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Thread: Gas Supply to Tankless Water Heater

  1. #1

    Default Gas Supply to Tankless Water Heater

    I'm installing a Rinnia tankless water heater that will operate on propane. I'm confused about the size of the gas line. The manufacturer requires a 3/4" line to the heater (up to 180,000 btu). I have a 1/2" copper line form the 500lb tank outside to the input side of the regulator on the side of the house. on the outlet side, the regulator is tapped for 1/2" npt fitting. I have asked the Propane company if this regulator is big enough to supply this heater and a 75,000 but furnace, a stove, and a gas fireplace and they said it will. It doesn't make sense to me to come out of the regulator at 1/2" npt and increase to 3/4" npt, unless the 3/4" pipe allows more volume to flow? can anyone explain.
    Thanks

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    Propane is denser than natural gas, and the tankless system is designed to accept natural gas, and by changing the jets and potentially other parts, run on propane. My guess is they sized it to handle natural gas, and may not make a distinction.

    You still would need to adapt to the size of the input fitting, but MIGHT not need to run a larger line. Best to call and confirm with the tankless manufacturer.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Master Plumber master plumber mark's Avatar
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    Talking call the company

    the larger line will allow for more volume...

    but I would be more concerned with the gas
    regualtor at the big tank not being big enough......


    with everything going on at once in the dead of winter

    I woud bet it wont cut the mustard..

    or it will squeal and moan like a pig when the propane tries to pass through that regulator all at once

  4. #4

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    Jad, MPM,
    thanks for your reply.
    The tankless heater came from the factory setup for propane use only, and in the literature and on a label at the connection it says to use a 3/4" supply. The gas company said the equipment outside the house will handle my furnace gas stove, fireplace and the tankless heater.

  5. #5
    DIY Member Squ1rrel's Avatar
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    It also has to do with distance and pressure. The closer you are to the tank, the higher the rating. For instance a 1/2" line, with a 10 foot distance, will supply 276KBTU, while at 40 feet will only supply 130KBTU. These numbers are based on an 11" WC supply pressure as well, so a difference in the pressure will change those numbers.

  6. #6
    Retired Defense Industry Engineer jadnashua's Avatar
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    Conversions typically only change internal parts (nominally just the jets or orifaces and the regulator). Thus, the external connection is 3/4" because if the internals were set for NG, you'd have to have that larger size input. From a code issue, the inspector would be looking to install things based on the manufacturer's instructions, so even if it might work on a 1/2" line, he might have problems if it isn't 3/4" because the installation instructions say 3/4".

    I'm not a pro, and these are just my observations, not the guidance you need...sorry.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  7. #7
    Moderator & Master Plumber hj's Avatar
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    Default propane

    Your tank should have a primary regulator which is sending a medium pressure supply to the secondary regulator at the house. The 1/2" outlet will supply an enormous amount of gas as long as the smaller size is not extended too far. Increasing to 3/4" immediately utilizes the Bournelli effect so the velocity increases for a short run and then slows down as the volume increases.

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