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ARTICLES - General - Chimneys - An Unexact Science

Chimneys—An unexact science !

Did You Know that a adequate chimney is actually more important for successful wood burning than a good stove or fireplace? In other words it would be better to have a good chimney and a poor stove than the opposite.

As the title implies, chimneys are an unexact science. The intent of this paper is to educate you so you can make the proper decisions concerning your wood burning systems. Problems with chimneys -If a stove smokes into the room , an inadequate chimney is the most likely cause. Creosote problems can often be lessened by good chimneys. Low heating efficiencies can also often be traced to poor chimney draft—In short, 90% of all wood burning problems (in our experience) can be traced to draft and chimneys.

Fundamentals- The function of a chimney is two-fold, first to carry the undesirable products of combustion (smoke) out of the home : secondly to supply the draft (pressure) to feed air to the fire. The draft of a chimney comes from the tendency of hot air to rise . This flow up the chimney is determined by many factors such as :
1. Chimney Height
2. Chimney Diameter
3. Chimney Location
4. Bends in the Chimney
5. Construction of Chimney
6. Tightness of House

Reversing Chimneys - a common problem Wind Related Downdrafts

By following some of the guidelines given in this paper you can learn which type of chimney to build or how to upgrade your present chimney to a higher standard.

Chimney Height- The taller the better ! A taller chimney will have more draft than a shorter chimney (all other things being equal). Its relatively easy to decrease the draft in a chimney which is too strong, but improving the draft on a short chimney can be more difficult.

Chimney Diameter- Chimney diameter also has an effect on the draft of your chimney. In this case, however, having a chimney that is too wide (as compared to the outlet on the stove) can be a problem since it allows the smoke to cool excessively, thereby slowing it down. If possible, the chimney diameter should be similar to the diameter of the flue outlet on the stove. In no case should it be more than 3 or 4 times the cross-sectional area of the stove outlet. If your chimney is oversized, you may consider relining it with a smaller diameter flue.

Chimney Location—The early settlers in this country knew some things we have forgot. They located the chimney inside the structure, where it would stay warm and draft better. This indoor location also meant that the warm chimney added some heat to the home. Take a hint from our forefathers…locate your chimney inside if possible !

Bends in the Chimney—Each turn of a chimney lowers the draft. A rule of thumb is that each 90 degree turn cuts 5 feet off of the effective height of a chimney. Therefore, if you have a 12 foot chimney with two turns, the effective chimney height is 2 feet !—surely not enough height for a stove to work properly.

Construction of Chimney—A round chimney flue drafts better than a square or rectangular one. In addition, an insulated chimney (either a metal one or properly constructed masonry chimney) will get warm and stay warm easier, making it easier for the chimney to draft properly. More on Chimney sizing issues.

Tightness of Home—All heating appliances need air to support the combustion. A house that is too tight may not allow enough air to feed the fire, and a smoking stove or fireplace can result. Many fireplaces and stoves allow for the connection a pipe which draws the combustion air from outside the home. In addition, a tight house can aggravate the problem of Flow Reversal.

Other Chimney Problems :
Flow Reversal or Down drafting chimney—This is the most common problem I have run into in my 20 years of business. A typical phone call comes in from a customer saying “I’m trying to light my stove and the whole house is filling up with smoke”. Of course, the problem is not with the stove, but with a chimney that has reversed. Cold air falls and warm air rises, we all know this and take it for granted…well, your chimney knows it too ! When it’s not in use, the cold air from outdoors falls down the chimney and tries to enter the house. Clothes dryers, furnaces and bathroom/kitchen fans agravate the problem further since they suck air from the home and push it outside. This air needs to be replaced from somewhere…and your chimney is just waiting to supply it. So, if you just open your stove or fireplace damper and light a fire, you may get a surprise.

There are a number of common sense solutions to this problem. First, make certain that your chimney is drafting upwards before starting your fire. See the document Starting a Fire for the lowdown on this technique. Here’s a few other tips on solving this problem: If you have an exterior masonry chimney, be sure the cleanout door is sealed well. If the chimney has a large area below where the stove enters (I call this a “cold air sump”), then stuff a piece of non-combustible rockwool or fiberglass insulation to close off this area. Crack a window near the stove or fireplace before you start it. This will help the chimney get its air.

Wind Related Downdraft—If your stove or fireplace spills smoke into the house only on certain days, then your problem may be wind related. In certain cases, installing a special type of draft increasing chimney cap (vacu-stak is one brand name) can help with this problem. Increasing the height of the chimney can also be of benefit.


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