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Thread: Flue pipe size

  1. #1
    Join Date
    Mar 2001
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    475


    I have noticed on some changeouts that the flue pipe is smaller than would be put in if it were new construction. For example, a 100,000btu furnace with a 40,000 water heater tied into a 5" flue. Is this an acceptable size. We only install 80 percent furnaces. Do you guys have a rule of thumb method of what size flue pipe to use on different furnaces?

  2. #2
    Join Date
    Mar 2003
    Location
    Scottsburg, Indiana
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    all furnaces come with detailed, information on flu venting, its very important to be vented correctly. and with the correct pipe, the amount ran matters, pitch of the roof, etc. and why do you only put in 80%, cause you dont know how to do a 90, or you just dont know what your doing???????????no, there is no rule of thumb method.

  3. #3
    Join Date
    Mar 2001
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    475
    Originally posted by customheating
    all furnaces come with detailed, information on flu venting, its very important to be vented correctly. and with the correct pipe, the amount ran matters, pitch of the roof, etc. and why do you only put in 80%, cause you dont know how to do a 90, or you just dont know what your doing???????????no, there is no rule of thumb method.
    No need to get pissy. I don't do work for myself. I can ask my boss why we don't. There aren't a whole lot of 90 % in central Florida, and there definitely aren't a whole lot of people that would spend the extra money to upgrade everything for a 90 percent furnace. We mainly use heat pumps down here.

    [Edited by dave r on 09-22-2004 at 11:37 PM]

  4. #4
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    Dec 2002
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    Houston,Tx.
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    15,825
    That was kinda rude of old Custom maybe he has just had a bad day and came here to vent.
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  5. #5
    Join Date
    Apr 2004
    Location
    Bartlett, IL
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    There is a vent table that you need to go by. Ask your supply house they should have something for you. It is important that you know the height of the chimney, (b-vent) the diameter and how many btu's the appliance is venting. The vent table will show you how many fan assisted and natural draft appliances you can connect to a certain run. If i'm not mistaken, 100,000 btu fan asssisted furnace with a 40,000 btu natural draft appliance can be vented with 5" b-vent minimum 15 feet high, as you go up in height, your btu allowance increases. But you should get yourself a vent table it makes life in "venting" much easier.

  6. #6
    Join Date
    Jan 2004
    Posts
    49
    The best reference for "B" vent sizing I've found is in the "National Fuel Gas Code". You may be able to purchase a copy from your utility, or perhaps even amazon. It also has charts for sizing gas supply piping, which comes in handy on larger jobs.

  7. #7
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    Mar 2003
    Location
    Scottsburg, Indiana
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    sorry , didnt maen to get pissy

  8. #8
    Join Date
    Apr 2004
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    7,680
    I didn't think a was a bad answer myself, I mean there is no rule of thumb for venting as he mentioned. There are specific vent tables and codes you need to follow. Did I miss something?

  9. #9
    Join Date
    Mar 2002
    Location
    Richmond, Virginia
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    4,264
    Originally posted by customheating
    why do you only put in 80%, cause you dont know how to do a 90, or you just dont know what your doing???????????no, there is no rule of thumb method.
    I think it may have been this part of the comment that offended.

    dave r, This may be the most concise and informative information I have come across regarding venting practices short of the GAMA venting tables. Read it, print it and save it. It's invaluable info. http://www.duravent.com/docs/instruct/shbook.pdf

    I printed it and placed it in it's own binder.

    [Edited by HVAC Pro on 09-23-2004 at 08:55 PM]
    There is nothing more frightful than ignorance in action....Mark Twain

  10. #10
    Join Date
    Sep 2005
    Location
    Cleveland
    Posts
    6
    A house I bought has a 7in vent running up 21 ft alongside the house. I got a Carrier 75k furnace and a small water heater on a horizontal 4in vent going out to the 7in. I bought 25ft of 5in B vent at the store. The inspector called and said to go back to the store and check the sizing chart before I install the 5in straight up through the house. He thinks a 3in water heater and a 4in furnace are ok with the 5in but will check the chart.

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