Renovation & Design

Seal unused chimney top and bottom

B-vent simplest type of old vent for homeowner to deal with

QUESTION -- This past winter I replaced my home's gas hot water tank with an electric model and also replaced my low efficiency gas furnace with a high efficiency furnace. My question concerns the now unused chimney, which has been dripping condensation into my basement all winter. I don't consider simply capping the basement end of the chimney, as the furnace installers have done, to be a proper way of dealing with the unused chimney. It seems to me that without sealing the chimney off and insulating at the attic level, my unused chimney is virtually like having a 5 inch diameter hole in my home's outside wall.

I would think this would be a common situation with furnace retrofits but solutions are hard to come by. It seems HVAC and government literature dealing with retrofits quite conveniently avoids the issue altogether or simply states "seal off the unused chimney".

What is the proper way of dealing with the resulting unused chimney from a furnace retrofit?

?--Roy Olynick, e-mail

Answer -- I agree that it is becoming more and more common for homeowners to discontinue the use of old chimneys after installation of high efficiency furnaces. You have raised an excellent point, which may not be dealt with properly by some HVAC contractors. I don't think that sealing an old chimney is a very difficult task and will give you my view on the necessary steps to stop your dripping.

The answer to your question will largely depend on the location and composition of the existing, unused chimney in your home. Older, brick chimneys with metal liners may have to be treated differently than metal B-vent chimneys. If an older brick chimney is located on the exterior of the home and if it goes up more than a single storey, it can be much more difficult to close up. Since you have not provided me with much of these details, I will explore possibilities for the various types of common, older furnace chimneys.

The simplest style of chimney to deal with would be an interior bungalow B-vent chimney, which is simply a double-walled metal pipe running from the basement, up through the house and attic and terminating above the roof line. The easiest option to seal this old vent would be to climb on the roof, remove the metal rain cap and install a properly sized flat metal cap in its place. This cap should be screwed in place and sealed with caulking to prevent blowing off or leaking. This cap, combined with the sealed bottom of the vent in the basement, should be sufficient to prevent most condensation leakage situations. By sealing the bottom, the HVAC contractor has limited the amount of warm air that can leak up the chimney from the home, but it does nothing for cold air intrusion from above. Sealing the top and bottom will prevent convective currents while preventing any warm air that does leak into the vent from the home coming into contact with very cold air entering near the top.

If this does not completely stop the dripping, there may be some additional thermal bridging going on with metal chimney itself. In that case sealing the floor of the attic, or the metal flashing in this location, where the B-vent goes through may stop the dripping. This can normally be accomplished by installation of rigid foam or blown-in foam around this area to insulate air seal the metal chimney. This may be difficult to accomplish, depending on the access to the attic and another alternative may be chosen. Complete removal of this type of chimney, or at least below the main floor ceiling of the home, is an option. This may be very practical if the roofing on the home is older and in need of upgrading. It may be little additional work to remove the old chimney and patch the roof sheathing, rather than properly sealing the old, unused chimney flashing to the new roofing.

If your home has a brick chimney that is not installed outside the exterior wall, it should have a metal liner inside the flue for the older gas-fired furnace. This metal liner may be treated the same as the B-vent and capped at the bottom top to prevent condensation. I did this last summer in my own home and it stopped some leakage I was experiencing, after upgrades identical to yours. Alternatively, this metal liner may only be held in place by mortar at the top and the bottom of the brick chimney and removal may be quite simple after chipping away the old mortar. If the liner is quite rusted in any location, it is a good idea to remove it and cap the top of the chimney with a metal flashing or thick mortar bed. The bottom opening in the chimney should also be patched with mortar or fine concrete mix to prevent air intrusion from the basement into the old brick structure.

If the metal liner from your old furnace exits through the foundation wall into an old brick chimney, especially on a two-storey home, further action may be required to prevent condensation. Follow the same recommendations for an interior brick chimney, but additional insulation may be required to completely eliminate the condensation. Installation of moisture resistant insulation, such as extruded polystyrene or blown-in polyurethane foam inside the bottom of the old cavity will do the trick. If the chimney is simply capped at the top and bottom, there still may be small openings that allow warm air to leak into the cavity. This moist air can quickly condense near the bottom of the chimney in the winter due to extremely cold outside temperatures. Maximum expansion poly foam may be an ideal solution for this area as it can be blown into the cavity and will continue to expand and fill any gaps after the bottom of the chimney is capped.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the President of the Canadian Association of Home Property Inspectors - Manitoba ( Questions can be e-mailed or sent to: Ask The Inspector, P. O. Box 69021, #110-2025 Corydon Ave., Winnipeg, MB. R3P 2G9. Ari can be reached at (204) 291-5358 or check out his website at


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