Calculating gas pipe size.

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by zimm0who0net, Feb 27, 2020.

  1. zimm0who0net

    zimm0who0net Member

    Jun 7, 2009
    So I'm confused a bit on how to calculate gas line size when the there are multiple different sizes of pipe in the mix.

    I'm feeding a 150k BTU tankless water heater (that is the only thing on this branch). I've already got 125' of 1-1/4" poly. I've got another 25 feet to go inside the structure. I'm thinking about necking down to a 1" CSST, but I don't know the proper math here to determine what size that last stretch needs to be. Could someone help me out here?
  2. wwhitney

    wwhitney Active Member

    Mar 17, 2019
    Berkeley, CA
    If you have charts that tell you "for pipe size X and flow rate Y, here is the allowable length" then what you can do is the following:

    1) Diagram out the whole gas piping system from the regulator to the appliances. Show pipe types, sizes and lengths (including fittings as their equivalent length), and the demand flow from each appliance
    2) Assume all appliances are using their full demand flow, and label each pipe segment with the resulting flow.
    3) For each pipe segment at the labeled flow rate, look up the allowable length. Divide the actual length by the allowable length and label the segment with the result, e.g. as a percentage.
    4) For each appliance, trace the route from the regulator to that appliance and add up all the percentages. If the percentage is under 100%, your piping system can provide the required flow rate to that appliance within the allowed pressure drop. If it's over 100%, enlarge a pipe segment and start over at step (3).

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  4. wwhitney

    wwhitney Active Member

    Mar 17, 2019
    Berkeley, CA
    P.S. As a simple example if there's just the one 150 kBTU/h appliance and just the two pipe segments you mention, then you need the allowable length L for 150 kBTU/h through 1-1/4" poly, and the allowable length M for 150 kBTU/h through 1" CSST. Then if the 125' and 25' figures include all the fitting equivalent lengths, you need:

    125/L + 25/M <= 1

    Cheers, Wayne
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