A well chosen chimney cap will not adversely affect your chimney’s draft, and it might even improve it. Under some circumstances, a poorly maintained or badly designed chimney cap could make your chimney draw badly. . If your chimney does not have a cap, there are many reasons why installing one is a good idea.
If your fireplace or woodstove draws well, you will enjoy burning wood efficiently without having to deal with smoke or noxious fumes backing up into your house. A poorly drafting chimney, on the other hand, can make you waste a lot of money because of inefficient fuel burning and can make your house unsafe because of smoke and carbon monoxide leaking into your house instead of being sucked up the chimney.
If you choose the right chimney cap for your chimney and maintain it well, it should not decrease your chimney’s draft. A chimney cap that is fit too closely to the top of the flue or chimney liner might inhibit the flow of gases from the chimney, so be sure your cap has adequate height above the chimney opening to allow free flow of smoke and gases.
Most chimney caps have screened side openings that double as spark arrestors while they keep animals and other debris from entering the flue. If you do not keep the screens clean and clear, they can reduce your chimney’s draft, resulting in a dirtier chimney and in smoke backing up into your house.
If you start fires using paper, be sure the paper ash flying up the chimney does not block the screen. Check your chimney frequently to be sure soot and creosote has not built up on the screen sufficiently thickly to inhibit gas flow.
If your chimney cap has sufficient clearance above the flue opening and you keep the screen clean, it should not decrease your chimney draft and it might even improve it. When the wind is blowing, the convex shape of the cap induces a slight vacuum at the top of the flue due to the Bernoulli Effect, so your chimney should draw better with the cap in place.
Some chimney caps are specifically designed to improve chimney draft. If your chimney system has chronic draft problems, maybe due to the flue diameter being too small or too large, or the flue being too short, a specialty chimney cap might solve your problem. The Vacu-Stack Chimney Cap is specifically designed to eliminate drafting problems in chimneys that tend to have down-drafts under windy conditions. There are also chimney caps with built-in electric fans that actively draw air and gases up the chimney and others with wind-driven blades that might help improve draft.
Installing a carefully selected chimney cap will make your chimney safer, cheaper to maintain and more attractive. The cap should not have an adverse impact on your chimney’s draft and might even improve it. Consult with your local chimney sweep to get recommendations for best type of chimney cap to choose for your particular chimney.