Q: I recently purchased a home that has a chimney right in the middle of it. The house has forced hot air and no central air conditioning system. The home was built in the early 1900s (1917, possibly).
I would like to remove the chimney and run a flue from the basement to the outside of the house and up the side of the home to at least two feet above the roof (as I have been told this is a must). I have spoken with two contractors, two architects and one plumber who have advised that this can be done. However, I am still unsure. My safety is a concern. I am aware the purpose of the chimney is to carry harmful gases out of the home, but it is truly blocking space on the second floor of the home. We are keeping the forced-air heat.
A: There are a lot of 'ifs' in your question that need to be addressed by a licensed, qualified heating/venting/air conditioning contractor, not a plumber or architect. In addition to transferring harmful gases to the exterior, the flue also carries large amounts of moisture, another by-product of combustion.
When you place a metal flue on the exterior of the home, the flue is much cooler and will condense the flue gases at a much higher rate than an interior chimney or flue. In other words, there will be a lot of water inside the metal flue and, over time, the water will lead to rust and damage.
To keep the flue warmer, it should be enclosed in a chimney, which can be made with a wood frame and covered with the same material that now covers the home or any covering that it is aesthetically pleasing. Inside the wood chimney, it is imperative to maintain at least six inches of clearance between combustible material and the metal flue pipe. It is also important that the metal flue be supported inside the chimney to support its own weight. I have seen metal flue pipes come apart at a joint and collapse sideways inside the chimney.
An HVAC contractor will size the flue pipe for the appliance(s) to be vented. If there is a gas water heater, that also has to be vented, and the chimney flue will have to be larger than the flue connector on the furnace. Other factors a qualified contractor will have to consider are the horizontal and vertical lengths of the flue and how many turns or elbows are needed to exit the basement. A single 90-degree elbow is equal to 10 feet of flue pipe when considering restrictions to drafting. The information about venting the flue two feet above the roof is partially accurate. The rule is a minimum of two feet or two feet above any portion of the home within 10 feet of the flue.
So it's possible to vent the furnace through an outside flue, but it will take a little more thought than first imagined.
Dwight Barnett is a certified master inspector with the American Society of Home Inspectors.