Automatic Sprinkler Lateral Pipe Size Reduction

Hi everyone. Just ran across this forum and hopefully someone here can give me a hand.

We have a 4 zone auto sprinkler system at our house. It was installed before we purchased the house. One of the zones is no longer landscaped so I replumbed that valve to another area that had piping pre-run under our concrete walkway for future expansion and landscaping. The piping going in and out of the valve is 3/4" but the pre-run line is 1/2". When I plumbed the line from the valve to the pre-run line I used a reducing 90* elbow to drop from 3/4" line to 1/2" line. When I tested the line, water started gushing up from the dirt below all the valves. After some digging, I found an uncapped pipe that the water was coming from. My guess is that this is some sort of pressure relief pipe. When any of the other 3 zones are on, no water comes from this pipe.

Could I be developing too much pressure with the 90* reducing elbow?
Could I reduce at a straight point in the line and not build so much pressure?
Can I simply cap this open pipe where the water is coming out?
Do I need to re-run that line as 3/4" all the way to the sprinkler risers?

Any thoughts would be greatly appreciated. Thanks!!!

First, there is nothing you can do that will increase or decrease the pressure. A large pipe will carry more water than a small pipe, but the pressure is determined by the pressure in the main water supply. There is no such thing as a pressure relief in an irrigation system. If there is an open pipe, that is your problem. There are automatic drains that are supposed to open when the water is turned off and close when there is pressure in the line, but I doubt that you would have several of these and even if you did, the probability of all of them failing at the same time is nil. If would not just cap that mystery pipe, it would remain full of water when the system is winterized and break when it freezes. If you can, find where it comes from and cap it there. I'd try the 1/2" pipe before putting in 3/4". It really depends on how many gpm will be needed. The price difference between 1/2" and 3/4" PVC is so small, it was pointless to use the 1/2" to start with, but it may be enough.

Gary Swart said:

First, there is nothing you can do that will increase or decrease the pressure. A large pipe will carry more water than a small pipe, but the pressure is determined by the pressure in the main water supply. There is no such thing as a pressure relief in an irrigation system. If there is an open pipe, that is your problem. There are automatic drains that are supposed to open when the water is turned off and close when there is pressure in the line, but I doubt that you would have several of these and even if you did, the probability of all of them failing at the same time is nil. If would not just cap that mystery pipe, it would remain full of water when the system is winterized and break when it freezes. If you can, find where it comes from and cap it there. I'd try the 1/2" pipe before putting in 3/4". It really depends on how many gpm will be needed. The price difference between 1/2" and 3/4" PVC is so small, it was pointless to use the 1/2" to start with, but it may be enough.

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Well luckily living in Southern California where a cold winter night is about 40 degrees, we don't really have to worry about freezing pipes. I'll see if I can do some more digging though to find out where it comes from.

I wouldn't mind running 3/4" to the new area, but since it is already burried and there are several concrete walks it goes under, I don't want to re-run it.

If you look at a lot of sprinkler heads, like Rainbird maxi-paws, they have two possible inlets: One is for a 1/2" fitting and one is for a 3/4" fitting. You keep the one plugged that you are not using.

Thus, as was mentioned, there's absolutely no need to convert everything to 3/4". Just cap off the mystery pipe or attach a sprinkler head to it, and you should be fine.

If the system was pre piped for expansion, they would have likely run that pipe all the way to the valve box. If you tied in somewhere other than the end of the line, you likely found the end. Without pictures and drawings, it would be hard to determine. Whenever I have wierd pipe / valve issues, I normally just hook up a garden hose to a head end, and back pressure the system to see where leaks pop up (literally). You then fix the leak and cap the ends until you get the pipe to hold pressure.

Something else that jumped out of your post, the 3/4 and 1/2 inch pipe must be dealt with carefully as far as capacity is concerned. You don't say what type of irrigation you are planning on putting on the 1/2 inch, but be very careful not to over allocate the available water by putting in too many heads.

Mr_Pike said:

If the system was pre piped for expansion, they would have likely run that pipe all the way to the valve box. If you tied in somewhere other than the end of the line, you likely found the end. Without pictures and drawings, it would be hard to determine. Whenever I have wierd pipe / valve issues, I normally just hook up a garden hose to a head end, and back pressure the system to see where leaks pop up (literally). You then fix the leak and cap the ends until you get the pipe to hold pressure.

Something else that jumped out of your post, the 3/4 and 1/2 inch pipe must be dealt with carefully as far as capacity is concerned. You don't say what type of irrigation you are planning on putting on the 1/2 inch, but be very careful not to over allocate the available water by putting in too many heads.

Click to expand...

You hit the nail on the head. I did some more investigation last weekend and realized that this mystery pipe is directly under the output for one of the other valves. It seems as though they did run the line all the way back to the valves and what I was tapping into was actually a riser for a sprinkler that happened to be near the valve. Hence the 3/4 and 1/2 issues. I was pressurizing the line at a sprinkler riser and not the rear hook up so the hookup was the path of least resistance, hence the gushing water.

Thanks for all your help!