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Gas supply line big - small - big

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seldoon
seldoon Member Posts: 2
in Gas Heating
I'd like to replace a failed DV hot water heater with on-demand hot water heater. The gas supply line in question goes from the meter (AC-250) 1' @ 1" to 3/4 (1x3/4 Tee then a 3/4x1 coupling) to 1" (10') to 3/4 (62'). I could repipe 31' of the 3/4 to 1" without opening walls, but I can't find a resource to help me understand if I get any benefit from doing this. I would probably also fix the small mess at the meter end as well. My understanding of the longest run method is that repiping 31' from 3/4 to 1" doesn't buy me any benefit. Do I have this right?

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  • mattmia2
    mattmia2 Member Posts: 9,702
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    no. the fuel gas code spells it out pretty well.
    Mad Dog_2
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,567
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    By "on demand" you mean a tankless water heater correct? Those take a fair amount of gas. What is the btu input of the new heater and all other gas appliances. And a sketch of the existing piping will help.
    Intplm.Mad Dog_2
  • hot_rod
    hot_rod Member Posts: 22,193
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    Like this 
    some if the Rinnais can run on 1/2” gas line and take a bigger pressure drop

    fill in a drawing like this to start
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Mad Dog_2
  • seldoon
    seldoon Member Posts: 2
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    Thanks for the feedback. The planned tankless is a 180k takagi, rated 5/10.5WC. I'm in Seattle, with PSE. The meter is an AC 250, " 250 CFH @ 1/2" Diff". Research suggests i have 7" WC at the meter. My appliances are rated to 5" WC, so i think I have 2" WC to work with.

    A drawing of the run:



    The current run is 1" rigid for A, then 3/4" CSST for B, C, D, E and G. F is 3/4" rigid. Also, B, C, D are actually a single piece, fwiw. The crux is that leg C runs inside a wall that I'd rather not open up.

    If I can use leg C as is, and replace B, D and E with 1' CSST, I get a WC at the tankless of 5.337" with this option (C3). Calculations:


    Per @mattmia2, it sounds like I can't do this option, but I'm unclear if that's what he's referring to above.
    If using 1" pipe after 3/4" pipe is not allowed, then I would replace B with 1" CSST and C with 1" rigid (run outside for 14', then back inside). With this option (B) I get 5.10 WC at the tankless. Calculations:

    Option B is a bit more work but still doable. There are other repiping combinations such as also changing D to 1" CSST (5.468" WC at the tankless) but these are the two I'm considering.

    Any feedback appreciated.


  • Intplm.
    Intplm. Member Posts: 1,977
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    You can also make up for some of your issues by using gas regulators.
    However, if this was my house, I would not use a tankless water heater.

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