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How to configure pipe layout for an irrigation system

Now we need to figure out our pipe layout. Later.  I have a few things to go over first. Your system should attach to your water line as soon after the water meter as practical. This keeps your pressure as high as possible. Examples of how to tie in are HERE.  Make sure you put in an isolation valve. This is shown in each of the samples on that page. An isolation valve/cut off valve/ shut off valve simply lets you turn off the system for repairs. It can be a ball valve or a gate valve. Please don’t forget it.

Each zone is controlled by a valve. Something to think about is that each valve takes wiring. You run two wires from the irrigation controller to each one. The ground, or common, wire runs from valve to valve to valve but each valve gets a separate hot wire from the controller.  There are two schools of thought on where to put the valves. One is to put the valves in the zones they control. Look at this picture. There are advantages and disadvantages to this design. The advantage is the valve is closest to the zone, letting you easily minimize pressure loss after the valve by proper pipe sizing. The disadvantage is you need more wire, which costs money, valves have a tendency go get covered by sod or the new barbecue pit and eventually you will forget where they are.

The other thought is to cluster your valves in one area, as in this picture. All the valves are in one area, hopefully low traffic, and are easy to find. They can be installed on a manifold, making installation easier. This uses the least amout of wire, saving money. However, it uses more piping and can make controlling pressure loss more challenging. Both ways work and have their fans and detractors. Your choice.

First, locate your utilities. You can call 811 and they will help you. You do NOT want to discover undergound power or gas lines with a shovel. I’ve done it. It’s not pretty. Or fun. So call 811 first.

Figure out your dig path. Walk it. Put markers down where your trench will go. Get someone else to walk it with you. Try to place as many runs of PVC in the same trench as you can. Less digging. Pay attendtion to trees, sidewalks, driveways, swingsets, fences, etc. Do you have an easy path? Have trees? The closer you go to a tree the more roots you have to fight. Need to go under a fence? Can you go through the gate area? Going under a sidewalk? You may have to. If so, look HERE. Use a trencher or hand dig? No matter what, there will always be some hand digging but trenchers can sure help.

Figure out where you want your backflow preventer. This may or may not be covered by local codes. You generally want it out of sight, which may add a little to your digging. It may be underground. Regardless, since it’s the next thing in line after the isolation valve you want to put some thought into it. It will need to be relatively accessible for maintenance and repair. Again, follow local codes. Now, mark your dig path and where your backflow will be on your drawing. Add where you think you will put the valves. Use a pencil. Things change. Color coding helps a lot.

Now we can talk about piping. I’ll go over some general ideas first then we’ll get into specifics and math. Don’t worry. Again, we’ll let you cheat with charts.

No matter what you’ve been told, BIGGER PIPE IS NOT ALWAYS BETTER. Having a pipe too big can cause a whole new set of problems. Too big is always better than too small but only if it’s a little too big, not a lot. An efficient design will usually use three or four sizes of pipe. We’ll get to that shortly.

Generally your main irrigation feed line will be the same size as the main supply line of the meter. If you have a 1” main supply line then tap into it with a 1” irrigation line. Any reducing, if needed, can come later. This line usually continues in size through the backflow and to the valves. This gives us the maximum amount of water available to the valves.

After the valves you can reduce your pipe size. After all, you’re not running the pipe as far and you are only feeding one zone.  This pipe is called a lateral. As a matter of fact, any line run after a valve is called a lateral. Before a valve it’s called the main. Laterals run from the valve into the zone area. Other laterals can run  to sprinkler head locations.

It’s about at this point that eyes start glazing over and I lose people. Don’t give up. It sounds so complicated but is so easy. Everything just involves simple steps. Right now you need pencil and paper.

Go back to your drawing. Pick a zone. We need to measure how much water is needed in each part of the zone so we know what size pipe to use.  Remember, the first section needs the most and it drops off from there.

Simple example: pretend Zone X1 has 6 heads using 2gpm each. Immediately after the valve you need 12gpm, 6 heads at 2gmp each.  Now, after you pass head #1 you have 5 heads left. So that section of pipe only needs 10gpm. After the next head you only need 8gpm. Get it? Simple.

Look at this drawing and the chart below. The black circles represent sprinkler heads with the gpm written beside them. Red is our pipe. Add up all the sprinkler heads. 2.5+2.5+2.6+1.3+1.3+1.3= 11.5gpm. Since it’s after the valve we can use class 200 pipe, if code allows. The first size that fits 11.5gpm is 1”. It will do 16gpm. So we know our line to the head has to be 1”.

Now go past the first head. The rest of the heads add to 1.3+1.3+2.6+1.3+2.5 = 9gpm. For 9gpm we can downsize to ¾” pipe. Now look at the Tee where the line splits. If you go up from there you have heads totaling 2.6+2.5+1.3 = 6.4 gpm. Still ¾”. Going the other way we only have two heads at 1.3gpm each. That’s 2.6gpm total and we can use ½”. Nothing to it, right?

Do this for each zone. Now you can total up how much of each type and size pipe you need. From the meter to the valves might be 1” schedule 40, so measure that. Include any need for above ground backflow installation, if needed. After the valves you have runs of 1” ¾” and ½”.  Add them up and you’ll know how much of each type to buy. Also pay attention to your corners and connections. You’ll need fittings to connect everything. You’ll get a rough idea of the fittings you need off your drawing. Always buy extra. Don’t forget the primer and glue.

That’s it. You now have your piping figured out. Now you should hear your shovel calling you.

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