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Monday, April 28th 2008, 7:04pm

Question on pipe size

Hi guys, I am starting my sprinkler install and had a question and wanted some comments. I live in New York and have 1" poly coming into the basement from the street. From there to the meter(5/8) the pipe reduces to 3/4" pex for about 2 feet. It then goes into a 3/4" rpz then I up sized the outlet to 1" for about 40' to the valves. From there, I plan to run 1" poly to the sprinklers in the yard. My question is does the 2' between the 3/4" and the 1" matter in regards to calculating gpm? I thought not, as I read on irrigation tutorial that under 5' was ok to use the larger pipe for the design. My static psi is 70 and I'm sizing for 14 gpm to be safe. I will have 8 zones for just under an acre of lawn. My other question is do you think the rpz is ok being 3/4"? I know it will support 14 gpm, and I factored in a 15psi loss for the rpz. Any comments would be appriciated.


Supreme Member


Monday, April 28th 2008, 8:32pm

Don't worry about the 3/4" rpz. The bulk of the pressure loss is going to be due to the rpz itself. There will be very little additional pressure loss because of the 3/4" compared to the 1". It's even possible that the 3/4" may have LESS pressure loss at your designed gpm. You'd have to look at pressure loss graphs for the rpz in question to know.

Otherwise, the 2' of pipe at 3/4" compared to 1" will have very little difference in pressure loss. The biggest difference will be the momentary increase in water velocity as it screams through the 3/4" pipe (it will be travelling around twice the speed in the 3/4" pipe as it will the 1" pipe). It's possible for that to lead to water hammer, but likely not.

My biggest question would be are you sure this setup can supply 14gpm? Just getting through the valves, rpz, meter, and mainline pipe is likely to steal something around 25psi or more. That leaves less than 50 psi of working pressure to attempt to push 14 gpm.




Tuesday, April 29th 2008, 2:48am

Thamks for the reply. I thought I calculated everything in regards to my losses. 6psi loss for 1"(70') coming into the house,15psi loss for rpz,3 psi loss for the water meter, 3 psi loss for the main line in the basement, and 3 psi loss for the valves. That leaves me about 35psi and I don't plan on having any rotors larger than 30'. Does this seem to close? Maybe I should size my zones to 10 gpm to give myself some more cushon? What are your thoughts on working with what I have?


Supreme Member

Posts: 5,383

Location: Metro NYC


Tuesday, April 29th 2008, 7:24am

Do the plumbing, then measure the flow and pressure, and rework the design if you need to.


Tuesday, April 29th 2008, 5:16pm

So what your saying is pipe in the backflow and piping to the valves, then check the flow?


Supreme Member

Posts: 5,383

Location: Metro NYC


Wednesday, April 30th 2008, 8:00am

Check flow and pressure together. You have that luxury, so take advantage of it.


Sunday, May 4th 2008, 4:58am

Check flow and pressure together. You have that luxury, so take advantage of it.
I agree

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