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Decommission existing chimney and go direct vent

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The house I am purchasing has two flues in the chimney (masonry) - both of the flues need repairs and have estimates for 8-9k.

The flues are used for a wood fireplace and gas water heater. I am considering putting in a gas insert with direct vent option instead of using the wood fireplace, and also replace the water heater with direct vent models (not power vent).

What do I need to do to make sure the chimney is safely decommissioned? Can I check flashing for leaks, and repair any masonry and leave it?

Please advise.


asked in House Chimney by chanjith (120 points)
edited by darekrudy
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1 Answer

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Hi Chandra,

Check my answer by following this link: http://checkthishouse.com/qa/27/seal-unused-chimney-after-replacing-furnace-water-heaters.

For the water heater flue just use methods described in the link above.

Assuming that the fireplace gas insert will go directly under the damper of the old fireplace you need to make sure that there is a sufficient amount of insulation above the damper to prevent any heat from that new fireplace from migrating into the old chimney stack and causing condensations.

Depending on the design of the chimney and fireplace’s damper you might be able to tightly roll 2’-3’ of blanket insulation, slide it through the damper, and let it expand inside the flue. If there’s enough room, insert another section of insulation flat on top of the damper.

There are also chimney balloons that you can insert into the flue and inflate it so it tightly seals the space. The manufacturer claims they stay inflated for years but I’m not sure if that would work on an exterior / cold chimney. Low temperature might cause the chimney balloon to lose pressure and break that seal. Since its access will be blocked by the insert there isn’t much you’ll be able to do.

To make the damper air tight add a bid of silicone (use RTV type / high temp.) along its edges (you’ll have to clean its surface first) or use aluminum duct tape to create a seal. Also, before installing the gas insert make sure that all the cracks and gaps inside the existing fireplace have been sealed.

You can also have a sheet metal plate precut and secured tightly / sealed along the edges directly beneath the damper (flue insulation still applies to this one).

Since there’s always a chance of condensation buildup and moisture inside the chimney, its top should be protected so the rain water doesn’t get in, but left open to allow any moisture and condensation to evaporate.

Therefore, install rain caps and protective screens (if missing) to stop rain water penetration and keep the birds / rodents away from the flues. Of course, you’d need to perform mortar and flashing repairs as well.

Let me know if there’s anything else. Thanks

answered by darekrudy (21,730 points)
My chimney is still being used for the water heater, does this still apply.  I am leaning towards the aluminum duct tape to make the damper air tight as all I am looking to do is to stop drafts coming in through the damper as they are old and have about 1 inch gaps on the 2 sides.  I guess the correct question that I should have asked is how to make the damper air tight safely with the gas fireplace insert.  I am especially mindful of making sure that whatever I do does not cause any noxious or poisonous smells when the gas fireplace is being used.  Thanks
I’m assuming that your chimney has two flues (two pipes sticking out from its top?), one for the water heater and the second one that is not being used anymore. If so, I would still first insulate the space directly above the damper to prevent any warm air from escaping and causing condensation. After that you should seal the damper with aluminum tape or even precut sheet metal section to fill the entire damper opening and then use the tape or caulking / silicon.