Hot water line size

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by keydash, Jul 15, 2010.

  1. keydash

    keydash New Member

    Dec 29, 2008
    Uncertain about what size of hot water line to use.

    Small structure: 1 shower, lav, toilet, washer, kit sink.

    The cold water line is 3/4 pex reducing to 1/2 at the bibs, then to 3/8 at fixtures other than the washer. I will be using an on demand water heater connected to the pex through a length of iron pipe.

    Question: Should I follow the same sizing for the hot water line as laid out for the cold water line or just run 1/2 pex for the hot throughout? The water heater has a 3/4 inlet and outlet.
  2. nhmaster3015

    nhmaster3015 Master Plumber

    Jul 30, 2008
    The granite state
    follow the manufacturers directions. On demand heaters are sensitive to flow
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  4. jimbo

    jimbo Plumber

    Aug 31, 2004
    San Diego
    Conventional wisdom says 3/4 copper ( 1" PEX) for the mains, reducing to 1/2" copper ( 3/4" PEX) for individual fixtures. These days, for low flow fixtures like lav faucets and showers, the smaller pipes have advantages. But for hi-demand fixtures like a tub or washing machine, you would quickly regret smaller pipes.


    Last edited by a moderator: Jun 20, 2016
  5. Gary Slusser

    Gary Slusser That's all folks!

    Aug 31, 2004
    Wherever I park the motorhome.
    I'd get rid of that piece of iron/steel pipe and since all tank less water heaters are flow controlled (already), I'd not run less than 3/4" to it. Steel/iron pipe rusts adding iron to the water and hard water voids the warranty on the tank less heaters.
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