The post below was sent as an issue of my newsletter, Ready for Rain.
Sometime in the middle of the house project, I learned a lesson about chimneys that has fascinated me ever since. We were talking about the metal tubes, or “flues” that would eventually stick out of our roof and vent our two wood-burning fireplaces. According to the fireplace company, the size of the fireplaces meant the flues needed to be eighteen feet high to work.
What? Eighteen feet? I thought there must be some mistake. An eighteen foot flue meant we’d have two 8 foot metal tubes sticking out of our roof. Why did they need to be so tall?
At the time, I received the end-all-be-all of answers to this question: physics. Unless we could somehow bend the immutable laws of physics toward our needs, the flues were going to be that tall. Apparently, it’s about “pull”, as in, “Without a flue that’s tall enough, you can’t get a good pull and it will be difficult to start a fire and keep smoke out of the house.” That sounded important, but I didn’t really understand the connection between height and effectiveness.
After a bit of research, I think about it like this… The flue is like a straw that sucks air out of a home and carries fireplace smoke with it. Like a siphon that sucks liquid, this kind of vacuum can be self-sustaining, but has to be jump started. Thankfully, we don’t have to suck the air from the top because we can push it from the bottom with heat from the fire.
This is why the length of the flue matters. Once the heated air starts rising, it creates a draft going up the flue that pulls fresh air into the fireplace. That supply of fresh air is what feeds the fire. To keep it going, the air needs to stay warm and travel upward for long enough to establish a flow to the top. That’s the “pull”. Once it’s going, the room can stay smoke-free because the fireplace is constantly sucking air up and out.
If the flue is too short, the flow is difficult to start because the warm air needs to travel upward for a while to get the flow going. That’s where physics comes in. According to a formula I don’t fully understand, the sucking action will happen reliably when the warm air travels about eighteen feet from the fireplace.
This article from the Chimney Safety Institute of America was once of the most helpful I found on the subject.
This was fascinating and I started to look at every chimney and flue I could find. They were all much taller than I remembered. The physics of heat and air flow were governing so many homes that flue height was unremarkable or even invisible. Like gutters and downspouts, they eventually just blend in.
Our Yurt-shaped house had one:
For a while I was concerned about the aesthetics of having two shiny metal tubes sticking out of our roof. Would it look weird? Would people wonder why they were there? My fears faded when I looked more closely at other homes on Orcas Island. They all had unreasonably tall flues that I’d never noticed before.
On Orcas, wood is a very common form of heating. Many homes, maybe a majority, have a wood burning stove that’s a major source of heat in the winter. Multiple companies cut, process, and deliver cords of wood to wood sheds throughout the year. One company, called Axe and Wedge has a well-designed website and a newsletter with nice photography and personal stories.
Before the house got started, we took out a few Douglas firs that were too close to the foundation. The wood was a much lower quality than you’d use for furniture and we needed to get it out of the way. So, we made a handshake deal with a wood processor. He removed all the logs in exchange for dropping off two cords of seasoned (ready to burn) wood in the future. A cord, if you’re curious, is this big.
In the Hunter House in Seattle, we had a natural gas fireplace that was stylish and came on with the push of a button. We thought we’d use something similar in the new house. But then we started visiting more homes and saw that wood stoves and fireplaces were part of island culture. Island homes burn wood. Thankfully, it’s abundant on the island, renewable, and mostly carbon neutral.
This realization helped me adjust to the idea of the big shiny flues. Along with being essential, they also told a story. This house has fireplaces. It burns wood. The flues became a design accent in my mind and something I looked forward to seeing on our roof.
For us, the fireplaces are not essential and won’t be used as a primary heat source. We have efficient in-floor radiant heat, so the fireplaces are for power outages, occasional heat along with ambience, sound, and smell. I can spend hours poking fires and I can’t wait to do so.
A few days ago, part of the flues were installed and I love how they look. Soon they’ll get taller and that’s OK with me. This is an island home.
Related: Designing our Blackened Steel Fireplace.