Drop in Water Pressure?

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.

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

rick52768 said:

Rain bird sprinklers states that if you have less than 7 GPM to call them?

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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/diy/DIYCatalog2009.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.

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.

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

rick52768 said:

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

Click to expand...

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.

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.