s outlet size, for safety reasons as well as performance. Canadian law requires a matching liner to serve any stove or insert vented through a fireplace chimney; in the US, the National Fire Protection Association recommends one if the flue is more than three times bigger (in square area) than the outlet on the stove or insert. Some localities enforce the NFPA guidelines as part of their building

codes.

Fuel

Even the best stove installation will not perform well with poor fuel. The best fuel is hardwood that has air- dried 12-18 months. Softwood burns, but not as long as hardwood. Fairly ‘green’ wood has a good amount of moisture in it; it will burn, but some of the heat potential is used to drive the remaining moisture off the wood. This reduces the amount that reaches your home and can contribute to a creosote problem. There are mois- ture meters available for firewood; you can also judge your wood by its appearance and weight. If you get it green, lift a piece and get a sense of its weight; it can lose a third or more of its weight as it dries. Also look at the ends of a log; as it dries it shrinks and often cracks. The more weathered and cracked a piece is, the drier it is.

Dry wood burns readily with a good chimney draft. But with modern stoves, especially catalytic ones, the wood can be too dry. While extra-dry wood has little creosote in it, the remainder can ‘gas out’ from the wood quickly and densely enough to overload the catalytic burner.

If you hear a rumbling or roaring noise, like a propane torch, from the stove, that is a sign the catalyst is seri- ously overfiring. The catalyst is a platinum film on a ceramic base; the metal can get to higher temperatures than the ceramic can take, and overfiring the catalyst can break it. Dry wood can also burn out faster than you want. If your dry wood burns out too quickly or overloads the catalyst you can mix in greener wood to slow the fire down.

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