We see this a lot: all kinds of different heads stuck almost randomly on the same lateral line. I guess some people think that installing irrigation heads is just like plugging things into a socket. As long as the circuit breaker doesn’t blow, everything is good.
Well, they’re not. Instead of something shutting everything off, you get poor performance. Some heads might just drizzle. They won’t apply water at the same rate, so you’ll get wet and dry areas. You’ll waste water.
This is a spray nozzle. It’s mounted on a riser adapter and screwed onto a cheapo riser.
This is basically the same nozzle as the previous image, except that it’s mounted on a pop-up body. A very cheap pop-up body, but apparently it still works. If any plants ever start to grow near this thing, they’ll soon be tall enough to block the flow. When operating, the stem barely raises the nozzle high enough to water lawn, its intended function.
As long as the nozzles are in the same series, the previous two solutions will work (aside from a lawn pop-up in a shrub/ground cover area).
This head shares the same line with the pop-up and the spray. This is the deal breaker, the 900 pound gorilla who eats all the bananas.
A typical impact head puts down 0.41 inches per hour (a measure of how fast it wets things), needs at least 30 PSI to work correctly, shoots over 30 feet, and uses between three and five gallons per minute.
A spray head (the other two examples) operates at 15 to 30 PSI. The spray head will wet things fast, over one inch per hour (up to two inches per hour). That’s a lot more than the impact head. The spray probably shoots about fifteen feet when all is well, although you can adjust this by changing nozzles. If it’s a fifteen foot radius nozzle (the most common), it uses about a gallon and a half per minute.
Note that if you don’t have exactly 30 PSI, things will go bad. The impact head won’t distribute water as designed, or the spray nozzles will fog, wasting water.
Here’s where things really go bad: the pipe is probably too small to support all this. There’s probably enough pressure, since we typically have around 60 PSI at our hose bibbs. It’s just that you can only move so much water through a little pipe.
So, let’s add up about what these things need to run: One impact head at 4 gallons per minute (GPM) + five spray heads at 1.85 (I’m assuming 30 psi) = 13.25 GPM.
Plastic (PVC) pipe comes in different wall thicknesses. Good contractors use Schedule 40. Bad ones use Class 200 because it’s cheap, but since it’s got thin walls it becomes brittle and is much more prone to problems later on. I’ll assume (unwisely in this case) that this was a good contractor, using SCH 40 pipe.
Pipe is sized to minimize friction loss. In a correctly engineered system, the water comes into a big pipe, branches into smaller ones and finally gets to the smallest size at the heads. The size of the pipes depends on how much water is running through them.
We have a bit over 13 gallons required just to make this setup shoot water. This amount of flow calls for one inch pipe, or better yet, 1-1/4 inch pipe to allow for future mineral deposits inside the line. Unfortunately, the pipe is almost certainly only 3/4″. It could be 1/2″. That would be too bad, because according to RainBird’s pipe sizing chart, the system would lose around 42 PSI due to friction loss in the pipe. So, 65 psi – 42 psi leaves us with a paltry 23 psi. Not good. Oh, and by the way, that 65 psi number is wrong, since water going through valves and hardware to get to the head will lose at least 5 psi. So, now we’re down to 18 psi with half inch line.
Lots of math in irrigation. Sorry.
So, what to do if you want to add heads? Well, for starters, don’t use impact heads unless the system was designed for them: your pipes are probably too small. New technologies like stream rotors use a lot less water and have matched precipitation rates so everything gets more or less uniformly wet. Better yet, they can easily be retrofitted onto pop-up bodies (check the threads first; one brand has male threads where the rest have female).
What else can you do? If it’s a shrub area, you can convert to drip by adding a pressure regulator and filter at the valve (or a similar solution). Drip systems are very efficient, and you can water a lot of plants with little water, thus avoiding the pipe sizing problem.
Another thing anyone with an automatic irrigation system can do is reprogram their controller every month to adjust the water requirements to the current season. Leaving the controller on a summer schedule in winter just wastes water. Turning the system off during winter rains is a no-brainer, too.
If the system is old, leaky, cobbled together into a Frankensystem, sometimes the best bet is to abandon it in place and start over. This gives you a more efficient system, lets you size pipe correctly, put everything where you want. You can also replace your controller with a weather-sensing model so you don’t even have to reprogram it as the seasons change.
You can also avoid environmentally unfriendly pvc pipe. Polyethylene (HDPE) pipe is now available for broadcast (stream rotor) irrigation systems, and it’s fast to install, too. No headache-inducing organic solvents needed, no pipe glue, no volatile organic compounds. That’s worth the switch right there!