It was the norm many years ago: They vented the fossil fuel furnace, the wood stove and maybe the fossil fuel powered water heater all into the same masonry chimney. Many homes like this are still around today. While this is may not be something to panic over, based on modern thinking, this is not a good arrangement. Today, at least where I work, the professional thought is that a masonry chimney is okay for a fireplace but fireplace inserts and wood stoves should have an approved metal liner inside the masonry (more on this below). And, if we are speaking of the masonry chimney being used instead of a B or L vent, for a fossil fuel furnace or water heater, then there are other considerations. This is listed as a safety issue in that a masonry chimney typically is too wide to provide the proper draw for a gas or oil appliance. Also, exhaust gases can leak through cracks in the mortar or masonry and that is a safety concern. If you really want the fine print, and the nitty gritty on all of this, below I have described in detail some of the problems with this arrangement. It better clarifies why an inspector will suggest a separate vent or a liner.
Sizing: A flue needs to be sized for the appliance it serves. The furnace and water heater in your home have a BTU (British Thermal Unit) rating usually listed in terms of thousands of BTU. It is the heat generated by the appliance that promotes venting. If you have a large, cold chimney and a small BTU appliance, it may not generate enough heat for the flue gases to rise inside the chimney. Adding a properly sized flue liner reduces the area to be heated by the appliance and therefore increases the venting capacity of the chimney.
Vapor: One of the byproducts of burning fossil fuels such as natural gas or oil is water vapor. If the chimney is unlined and cold, the water vapor will condense inside the chimney and the chimney will become wet. A wet chimney can stain the walls and ceiling of the home and rust out the metal flue pipes.
Gases: Other byproducts of combustion are nitrogen dioxide and carbon monoxide. When the water vapor produced by combustion combines with nitrogen dioxide, the result is nitric acid. Acid vapors can eat away at the mortar in the brick chimney.
Efflorescence: This is a whitish mineral powder that forms on the inside or outside brick of the chimney that is exposed to the air. When water vapor soaks into the brick, the moisture migrates through the brick, usually to the exterior, where it evaporates, leaving behind minerals collected by the water. The minerals form a whitish powder that can usually be brushed off. In winter, the migrating water freezes inside the brick, resulting in damaged brick or a cracked chimney.
Wood: A masonry chimney that is serving a wood burning fireplace insert or wood stove should have an approved metal liner (typically stainless steel). A clay liner, in good condition, is acceptable in a fireplace but not for wood stoves or inserts. Without a liner, the flue is generally too wide to provide proper draw for a wood stove or an insert. And, with such an arrangement, it is difficult to establish a positive connection to vent all exhaust gases. This can lead to a danger of exhaust gases getting into the home, and there is also a risk of a spark getting through the masonry and that could lead to a fire.
If you are interested in a related article, click here.
Top end Bottom end: Left, Gas furnace; Right, wood burning stove
Thanks for taking the time to read all the way through this. You are a scholar!
Steven L. Smith
Bellingham WA home inspections