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Thread: Question on installation methods regarding pressure and volume/

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  1. #1
    In the Trades
    Join Date
    Feb 2009
    North East PA / New Jersey

    Default Question on installation methods regarding pressure and volume/

    I have a question regarding increasing water volume in a residential plumbing installation.

    My situation.
    I am plumbing 2 bathrooms, the master bath (9 Cold FU, 60' from source and 6 Hot FU, 20' from source) and the second floor bath (7 Cold FU and 4 Hot FU, same distance from source.) 46 to 60 psi.

    The shower in the master bath is on the same wall as the toilet and double sinks for the second floor bath. I was going to feed both bathrooms with a 1" for the cold water and 3/4" for the hot water wich according to the code is okay however I am worried about two possible problems.

    1. I think there will be a major pressure drop once the water is on for say the shower and someone flushes the toilet.
    2. The hot water will need to run for a while before the hot water reaches any of these baths.

    So here is what I am thinking of doing and please let me know if you are in agreement.

    For the pressure drop I was going to run a 1" pipe for each bathroom's cold water and a 3/4" pipe for each bathroom's hot water. Possibly use 1-1/4" pipe as a header and feed the fixtures off this header with 1/2".

    For the hot water I am thinking of tying in the farthest hot water pipe to a gravity loop system back to the hot water heater's drain valve port. This should at least keep the hot water flowing to minimize time.

    Your help is most appreciated and I thank you in advance.


  2. #2
    Consultant cwhyu2's Avatar
    Join Date
    Aug 2007
    Cincinnati OH


    The pressure will be the same no matter the size of pipe.
    larger pipe more volume.Pressure remains the same.
    Then you have static pressure and working pressure.
    60psi when all fixtures are closed is static pressure.
    40 or less is working pressure would be when a fixture is open.
    this is just an example.

  3. #3
    In the Trades Gary Swart's Avatar
    Join Date
    Sep 2004
    Yakima WA


    You may want to consider a recirculation pump for the hot water. This would give you instant hot water at all locations. You need to understand the difference between pressure and volume. While there is a relationship, they are not the same. You may be on the right track with increasing pipe size, but you are going overboard on sizing. There are charts available to help you with this.

  4. #4
    Plumber jimbo's Avatar
    Join Date
    Aug 2004
    San Diego


    Certainly no need for anything larger than 1" in your picture.

    And while larger pipes minimize pressure drop problems, a larger hot pipe means it will take even longer to purge the cold and get hot water to the fixtures. Recirculation will fix this. Gravity recirc is good. The return line must run continously up hill....no dips or humps.

  5. #5
    Moderator & Master Plumber hj's Avatar
    Join Date
    Aug 2004
    Cave Creek, Arizona

    Default pipes

    An downhill once it reaches the far end of the system, again without any dips or rises. There is a point at which it makes no difference how much larger you make the pipes, it will not have any effect on the dynamic pressure or volume. That point is when the maximum flow is passing through the small openings in the faucet's seats. 1" pipe might even be too large to make a difference. In addition, you MUST use a pressure balancing shower/tub valve so the effect of slightly undersized pipes would be minimized and unnoticeable.


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