Look After Your Lungs
Posted August 29th, 2007 by Sarah N
Here at Lehman’s, we’ve sold and installed thousands of wood burning stoves over the past 50-plus years. As a result, we’ve had to diagnose and fix problems such as low heat output, fire starting difficulty, dirty viewing glass and smoke leakage. Our vast experience has shown there are a number of factors which influence the satisfactory operation of a woodstove.
These factors are:
1. Chimney issues
2. Stove
3. Operator
4. Fuel (wood)
Of these factors, we’ve found that chimney deficiencies account for the vast majority of potential problems. Chimneys are the “lungs” of your woodstove installation. Just as with your own set of lungs, the chimney is responsible for delivering oxygen to the heart of your system (your stove) and then for disposing of this used-up air and other byproducts of combustion back outside. By following the guidelines given in this article, you can learn which type of chimney to build or how to upgrade your present chimney to a higher standard.
There are three types of chimneys commonly used for wood stove installations.
* Masonry free standing chimneys - These are usually lined with a clay flue tile, and are typically sized from 6×6 to 8×12 inside diameter. The outside wall is either cement block or brick.
* Pre-fab metal chimneys - Stainless steel insulated class A chimneys are popular because of their light weight and relative ease of installation. The common sizes for wood stoves are 6″ and 8″ round.
* Fireplace Conversions - Most fireplaces have larger masonry flues, usually 12×12 clay tile. Some fireplaces are factory built (also called zero clearance and pre-fab). Special care should be taken to properly covert these fireplaces to a wood burning stove. Please contact the Lehmans staff if you have a pre-fab fireplace and would like to convert it.
Fundamentals
The function of a chimney is two-fold: first, to carry the undesirable products of combustion (smoke) out of the home and second, to supply the draft (pressure) to feed air to the fire. Draft 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
Chimney Height- The taller the better! A taller chimney will have more draft than a shorter chimney (all other things being equal). It’s relatively easy to decrease the draft in a chimney which is too strong, but improving the draft on a short chimney can be 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. This problem is most common when connecting a stove or insert to an existing masonry fireplace.
Chimney Location - A warm chimney drafts better than a cold one. Chimneys located partly or totally inside a structure will perform better than those exposed to the cold.
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.
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 a very common problem. A customer might tell us, “I’m trying to light my stove and the whole house is filling up with smoke.” In almost all of these situations the problem is not with the stove, but with a chimney that has reversed. Cold air falls and warm air rises. When your stove is 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 aggravate 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.
Here are 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 pipe enters, try stuffing 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 may help the chimney get its startup 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.
Marginal chimneys
A marginal chimney may work most of the time and then suddenly become troublesome when you have several aggravating conditions at one time. For example, improperly sized chimneys may work fine except in high altitude installations, or when exposed to high wind or a low-pressure systems, such as rain or fog. Here’s how to bring your marginal chimney up to snuff:
1. Tighten up - Make certain that all of the stovepipe joints, from the stove conection to the chimney, are tight and sealed with furnace cement and screws. If these joints leak air, this “spoils” the draft of the chimney through the stove, much like a vacumn cleaner hose with holes in it would not suck well at the nozzle end.
2. Straighten out - If your stovepipe uses two or more 90 degree bends, you may be able to replace them with 45 degree elbos and sloping pipe. This will carry the draft better.
3. Grow Up! - Adding to the height of the chimney will help increase the draft. Ask Lehmans or your local installer/sweep about options for increasing your chimney height.
4. Check the Cap - Some chimney caps can actually restrict your draft. Consider replacing your cap or removing it completely (on masonry chimneys). You can also buy draft increasing chimney caps which use wind to increase the draft.
5. Give it some Air - If your home or room is very tight, bringing outside air into the room or directly to the stove (many models allow this hookup) will help with chimney draft.
6. Dial-a-Draft - There are some situations where the above remedies are difficult or impossible to achieve. In these rare cases, an electric draft inducer can be added to your chimney. This device uses a small fan to suck air up your chimney. Draft inducers are 
available in models which install on the interior stove pipe or on the chimney top.
We at Lehman’s hope that this article has given you some brief insight into common chimney situations. If you think you have a chimney problem, please discuss it with our staff so we can help you diagnose the best solutions.
Lehman’s has been selling wood stoves for over 50 years. You can get a personal stove recommendation from Lehman’s by visiting www.mostwoodstoves.com, or email our non-commissioned sales staff directly at stoves@lehmans.com.
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