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Thread: Drop in Water Pressure?

  1. #16
    Retired Defense Industry Engineer jadnashua's Avatar
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    An elevation change can easily account for a couple of pounds of static water pressure. 3/4" cpvc pipe is about the same ID as 1/2" copper, and that is generally too small for a normal house with today's codes. How long is that 3/4" cpvc section?
    Jim DeBruycker
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  2. #17
    Homeowner Thatguy's Avatar
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    FWIW,
    if 5 psi [12' of head] @ 4.5 GPM is bad
    and
    40 PSI [92' of head] @ 16 GPM is good
    then a single number figure of merit for resi. water supplies would be how much hp is available at a sillcock with nothing else running.

    12 x 4.5/3956 = 0.014 hp
    and
    92 x 16/3956 = 0.37 hp

    I don't know what WaCo's benchmarks are except for this 40 PSI and 16 GPM.

    If anyone out there has pressure & GPM gauges I'd like to hear your values along with how satisfied you are with your city water supply. This will sharpen the line between a good resi. supply and a bad resi. supply.
    Last edited by Thatguy; 10-25-2010 at 04:45 PM.

  3. #18
    DIY Junior Member rick52768's Avatar
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    A lot of science going through Thatguy's brain. I wish I could get to my old house as I used four sprinklers in the back yard (almost 1/3 acre) using a bunch of 5/8 water hoses all up hill supplied by 40+ yrs old copper pipe with not a lot of problems. Run-on sentence of the week.

    Jadnashua are you asking how long the 3/4" line is until it splits off to 1/2" to feed the hose spigots or dishwasher?

    Here is another possible issue. My water line comes in the house in the basement and shoots straight up 6'. I bet that does not help the issue.

  4. #19
    Retired Defense Industry Engineer jadnashua's Avatar
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    You said "very short run of copper, then cpvc the rest of the way"....how long is 'the rest of the way'? The house I grew up in had a 1/2" copper supply line (don't think it ever got changed), but we only had 1.5 bathrooms. Today, that would not pass code. 3/4" cpvc is much closer to 1/2" copper than 3/4". So, if that run is more than a few feet, it is restricting the water flow, or could be, depending on how many taps you have opened. Static pressure (no flow) would be the same with a 1/4" line verses a 3" line, but once you add flow, the smaller one would realy mess with the volume or flow.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  5. #20
    DIY Junior Member rick52768's Avatar
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    Default Foot of copper, feet of CPVC

    The switch to CPVC is quick, water meter to a foot of copper to 30' of 3/4" CPVC. Then the 3/4" shoots up 6', over 35', then switches to one hose spigot with 15' of 1/2" CPVC. That was all from memory so the distance is not 100%, but close. Going to try the bucket GPM test again with a bigger bucket to decrease my error rate. Rain bird sprinklers states that if you have less than 7 GPM to call them? I was wanting to try the in ground sprinkler system as they are much more effective.

  6. #21
    Retired Defense Industry Engineer jadnashua's Avatar
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    A short section of smaller pipe acts like a venturi, and doesn't significantly affect the overall flow rate, but 30' of a smaller pipe definately will. Once in the house, branching off to smaller, single use connections is fairly normal. It's just that your supply is marginal, in my opinion, if you are looking for high flow without loss of pressure. When you try to get high flow out of a pipe, the higher you go, the more friction. Plus, you don't want the fps to be too high, either. a figure in the range of 5-6 fps is the range you want for the maximum.

    On copper tube sized cpvc, the id of a 3/4" pipe is nominally around 0.690", so at 6fps, that comes out to 7 gallons/minute. Now, your supply pressure will affect that flow as will any other restrictions (such as branching off to smaller pipes). While 3/4" copper is about 3/4" id, at the same velocity flows about 8.25gpm, or about 18% more. Bump that up to a 1" pipe and you have 14.7gpm, or 78% more. So, as you can see, changing the ID of the pipe just a little means (at the same pressure and flow rate) you can supply a lot more water. And, because the friction losses are smaller with larger pipe, you'll see closer to that theoretical maximum with less pressure loss.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  7. #22
    DIY Junior Member rick52768's Avatar
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    Default Not liking that answer

    Thanks for explaining the possible "why" in the issue I am having. I guess you are saying that the fix would be to dig up my yard and install 1" copper into the house? Anybody have any "better" ideas. How about if I also told you that the the WaCo tapped in to the water line that supplies the houses right behind me to boost the pressure of a neighborhood the is 1000 or so yards behind us. Could it be that my pressure was marginally okay but really sucks now due to this action? Thanks
    Last edited by rick52768; 11-01-2010 at 08:28 PM. Reason: Spelling

  8. #23
    Homeowner Thatguy's Avatar
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    Quote Originally Posted by rick52768 View Post
    Rain bird sprinklers states that if you have less than 7 GPM to call them?
    If you can get RainBird to tell you what input PSI will give you 7 GPM through their system, this is one point on the sprinkler "system curve".
    The city water "pump curve" as seen at your house should be above this point.

    Never mind. Look on page 32 of this link.
    http://www.rainbird.com/documents/di...atalog2009.pdf
    From these numbers it seems like you'd need a 1/2 hp to 2 hp pump to simulate city water, depending on your meter size, pipe size and pressure.

    For my house it's a 3/4" meter, 1" pipe and 60 PSI @ zero GPM so it must be 15 GPM at zero PSI.
    Last edited by Thatguy; 11-02-2010 at 03:29 PM.

  9. #24
    Retired Defense Industry Engineer jadnashua's Avatar
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    A bigger supply line will have less pressure loss and provide greater flow capabilities. It is fairly normal to slap a smaller meter on a bigger line...the path through it is fairly short, so it doesn't have a huge pressure drop across it. Often, you pay a fixed price or have a different rate structure based on the size of the meter - that is sometimes called a demand charge...a larger meter can demand more from the supply than a smaller one (and the supplier may have to do things to ensure you CAN demand that much, even though you might not). Then, you often pay for what you use, but a larger meter may start at a higher $$ amount than a smaller one.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  10. #25
    DIY Junior Member rick52768's Avatar
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    Default Jumping to action

    Maybe not the best plan of solving the issue, but I am planning to replace 15' of 1/2" CPVC pipe to match the supply line 3/4" in size to see if I get any difference at that hose spigot. Easy to get to and if I see any results it will be a quick fix. Could this cause any kind imbalance as the other size of the 1/2" tee feeds a half bath? Thanks

  11. #26
    Homeowner Thatguy's Avatar
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    Quote Originally Posted by rick52768 View Post
    Maybe not the best plan of solving the issue, but I am planning to replace 15' of 1/2" CPVC pipe to match the supply line 3/4" in size to see if I get any difference at that hose spigot. Easy to get to and if I see any results it will be a quick fix. Could this cause any kind imbalance as the other size of the 1/2" tee feeds a half bath? Thanks
    Post your proposed hydraulic circuit with lengths and IDs, and readings if you have any. Somebody here should be able to give you a yea/nay answer to the imbalance question, but actual values for pressure and GPM are harder to come by.

  12. #27
    Retired Defense Industry Engineer jadnashua's Avatar
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    Keep in mind that (elevation changes excepted) when there is no flow, the pressure in your water distribution system is the same everywhere. Then, only when you try to draw more water than the line can handle does it drop. So, running a larger line to a high volume use point should have no effect on other tap points in the system unless that tap point is using the whole capacity of the system. The goal should be to have the main pipe large enough to meet the maximum demand you anticipate from your whole house (it would be rare to have every tap open, but if you did plan that, then you'd have to size appropriately). If you are using a branch type system, the main trunk needs to be big enough to supply all of the branches that you anticipate will be open at the same time. There are codes and guidelines that specify this. Earlier, it was discussed on how much volume/time you can expect out of a pipe without exceeding the maximum recommended velocity. So, you can determine what size the main trunk needs to be, then how much you can expect out of any branch, then add it all up to see if the trunk is large enough. If it isn't, you have to either make it bigger, or live with a lower expectation, or reduce simultaneous uses to lower the maximum flow to what your system can handle. Because this isn't always done right, over the years, places where it could be dangerous, like in a shower, they've mandated safety features (the anti-scald shower valve) to prevent less than optimum flow from creating an unsafe situation.
    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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