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  1. #1
    Join Date
    Sep 2008
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    Default Flue Pipe Insulation

    I have added extra insulation in the attic and made covers for the whole house fan and the drop down stairway. This has eliminated the ice damn problem but has created another one. Down by the furnace I noticed some white crusting around the joints of the elbows at the bottom of the flue pipe. This seems to be a moisture problem. Is there an insulation that wraps around the flue pipe in the attic space? The home improvement stores are trying to sell me duct insulation. The package does not say that it is rated for high temperatures. I heard there is a foil only type wrap but can not seem to find it. It is a 6" b vent. Would it be possible to snap an 8" stove pipe around the flue and then a 10" around that to create an air space? There is about 12' of b vent exposed in the attic.
    Thanks for any help on what to do or where to get the wrap.

  2. #2
    Join Date
    Aug 2007
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    Default Re: Flue Pipe Insulation

    A 100,000-BTUH furnace burning for one hour will generate a gallon of water in the form of vapor in the flue gas. The vent, through proper design and sizing, must keep the moisture in the vapor state until it exits the vent. This is one reason why the B vent sizing is so important!

    Condensation will form in the coldest part of the vent (near the termination) when the flue gas cools to its dew point temperature. This is the temperature at which the flue gas, with its heavy load of moisture in the form of water vapor, starts to condense on the cooler walls of the vent.

    The coldest run will be near termination in the attic.

    Aside from adding insulation …. has anything else changed ?
    Any changes to the combustion equipment ?

    Have you increased the openings for attic ventilation?

    The B vent sections from the attic to outside need to be a double walled insulated B vent .

    Other causes can come from short cycling of the combustion equipment. The combustion equipment doesn’t run long enough to maintain warm enough temperatures in the flue pipe …. which will create condensation.



    Just some thoughts.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  3. #3

    Default Re: Flue Pipe Insulation

    They do make fire rated rock wool insulation for commercial used like kitchens etc. but canuk has a point you need to find out why you have so much water.Maybe you need install a power venter for the flue or a power damper to only open and close for heating operation .Some systems have a "t" in the flue run to collect debris and or condensation.
    have your combustion analysed for proper operation.

  4. #4
    Join Date
    Sep 2008
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    4

    Default Re: Flue Pipe Insulation

    About 7 years ago I had 2 furnaces installed(we only had 1 before) A 100,000 for the 1st floor and an 80,000 for the 2nd floor. The house is over 3300 sq. ft. At the time I ? the flue size and was told it was ok. I asked a company that just does flue pipes in commercial buildings to calculate it for me. They said that with the 2 furnaces and the 40,000 btu hot water tank that it was close but still ok. I did not notice this problem until after the insulation was installed. Figured that with the warm air not getting into the attic now that the temp difference is what was causing the condesation problem. Other than that nothing has changed. Is my flue too small or is it ok like I was told?
    Thank you

  5. #5
    Join Date
    Sep 2008
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    4

    Default Re: Flue Pipe Insulation

    Yes I did add 2 more roof vents.

  6. #6
    Join Date
    Aug 2007
    Posts
    1,131

    Default Re: Flue Pipe Insulation

    About 7 years ago I had 2 furnaces installed(we only had 1 before) A 100,000 for the 1st floor and an 80,000 for the 2nd floor. The house is over 3300 sq. ft. At the time I ? the flue size and was told it was ok. I asked a company that just does flue pipes in commercial buildings to calculate it for me. They said that with the 2 furnaces and the 40,000 btu hot water tank that it was close but still ok.

    Are you saying that both of these furnaces *and* the WH share the same flue?

    Are both of these furnaces (and the WH) in the basement? ...in the attic? Am confused. Or perhaps there is no basement and they are on the ground floor in a utility room there. ???

    Do any of these have a standing pilot? Not likely, but checking nonetheless.

    Where/how do these appliances get their combustion air? Piped in per manufacturer's instructions...or ??? If located on ground floor or basement and no provision has been specfically made for combustion air...it may have previously been drawn thru the leaks around your staircase and whole-house fan...which you have sealed off. This could leave you with inadequate make-up air resulting in flame roll-out, poor combustion and poor exhaust draft (more condensation)....not to mention CO spilling inside the house. Got a CO detector in place?

    Passive venting or power-vented?
    Last edited by goldhiller; 09-07-2008 at 03:16 PM.

  7. #7
    Join Date
    Aug 2007
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    Default Re: Flue Pipe Insulation

    While the 2 added vents will make the attic that much colder .... I'm with ****hiller's questions and comments.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  8. #8
    Join Date
    Sep 2008
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    4

    Default Re: Flue Pipe Insulation

    Yes. There are 3 co detectors in the house. None of them have ever gone off. Yes they are all working. The furnaces and hwh are in the basement. The hwh does have a pilot. The furnaces do not have a pilot. The basement is 24' x 40'. The joists are 7' off the ground. There is also a crawl space that is about 14' x 20' x 3'6" tall. There are no walls that divide the basement but there is a wall that separates the crawl from the basement. The joist space over the wall that separates the crawl is open.

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