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Should an unused chimney flue be closed off at the top?

user-1046359 | Posted in Energy Efficiency and Durability on

I have a 90 year old external brick chimney that has three separate flue passages. None have been used in the nearly 30 years that I have been in the home but I am contemplating installing a wood burning stove in one of the fireplaces and using one of the flues. There is definitely a stage 3 glaze creasote build up in the flue in question which will have to be addressed as well as installation of a double insulated 6 or 7 inch steel liner to accommodate the 6 inch stove exhaust opening. There is no current liner in place. I can see the layer of creasote from the damper opening.The upper two feet of the chimney (it is about 33 feet above ground level) is in very bad repair and will either need to be repointed or replaced. My questions are the following
1. Would it be advisable to pour TherMix between the new steel flue and surrounding masonry to further maintain gas temperature ? The space will probably be an inch or so.
2. Should I cap off the two flues that will never be used? Capping off prevents moisture from coming in or cold air that may come in due to any possible negative pressure stack effect that could intermittently exist in the house. On the other hand it will prevent ventilation and drying in the two flues that will be capped. A chimney mason in my area Zone 5 told me that capping a flue in my area is very common but that he advises against it because he has seem many cases in which the chimney seems to deteriorate more rapidly after being capped

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Replies

  1. wjrobinson | | #1

    I'm with your mason and sounds like you have a certified chimney person to set up your chimney for the woodstove which is the right way to go.

  2. GBA Editor
    Martin Holladay | | #2

    W. Timothy,
    Q. "Would it be advisable to pour TherMix between the new steel flue and surrounding masonry to further maintain gas temperature?"

    A. This step is unnecessary but not a bad idea. It only makes sense if you have located a contractor who is comfortable with the procedure, however.

    Q. "Should I cap off the two flues that will never be used?"

    A. I would answer yes to this question. Again, however, this work should only be performed by a contractor who agrees with the decision to cap the flues and who will stand behind the work.

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