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Thread: Am I on the right track?

  1. #1

    Default Am I on the right track?


    I'm new to this site and have searched and read past articles on water pipe sizing - go easy on me. I'm in the process of performing extensive remodeling on a small 3 bedroom 1 bath single story house (1100 Sq. Feet) and have a question regarding water pipe sizing. Home is built on a slab and currently all the piping exposed. I have a 3/4" copper main coming in to the house. Immediately after the shut-off it splits off w/ 1/2" copper to two branches.

    On one branch I have a Tub, Lavatory Sink, Water Closet and the branch terminates at an instant hot-water heater. (7.8 Fixture Units)

    On the other branch I have a Kitchen Sink, Hosebib, Dishwasher, and Washer Machine. (9.5 Fixture Units)

    The 1/2" copper hot water supply leaves the Hot Water Heater and goes to each unit in sequential order (no branching).

    If this is confusing, please see my cryptic picture below:

    O = 3/4" Supply

    O---Sink (1.5)---Bib (2.5)---DW(1.5)---Washer (4.0)
    Lav (1.0)
    WC (2.5)
    Hot Water Heater

    My thinking is as follows:

    I should be OK with a 1/2" hot supply throughout the house. Yes/no?

    A 1/2" cold supply to the branch that contains the bathroom and Hot Water Heater as it stands now should be OK.

    My concern and question is regarding the other branch. The kitchen/Laundry room branch looks to me like it should be 3/4" up to the dishwasher and then safe to reduce to 1/2" to feed the laundry area. So in this case I would have only a small section of 1/2" to replace with 3/4".

    Since everything is exposed, I sure would like to make it right before I button up the walls. If anyone has any suggestions/recommendations I would appreciate them.

    Thanks in advance! (sorry for the long message)
    Last edited by dowop; 05-19-2005 at 07:32 PM.

  2. #2

    Default Rule of thumb

    Go with 3/4" minimum from the shutoff by the meter to the hot water heater. Run as much 3/4" as you can and only pull one fixture from each 1/2" branch..try to keep the 1/2" stuff under 5'. Picture your water supplies as a tree..the closer to the trunk..the larger they get, and vice versa. I personally start at the fixture end, and size accordingly back to the main.

  3. #3


    The only reason I was suggesting a 1/2" from the hot water heater was because the Bosch Instant Hot Water heater has 1/2" inlet and outlet. My thinking was that it would be limited by this outlet. Is that not the case?

    Thanks in advance.

  4. #4
    Moderator & Master Plumber hj's Avatar
    Join Date
    Aug 2004
    Cave Creek, Arizona

    Default pipe

    That is a "pinch point, but pressure and volume are also affected by the length of the pipe and a long 1/2" pipe feeding through the water heater will deliver considerably less water than a 3/4"one feeding through that same heater. What kind of pipe are you using, because that will also affect your decision. I never run 1/2" for anything other than final terminations.

  5. #5


    Thanks for everyones help so far!

    As far as piping is concerned, the plan was to use CPVC (CPS). So if I have this correct, I should run the cold as follows:
    'T' off from the supply and carry 3/4" on the branch that contains the hot water heater all the way. On the Kitchen branch carry 3/4" to the dishwasher and then 1/2" to the washer (about 3').

    On the Hot side: Exit the hot water heater with 3/4" and carry that all the way to the dishwasher (feeding the bathroom and kitchen) where I'll then reduce it to 1/2" at the same point where I reduced the cold water supply line to supply the washer.

    Sound about right?

    If I was to use copper, that wouldn't change anything would it?

  6. #6


    personally I think 1/2 cpvc is useless for anything except filling up water ballons for the kids..do it in copper


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