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Thread: Gas line size

  1. #1
    DIY Junior Member rsten's Avatar
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    Nov 2012
    Portland, OR

    Default Gas line size

    My house currently has a gas furnace, water heater, and clothes dryer. I would like to add a gas kitchen range. The drawing below shows the proposed piping layout. Segments AB, BH, HF, and HG are existing 3/4" black pipe. Right now, before adding the range, Segment BD is 1/2" CSST. I am considering installing a tee fitting at point C, replacing Segment BC with 3/4" CSST, and running 1/2" CSST to the new range. Segment CD would remain 1/2" CSST.

    The distance from the meter, point A, to the farthest appliance, point D, is 41 feet. The total demand from all appliances is 227 CF/HR. From reading some sizing tables I gather that the pipe for Segment AB ought to be at least 1" diameter. I don't want to replace it right now, however, because the entire plumbing system is going to be redone next summer as part of a major remodeling; replacing that section of pipe would be a lot of work that would be nullified in a few months. I would prefer to wait to install the new range as part of the remodeling, too, but our existing electric range is dying right now and I'd rather not pay to have it repaired.

    My question: what would be the potential consequences of leaving the undersized pipe at Segment AB? Let's say that the furnace and water heater are both running, there's a load of clothes in the dryer, the oven is at 450 degrees, and four burners are lit. There won't be enough capacity in gas line. How will the various appliances respond? Is there a potential safety hazard? Is the situation manageable if we just pay attention to how much demand we're placing on the system at any given time, and cut back as necessary?

    Thank you.


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    Last edited by rsten; 11-06-2012 at 12:24 PM.

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    New England


    Gas pipe sizing is one part of it...is your gas meter and main line regulator capable of supplying the desired peak load as well? The pipe run distance and number of elbows can make a big difference and it's the 'effective' length that's important - each elbow increases the effective length of a run.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    In the Trades godsplumber's Avatar
    Join Date
    Nov 2012


    Great drawing just like in school 20 years ago. Here in my town, we have to stay with 3/4" all the way to the range, even though the cfh would dictate 1/2" is ok. In your case you could run 171 cfh with 3/4 at 40 feet. I would also say you could manage your demand and be ok waiting to up size in the months to come. Thanksgiving and Christmas dinner is an example of potential demand issues with showers, dryer, cold weather,mushroom gravy, steamed green beans, on the burners and oven all going. Try to celebrate at friends home (he-he). Best to you.
    (I think I'm hungry)

  4. #4
    DIY Senior Member Hammerlane's Avatar
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    Oct 2011


    Have you checked the sizing tables in the National Fuel & Gas Code ?

  5. #5
    Moderator & Master Plumber hj's Avatar
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    Aug 2004
    Cave Creek, Arizona


    The gas sizing tables refer to things in an ideal world. As a practical matter, you will have no problem with the system as you intend to do it.
    Licensed residential and commercial plumber

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