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Hopworks 10-04-2011 06:33 PM

Sprinkler design question / clarification
I'm in my new house and going after designing my sprinkler system for the back yard. I have done a few before but back then everyone told me one thing was best, and now I cannot find any designs that support that. I figured here was the best place to get a clarification.

I will try not to be long winded. :eek: Also, please pardon my lack of knowledge of sprinkler design terminology.

Should I run heads off the zone feeder lines or should I run a short length of pipe off the feeder lines and then to the heads? I did some crude illustrations to explain what I mean and will post them if needed.

All the designs I googled show threaded tees in series with the zone lines and the risers/heads coming directly off them. It was my understanding that running a small length of pipe off a tee in series with the zone line and then attaching a one-end-threaded/one-end-slip elbow to that short length's other end, then ultimately the riser/head to the threaded end helps to stabilize the PSI and gives a more even head pressure throughout the zone. It's a bit more work and cost, but I'm willing if it will help me avoid weak heads. I know they are adjustable. I just want to do all I can to give as close to equal PSI to the heads at the riser as I can.

Back in 1998, the helpful chaps at Home Depot helped me with the layout of zones and head placement. In the end it worked very well, but, it was the short length of pipe off the tees on the zone lines that we went with.

Was that just to help with low-psi situations? Or is that a standard practice that isn't used anymore (at least on google searches)? :laughing:

Thank you all for your time and help with this clarification request.


piste 10-10-2011 12:53 PM

check out the rainbird web site. they offer a free design service!

user1007 10-10-2011 02:09 PM

Before you do anything you need to know the PSI (pounds per square inch) of water pressure flowing to the start of your proposed irrigation system and the flow rate you have available as well. Sprinkler heads are rated by the PSI and Gallons per minute (GPM) they need to work.

With these numbers in hand, and knowing the ratings of the sprinklers you have in mind you can at least begin to design your system idealistically. For example, if you have 12GPM per minute availed and each one spits out 4GPM like some giant golf course versions. You only can have three per circuit.

If each one needs residual PSI of 35 to start and you only have 70 availed, You can only in dreams fit two per circuit. The one at the end may end up looking like it needs Viagra.

Actually residential sprinkler systems require much less in terms of both pressure and water flow. But you get the idea. Each bend in irrigation piping steals some hydraulic energy also.

The best method for positioning your sprinkler heads? For a residence with with reasonable water pressure and flow rate and not like acres of land? Take the manufacturer's top specs for the radius the heads can spray at peak performance and substract 10 percent. Get your self a triangle, some transparent graph paper or scaled architect's ruler and layout a series of equilateral triangle all over the page set to the performance number I had you decide. I personally find it helpful to swing circles with a compass using the same number from each point in the triangle.

Lay these triangles and circles over the scaled drawing of your property and you will know exactly where your sprinkler heads should go and the minimum amount of piping should also be obvious. You will see where you have to buy geometric patterned sprinklers so you are not watering sidewalks and stuff.

I know this sounds complicated but it is really easy. Triangles, circles, knowing or find your water pressure and flow rate. Super simple.

rkanemeier 11-20-2011 01:55 AM

i'm taking an irrigation design class myself (although it's geared towards Texas law). a lot of stuff seemed to be covered in sdsesters post, so i'll add my own 2cp.

probably the 1st thing you should check into is city/county/state laws. for texas, you HAVE to have a backflow preventer between the meter and your valves. these things are not cheap ($100+ for the preventer + more for the pipes going in and out). they also have to be certified. i have a Pressure Valve Breaker (PVB) which needs to be installed at least 6" higher than the highest head on the property. So if you use a 12" riser pipe, your PVB needs to be 18" above ground (again this is in Texas, not sure elsewhere). Also, since the PVB will be above ground, you have to use schedule 80 PVC pipe OR heavily insulate schedule 40 or class 315 (usually only used for 1/2") PVC

we also have what we call "head to head" coverage (meaning if you use 8' heads, then your heads will be no more than 8' apart from one another). If you have full coverage like this, then your watering times can decrease.

we also have some laws that state what your working flow is supposed to be. it's the lesser of the following 3, with the 3rd rule being the end-all-be-all maximum:

1. pressure loss through the water meter should not exceed 10% of the minimum static water pressure available
2. maximum water flow through the meter should not exceed 75% of the safe water flow of the meter
3. velocity of the water should not exceed 9fps through copper service line

you mentioned you already know how to install some irrigation systems, but in case you were not aware, in order to determine if the entire system is going to work, you'll need to calculate a hydraulics worksheet. you need to take the calculate all the pressure losses from the "furthest head of the largest zone" all the way back to the meter (include all heads in the zone, all piping, the valve, any pressure regulators, backflow device, the meter and the service line). If the remaining pressure is more than the pressure needed for the head to work, then every other head in all the other zones will work. However, if the pressure is too high (i think more than 5-7psi) you may need to install a pressure regulator or a pressure compensated nozzle. this may not be required but you could get a lot of misting (which i am currently having some trouble with).

You also would need to take into consideration any elevation changes.

There is some cad software called Pro Contractor Studio (i'm not a paying subscriber and have no affiliation with them) that allows you to do everything on computer if you don't like drawing stuff on paper.

Anyway, for what it's worth.

Msradell 11-20-2011 01:06 PM


Originally Posted by rkanemeier (Post 775127)
i'm taking an irrigation design class myself (although it's geared towards Texas law). a lot of stuff seemed to be covered in sdsesters post, so i'll add my own 2cp.

probably the 1st thing you should check into is city/county/state laws....

Other than backflow preventers I've never seen any place with laws regulating sprinkler systems. I've seen them that regulate water usage (restrictions) but not that really govern the installation of the system. Is this because of the severe water restrictions that are quite common in Texas or is there some other basis for it?

In answer to the OP's question regarding installation. I prefer installations that have the section of flex line coming off the mains. These make it much easier to adjust the height and to a certain degree that position of each head to meet changing conditions.

rkanemeier 11-20-2011 05:59 PM

well, i cannot speak for other states, but our instructor told us that these things are common irrigation practices. some things you cannot get away from (pressure losses, backflow prevention, working pressures etc). but other things can be dictated by law (ie. you cannot cut through a root that is more than 1" in diameter).

Msradell 11-20-2011 07:03 PM

Certainly agree that the first group of things you mentioned are just how the systems operate mainly because of physics and happen whether you allow for them or not. Other things, like your example of cutting roots are local mandates in in this case one I've never heard of before. Besides, when you're running a ditcher how do you know how big a root is?

rkanemeier 11-20-2011 09:53 PM


i would think if you're near a tree then you wouldn't use a trencher. and if you use a shovel, be careful not to dig too hard near trees. of course call before you dig.

as a side note to everything i'm telling you, 90% of the people in this class are already designing and installing irrigation systems (illegally). lol.

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