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Venting into exterior chimneys

Anthony D.
Anthony D. Member Posts: 63
From what I can see in the manufacturers lit., the code books, and aga manual - new mid-effieciency gas furnaces should not be vented into an exterior masonry chimney (3 sides exposed)without the use of a liner. Is this true?, because most of the contractors in my area are not installing liners.

Also when venting into an interior masonry chimney, using a linerless masonry chimney kit, should the breaching be double wall, or can it be single wall pipe?


  • frank s
    frank s Member Posts: 64
    chimney liners

    Hey Anthony, it's been my experience that if the chimney does'nt have a tile liner regardless of where it's located ...inside or outside(3 sides exposed) it should be lined, the flue gases will eventually corrode the mortar. If the chimley is tile lined no need for a metal liner if it's interior, if it's exterior it will depend on the size of the chimney and the height,on very cold days the flue spill switch will trip...don't want that, but if it happens I suggest a liner....Z-flex or whatever. It also helps if there is a water heater vented into the chimley as well. Hope this was helpful,
  • Herb
    Herb Member Posts: 31
    chimney liner

    I just attended a class about venting requirements put on by our local utility (RG&E) in Rochester NY. All exterior chimnies must be lined and B-vent used to connect to that liner. Likewise interior tile lined chimnies should be connected with B-vent. Vent tables should be consulted to determine sizing etc.... to see if the interior flue should be lined.
  • Dave Flood
    Dave Flood Member Posts: 14
    Chimney Liners

    Hello Tony,
    If a chimney is not lined, it must be lined before installation of a new heating system. The flue gasses will break down the mortar in the joints. Before you know it, your bricks will be falling in. Look in the NFPA 54 for the codes. The NFPA 54 does an excellent job in explaining how to design a new chimney, for gas fired sysems.

    Dave Flood
    Dir of Tech Ed
    Wallingford, Ct

  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928
    House across the Boulevard from me

    c. 1901, solid masonary, 2 1/2 story, triple wythe walls. Original lime mortar. North-facing chimney exposed on one side only. I believe brick is only one layer at the chimney. Older gas forced air.

    Bricks are decidedly discolored (darkened) along the run of the flue in use. Discoloration noticeable regardless of being wet from rain. Mortar seems fine from the outside, but I'm not going to dig into my neighbors' mortar to find if it's unduly soft.

    My own chimney (1903) is internal, soft brick, lime mortar and unlined. I've inspected it with a mirror and the mortar is about 3/4" back from the internal face of the brick. Other flues in this triple chimney (where coal only burned) have mortar only slightly recessed behind interior face. Older gas boiler. Exposed brick and mortar in attic is fine except where children/drunk frat boys have "played."

    Higher efficiency = lower flue temp = greater potential for condensation inside the flue. Exposure to outside temperatures only increases this potential. I believe they "rot" from the inside-out.

    Old brick and lime mortar are strange in how they deal with elements. My brick foundation is parged above grade. When I dug an inspection hole against the foundation the brick and mortar both looked like new despite almost 100 years of earth and water contact. Yet I was able to remove the 2nd 30'+ chimney (it was lopped off just below the roof previously) using only my hands lifting bricks from the top down. The only burning this chimney had seen was coal.

    I sometimes take exception when "new" rules are applied to old homes. I do though believe that requiring a three-exposure chimney to be lined is utterly reasonable.

    Sorry to ramble.
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