Chimney Construction, by PrepNow

I would like to offer some information about my experience with chimney construction and creosote build up. This information does not apply to the typical suburban open fireplace. What I’m talking about is a wood-burning stove designed to heat your home or shelter. There are a number of manufactured fireplaces available that are designed to regulate the amount of combustion air traveling into the firebox and consequently the control the actual burn. These are the most efficient and are the type that we would be using in a structure designed to ride out the future storm.
Construction of the chimney is extremely important. In this case the old ways are not the best. Fire brick and chimney tile will eventually burn out and will not handle many chimney fires. I heated my two-story log home in Montana for years primarily with split pine, which is very susceptible to creosote build up. Due to the construction of the chimney and fireplace I was able to regularly “burn out” the creosote safely.

I constructed the chimney using high quality triple wall stainless steel chimney pipe that was designed with separate air spaces between each layer of tubing. This allows the inner tube to dissipate heat. (Never use the double wall insulated pipe because it will contain heat and can cause extremely high temperature build up in the wall of the tube). The triple wall stainless steel (SS) chimney tubing was then encased in a framed shaft lined with fire rock all the way to the roof. The SS tube extended through the metal roof cap. This cap was removable so that the tubing could be pulled out and replaced if necessary without disassembling the chase and associated walls. Of course a spark arrester was installed on top of the chimney. I installed a vent in the bottom and top of the chase to capture the heat from the chase and reduce any heat build up. The vents incorporated at lead link controlled fire damper so that if there was a fire in the chimney chase they would automatically close. The bottom of the chimney was located directly above the fireplace and connected by a single wall SS pipe open to the room. The entire corner walls and floor where bricked and the stove set on the brick.
The fireplace was a plate steel enclosed box lined with firebrick. There were controllable air intakes on the front doors and also a combustion air vent piped from outdoors with a control damper built in near the stove. These allowed me to shut down the air supply and control the fire level. Most of the time the fire was kept and a fairly low level and consequently contributed to creosote build up in the chimney.
About once a week during the main heating season I would open the air intakes and allow the fire to build up enough to burn the creosote out of the chimney. This can be a little spooky the first time you do it because it sounds like the chimney is going to blast off into space. I chose days when there was adequate snow cover or wet weather in order to eliminate the chance of fire from sparks emitted from the chimney. These chimney burnout’s were generally very small and short-lived due to repeating them on a regular basis. During the learning curve I did have a couple of fires that emitted a large amount of flames and smoke from the chimney. I monitored the heat coming from the chase vents and it never exceeded an uncomfortable level. I also inspected the flue system and no damage was done other than a discoloring of the spark arrestor.
The weak link in a system like this is the single wall pipe between the stove and the chimney. This must be stainless steel, have adequate spacing from combustibles and be inspected regularly.
Another thing to remember is that a small hot fire is much better than a large cool fire. This is accomplished through the control of intake air and will become easy to maintain with practice. More of the gases that create creosote are burned in the hot fire. The diameter of the chimney flue is also important. If sized too large the velocity of the smoke and gases will move up the flue too slowly and will cause build up. Some of the older large chimney’s actually set up a convection current inside the flue drawing cold air from above, heating it and moving back up and out. This also opened the door for an uncontrolled chimney fire because it was self-feeding. A smaller diameter flue creates a higher velocity current fed only by the controlled combustion air thus keeping the smoke gasses a little hotter, moving them out of the chimney and reducing creosote build up.

The important element of this type of heating system is the ability to shut off the supply air. You can literally kill a fire in this manner. A back-up dry chemical fire extinguisher released into the front air damper opening should solve any out of control problem. I never found this necessary but kept one on hand, just in case.
Another point that goes along with wood heating is having a metal roof on your house. This is the simplest way to fire proof your roof and a good standing seam system, (not a screw down), is easily a 50-year roof. I had hand-split cedar shake shingles on mine and was always paranoid about the possibility of it catching fire from either a chimney spark or a forest fire. My next home will have a standing seam galvanized aluminum roof. Pricey, but worth it.