QUESTION — I read your piece on capping redundant flues, which involve chimneys. My high-efficiency furnace has made a straight-up-thru-the-roof flue superfluous. It’s capped at the bottom, in my basement. I’m about to have my asphalt shingle roof replaced. I first thought I’d have the unused flu sealed again at the top, below the new shingles. Now, I realize that complicates the choice of furnace by future owners of my home.
How does a future homeowner, or inspector, know where to find the top of the old flue, if necessary? Should I have the redundant flue sealed at the top of the pipe as it now stands? Thanks for your time and expertise.
Tony Sargent, Annandale, Va.
ANSWER — Sealing an old chimney, and removing it above the roof, is not critical but an ideal time to do it is when you are planning for a roofing upgrade. The main reason to do it is to make future repairs or maintenance on the old pipe a non-issue.
I have written on this subject many times and most homeowners understand that keeping any redundant item sticking out of the walls or roof in place is a recipe for future problems. This applies to vent hoods, antennas, satellite dishes, and especially chimneys and furnace vents. Once these components are no longer in use, removing them will prevent repairs or maintenance over time. Since they are no longer of any practical significance, getting rid of the potential repair makes sense. With a simple metal chimney, it is almost as easy to remove the portion above the roofline and seal it in the attic as it is to repair the older flashing and install the new shingles around the old vent. This may be much more difficult with a true masonry chimney, but certainly is warranted with a simple metal vent pipe.
All older natural gas-fired heating appliances required a metal vent to discharge the flue gasses to the outside of the building. This is to help prevent toxic gasses from entering the living space, but also to prevent condensation and damage inside older brick chimneys. Stand alone metal chimneys for older gas furnaces are typically a double-walled galvanized pipe, called a B-vent. These will last many years, due to the corrosion resistance of the galvanized metal, but will eventually rust. Usually this will first occur at the portion near the top, above the roof, due to outside environmental conditions. Once this type of unit rusts through, the only option is replacement with new piping. If a furnace or gas-fired water heater is still using this flue, then removing it is much more complicated and replacement of the damaged component makes sense. If there is no longer any gas-fired appliance connected, replacing any damaged sections to prevent moisture and pest intrusion, is not feasible. Removal and/or sealing the old pipe is the only sensible option.
If you have an older masonry chimney the decision to remove the chimney, rather than sealing it, becomes more difficult. The flue you are referring to would likely be a single-walled A-vent, commonly called a chimney liner. This could be a B-vent, but for practical purposes that makes little difference for our discussion. Since it is no longer in use, this metal pipe may be in reasonable condition inside the brick chimney, or may be corroded. If it is still in good condition, and the chimney also is not damaged above the roof, then simply capping the metal vent pipe would be the easiest option. If the brick or mortar is deteriorated, or the vent visibly rusted, removal of the entire assembly will be best. That will require more work and expense, but will save having to remove the entire damaged liner and hiring a mason to climb up on the roof, or assemble scaffolding, just to keep the brick chimney from further falling apart. That money would be better spent having the whole thing demolished, at least above the roof, and the opening in the roof sheathing patched before the new shingles are installed.
I applaud your concern for any future owners and their limitations on heating systems, but that ethical worry is misplaced. I’m not certain this is the situation in your part of the world, but in Western Canada all new natural-gas-fired furnaces are high-efficiency units. That means they are all vented out of the living space with PVC pipes, typically exiting horizontally through the exterior foundation or house walls. It is rare to have these exit through the roof, but sometimes the old chimney piping is used as a chase to run the new plastic pipes, when no other option is available. So, retaining the older vent for a future gas furnace is not necessary. Your best option, in my opinion, is to remove the older piping above the roofline, insulate and seal the top of the remaining flue in the attic and have the roofers properly patch the hole in the roof sheathing before installation of the new shingles. Alternatively, you could remove the entire length of pipe down to the basement, but that would require patching, sealing, and insulating the attic floor where the old pipe was.
Taking out an unused furnace vent or chimney above your roof, rather than sealing it, makes good sense when doing roofing upgrades. Saving it for future heating options is not required and eliminating current or future maintenance on the older components is the main reason to remove it, along with installation of the new roofing.
Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the past president of the Canadian Association of Home & Property Inspectors — Manitoba (cahpi.mb.ca). Questions can be emailed to the address below. Ari can be reached at 204-291-5358 or check out his website at trainedeye.ca.