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Thread: water volume question

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  1. #1
    DIY Junior Member monkeybrains's Avatar
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    Nov 2010
    new york

    Default water volume question

    I am trying to increase the water volume to my house. I currently have a 5/8 water meter to a 3/4 main that runs about 30 feet to the prv which ive replaced with a watts 223 and then goes to rinnai instant heaters. I cannot upsize the main or meter due to ridiculous charges by our local municipalities. H2O pressure from the street is 125 psi. Do you think I could increase flow by increasing to 1 inch as the main enters the house upstream of the prv, then going with a watts 223 1 inch prv and 1 inch pipe to the rinnais, theen i could plumb 1 inch pipes to the risers going upstairs. I am currently only able to get about 7 gpm out of our master shower and I plumbed 3/4 all the way to the drop ear elbows through a hansgrohe volume control and 3/4 thermo mixer, it just seems like we should be getting much more water flow. Also we got about 10gpm before the rinnais were installed and just had a 50 gal h2o water heater.

  2. #2


    Instead of replacing all that piping see if you can find a PRV that will allow you to adjust the pressure up higher than what you have now, this will give you a faster flow which will equate to more gpm, but, I am not sure if they make PRV's that are adjustable to a 100 PSI. Or for that matter, remove the PRV, the only real concern would be if you had a water softener because the tanks are only rated for 125 PSI. Removing the PRV may go against your local codes, and is not really a good idea but is a way to boost the GPM.

  3. #3
    Master Plumber Redwood's Avatar
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    Dec 2007


    80 Psi is the max...

  4. #4
    Homeowner Thatguy's Avatar
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    Aug 2008


    Does your setup contradict the results you should get according to pg. 32 of this link?

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


    If you had good flow before installing the Rinnai and poor flow afterwards, then the problem seems to be the flow regulator IN the Rinnai, NOT your supply lines. If that is the case, then the shower valve would automatically reduce the cold volume/pressure to match the hot. Heaters are rated for 150 psi and the standard relief valves release at that pressure. From your description we cannot tell what you would have to do to increase the general volume of the system because the restriction could be anywhere between the city main and the faucets. Most PRVs can be adjusted to higher pressures which would help the rest of the system, but NOT the hot water if the problem is indeed in the Rinnai heater.

  6. #6
    Retired Defense Industry Engineer jadnashua's Avatar
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    Sep 2004
    New England


    One way some tankless systems try to ensure they can meet their delta T (temperature rise) figures is to have a flow restriction in them. Don't know if yours does, but it is likely the cause of your reduced flow. Some of them can be cascaded (i.e., installed in series) and then the flow restriction probably can be eliminated.

    A larger main line and meter in many places means a greater 'demand' charge (the ability to draw more water, even if you don't use it, you could). So, what you were seeing is fairly common. Now, whether it is ridiculous or not, if you think about what you are demanding of the system, it means the whole supply system needs to be able to meet your demand - pumps, pipe sizes, supply, etc., without impacting any other customers.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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