Smoldering Wood

Photo by: Skylare Klang, Creative Commons Licensed photo Smoldering wood in hot burning fireplace.

I heat my home with wood, which I imagine a lot of you do as well, and this week I came across a problem with my double barrel stove. It wants to smoke unless it is burning extremely hot. I know it is a problem with the draft from the chimney, but I was not sure exactly how to solve it. I was always told that if there was not enough draft, your chimney was not high enough, but is that the real science behind how the draft works?

I began reading articles on chimney design and what makes a good chimney. I learned a couple of things that surprised me just a little. The first was that the height of the chimney is only one factor in the design of a proper chimney. There are four factors that cause the smoke to rise out of the chimney and both are referred to as the draft of the chimney. The first is the temperature difference between the top and bottom of the chimney. If the top of the chimney manages to get too hot, the draft will decrease and you may get smoke in your house.

The second factor that contributes to the draft is the pressure difference between the inside and outside of the house. Hot air is less dense than cold air, meaning heating the air in the house will result in a pressure difference between the inside and outside of the house. The larger the temperature difference, the stronger the draft. This explains why a fireplace with a low chimney may burn perfectly on a really cold day with high atmospheric pressure but smoke a lot on a warmer day, or before a pending storm when the atmospheric pressure drops. This is the biggest contributing factor to a good draft. The height of the chimney helps to increase this pressure difference and a leaking chimney will require more height to compensate for cool air entering the chimney.

The third factor is one that surprised me and is likely one of the main problems with my chimney. A well-insulated house that has little area for outside air to enter will cause a smoky fireplace. If your house is sealed too well, there is no way to replace the air rising out of the chimney rapidly enough to keep a strong draft. This will cause a vacuum effect in the house, lower the indoor air pressure and cause a strong downdraft in the chimney. My problem started when I added a second wood burning stove in my living room. This increased the amount of air leaving the house via the draft of the chimneys and both wood stoves started smoking more than usual. I found that if I opened a window near either one of the wood stoves, the smoking was drastically reduced. This creates a new problem of letting cold air into the space I am trying to heat, so I had to look for another solution.

The fourth factor impacting the draft of the chimney is the route from the fire to the chimney. The entire route needs to be smooth, straight and vertical wherever possible. If bends cannot be avoided they should be at no more than a 45 degree angle, and additional height must be added to compensate for the non-vertical section. This turns out to be the main problem with both of my wood stoves. I am using insulated stove pipe on one, and non-insulated stove pipe on the other. I see more smoke issues with the non-insulated pipe, but both have two 90 degree elbows in the pipe. Solving this issue will prove to be difficult considering that I do not want to run the chimney vertically through the two story house, and I have not seen anything other than 90 degree elbows for the insulated stove pipe. I may try and fabricate a chimney of my own design.

Until next week, stay safe and learn something new.

Scott Hamilton is an Expert in Emerging Technologies at ATOS and can be reached with questions and comments via email to shamilton@techshepherd.org or through his website at https://www.techshepherd.org.

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