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Renovation & Design

Discarding chimney a common dilemma

High-efficiency furnaces eliminate venting from basement through roof

QUESTION: I read your column every weekend and am usually quite pleased with the thoroughness of your responses. My question is about closing off or removing my chimney. In the past few years, I have had my water heater replaced with an electric water heater and my furnace upgraded to a high-efficiency model, making the chimney unnecessary. To reduce air flow around the chimney stack and improve heating efficiency, should I consider removing as much of the metal chimney stack as possible, and closing off the chimney openings on the roof and in the ceiling/attic cavity? I had water leaking around the chimney opening during a severe rain storm last summer, and wonder if I should go the extra mile and remove everything possible, and seal the openings. My 1,650-square-foot bungalow in St. Vital is 23 years old, and I have easy access to the attic and basement below the chimney.

 

— Duane Drayson

 

ANSWER: Your dilemma is one faced by many homeowners when they upgrade to a high-efficiency furnace, which doesn't require the large, metal chimney that goes from your basement through the roof. Because the new furnace is typically vented horizontally through the side of the foundation or exterior wall, the only thing that still required the large chimney was your old, gas-fired water heater. Replacing that with an unvented electric unit has offered a potential benefit, but also requires making a tricky decision.

The obvious choice you are faced with is whether to simply cap your old chimney, leaving it in place, or removing it. One of the main factors that should be considered is the condition of your roofing. If the roofing is worn and nearing time for replacement, removing the old chimney may be much more desirable. In that scenario, the chimney may be easily removed from the attic upward. The opening in the roof sheathing can be easily patched before covering it with a layer of the brand new roofing, sealing it well against the elements. If your roofing is newer, removing the total chimney may be more difficult, as matching the shingles to patch the hole can be tricky.

The other option, which you have wisely asked to explore, is the possibility of a partial removal. It is certainly possible to remove only a portion of the old stack, which is most likely a double-walled B-vent. This type of chimney is made up of several small sections that are connected together. Disconnecting enough individual sections to leave only the uppermost part will save the need for insulating and sealing the old pipe and patching the roof. If the bottom of the partially removed unit is above the ceiling, leaving the top sticking through the roof may cause no harm. Sealing the top may not even be required, as long as the rain cap is in good condition. Also, installing a metal screen near the top may be a good idea to keep insects and critters out of the attic.

It is also possible to remove only the upper sections, leaving the rest of the metal pipe installed in the walls and protruding slightly through the floor in the basement. In that case, you would definitely have to seal and insulate the bottom and top of the vent to prevent warm-air intrusion and condensation. That may be easily accomplished with a combination of a few cans of blow-in polyurethane foam and fitted metal caps. The benefit of that method is limiting the need to seal the areas around where the old pipe went through the floor and ceiling.

If you decide total removal is your best bet, attention to the transition areas between the floors of the home and attic is warranted. Patching these areas with wood or drywall sheathing will not be enough to prevent a problem. Air-sealing and insulation will also be needed, especially at the upper ceiling, which doubles as the floor of the attic. This could be done with standard 6mm polyethylene sheathing, acoustical sealant and insulation, or other materials. Covering this opening with a few layers of extruded polystyrene sheathing will ensure warm air will not leak into the attic where the old chimney used to be. Blown-in foam would also work, but would be a more costly method and may require installation in several applications to ensure enough thickness to prevent air leakage and match the insulation level of the surrounding attic.

Taking that extra step when doing upgrades is always a wise choice, but one that is not always critical. It may be less complicated to properly air-seal your home after complete removal of the entire chimney, but it is not completely necessary. Partial removal will be adequate, as long as precautions are taken in several areas. Air-sealing both the open ends of the partially removed vent and the attic floor will be necessary to prevent warm-air exfiltration and condensation. Either way, careful attention to those details will prevent a problem that didn't exist with the old furnace and chimney intact.

 

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 trainedeye@iname.com. Marantz can be reached at 204-291-5358, or check out his website at trainedeye.ca.

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