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5000 MBH Condensing boiler flue chimney: 1'' air gap in lieu of insulation between SS layers

SS_YVRSS_YVR Member Posts: 1
Have anyone installed 80 ft of flue chimney where 90% of flue is in the room (last piece is 10 ft above the roof finished with the velocity cone).
Layers: AL-29-4C inner, 1'' of air gap, 304 SS outer.
I've never spec'd an air gap as an insulation between metal shells.
Have anyone experienced an overheating of the boiler room atmosphere when 1'' of air gap is used for condensing appliances in lieu of insulation. (flue gas temp 140-170F max).
I have concerns that it overheats the room. Heat transfer calculation gave me 1760 BTU/ft of the heat loss, so total is 123,000 BTU per 70 ft of an indoor flue run....


  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 7,099
    What about insulating the flue pipe? Probably a good idea on a 80'run
  • Jamie HallJamie Hall Member Posts: 12,956
    Nothing wrong with your calculations. Welcome to the wonderful world of physics... the air gap is helping a lot, believe it or not -- but external insulation as @EBEBRATT-Ed suggests would also help. Another approach -- if you don't mind the heat loss -- is to make sure that that air gap has somewhere else to vent other than the room.
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
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