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It depends upon which model and size you buy AND what the flow rate is.

You can look at http://www.watts.com/pdf/ES-009.pdf to find the pressure loss charts.

If you installed a Watts 3/4" RPZ Model 009, you will have about 14psi loss for flows between 1-12 gpm. But if you install the 1" version, you'll only get a 10psi loss at 20gpm (but it's 14psi for 5-10gpm).

I believe Febco is one of the other popular makers of RPZ for irrigation, but I don't have a link to thier specks.

The zones have no more then 4 heads and most have 3. Will the pressure loss be that noticable?

Depends upon how much pressure you have to start.

As a general guess, your heads need about 20psi to operate. What you have to do is start with the static pressure of your system and subtract all your pressure losses. If you still have about 15-30psi left, your system should be ok.

While you really need to take the time to review everything at www.irrigationtutorials.com, here's the basics.

Get a pressure guage and use a hose bibb source by your water line before any pressure regulator and read your static pressure. Start with that and here goes some estimates.

Drop 3 psi for the watermeter.

Drop 5 psi for the irrigation valves.

Drop 12-15 psi for the RPZ.

The last part is friction losses throught the pipe. Assuming everything is 3/4" Sch40 PVC, flows of 5gal/min = 5.7 psi loss per each 100 foot. 7gpm = 10.5psi/100' and 10gal/min = 20psi/100'.

So you've got to know how much water is flowing through each pipe, it's length, and do the math (so you need the gpm figures for the spray heads).

The last thing to consider is that water losses 4psi for every 10ft of elevation change.

But as a basic follow-up, if you have lots of static pressure to begin with (like 100psi or more), then the loss in the RPZ won't matter much, except that you would need to adjust the pressure from any pressure regulator you should have if your static pressure was over about 80psi to begin with.

But if your system was designed with just enough pressure, the RPZ could be enough for the system to not provide enough pressure at the heads. If that happes, you might need to look into replacing your spray heads with MP-Rotators. They use low gpm compared to regular heads. This would greatly reduce your friction losses. When you drop the flow by half, the pressure losses reduce by MORE than half.