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What diameter PEX is most hot water efficient?

Ine | Posted in Green Products and Materials on

This is a follow up to the recent Q&A on tankless water heaters. One suggestion was smaller diameter pipe. I’ll be doing a PEX home run system and was thinking of 3/8 tube for all the hot fixtures. We are currently using a 1.6 gpm shower head and are happy with it so I’d think 3/8 should work for the shower and sinks but will this be enough to supply a dishwasher, fill a bath tub?
Thanks, Don

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  1. Riversong | | #1

    You'll want 1/2" PEX for any fixture except a sink or toilet. Among the many advantages of PEX domestic water supply piping over copper is that it is highly insulative, so simply changing to PEX will cut down on both water heating costs and waiting time for hot water. If the water heater is located reasonably close to fixtures, then waiting time and pipe losses are reduced.

    One of the advantages of a home run supply system is that 3/4" trunk lines are not required and water volumes between heater and fixture are reduced. A 1/2" pipe has less than half the volume of a 3/4" pipe and 2/3 the surface area.

    I would use 1/2" PEX for all hot and cold supply lines, keeping the system simple.

  2. GBA Editor
    Martin Holladay | | #2

    I ran 3/4 inch copper to my bathroom, and I regret it. Whenever I get some spare time, I'm going to run a dedicated 3/8 inch PEX line from the water heater to my bathroom lavatory.

    A lot of once-hot water runs down the drain when I wait for hot water to reach my bathroom sink. So 3/8 would be better.

    That said, I think a 3/4 inch supply makes sense for bathtubs. If you or other members of your family prefer baths to showers, you might not want 1/2 inch for the tub.

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