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Thread: supply line for tankless- do i have 1/2 or 3/4"?

  1. #1
    DIY Junior Member policestyle's Avatar
    Join Date
    Dec 2010
    toronto, ON

    Default supply line for tankless- do i have 1/2 or 3/4"?

    Hi everyone-

    I called the tech assistance line about my installation but they were unable to answer me, so I thought I'd go straight to you...with 2 related questions.

    Question1 : I have a Rheem Eco Sense 180 DVN tankless I'm about to install. It says that the water supply is to be 3/4". The City of Toronto (where I live) has a 3/4" supply line into the house, but it is commonly reduced to 1/2" inside the residence. Does this still count as 3/4" (I'm thinking about Pascal's Principle here- pressure is still the same, etc...) since it began as 3/4" when it entered my house?

    Question2: If the output (hot) line is only partly 3/4", with 1/2" down the line at some spots, is that ok?

    The tech person on the phone just kept saying "3/4" to me, and wasn't really aware of Pascal's Principle. I probably sounded like a jerk. Oh well.

    Thanks anyone, everyone!

  2. #2
    Moderator & Master Plumber hj's Avatar
    Join Date
    Aug 2004
    Cave Creek, Arizona


    Pressure is the same everywhere in a "STATIC" system. Once water starts to flow the volume is a function of the flow resistance created by the pipe size and velocity. Using your theory, we could reduce the 3/4" to 1/4" and still have 3/4" pipes. As a rule, the final volume is a function of the smallest pipe in the system, although Bournelli's Principle comes into play for short sections of reduced pipe size.

  3. #3
    In the trades Dana's Avatar
    Join Date
    Jan 2009


    Even if the water supply line is only half-inch that may not present much of a problem, as long as your static water pressure is well above the operating minimum of the Rheem (probably ~15psi) and the distance from the reduction point modest. At higher flow rates it's possible with very long undersized supply runs to have sufficient pressure drop at the water side of the heat exchanger to induce some sizzle & micro-boil, reducing the efficiency of the heat transfer. If the output side is all 3/4" & short, and the input side all 1/2" & long the pressure at the HX will be lower than if both runs were 1/2". But it's mostly just a performance limitation, and only at the higher flow rates. If the pressure gets too low at higher flow the combustion efficiency will drop, and the heater won't quite meet it's flow x delta-T spec. Only in extremes would this be enough for the heater to self-diagnose a problem a spit out an error code.

    Some shower-mixers may have issues with the combined head loss of the plumbing + water heater though, but those won't usually be fixed just by upgrading the pipe diameters.

    You could go all techno on this, measure the pressure and calculate the drop due to the head of the plumbing on the input side, bla bla, but just installing it as-is and fixing it only if there's a performance issue would be reasonable. The MOST important thing to get right is the fuel line plumbing capacity, which can affect more than just heat transfer efficiency.

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