Sprinkler supply line, 3/4 vs 1"

So right now my sprinkler supply line is 3/4" and travels 200' from a well to my furthest zones.
Sprinklers also use 3/4" PVC. My water pressure is not that great. There is an unused 1" pipe coming from the same well that never got used for anything so I could tap into it to give me a larger supply line, but this pipe would add another 100' to my total supply line on the furthest zone, less on others though. Would I gain anything by going with a larger pipe and a longer distance or would they cancel each other out?

 

Each 1' of 3/4" Schedule 40 PVC pipe (inner diameter of 0.804") is equivalent to about 3.3' of 1" Schedule 40 PVC pipe (inner diameter of 1.029") in terms of pressure drop. This comes from the empirical Hazen-Williams formula, which says the pressure drop varies as the the inside diameter d^(-4.8655). And (1.029/0.804)^4.8655 = 3.3.

So you can compare your two options with different sizes by converting one size of pipe to the equivalent length of the other size of pipe, and then comparing the total lengths after conversion. Be sure to include the equivalent lengths of fittings.

Cheers, Wayne

 

Your piping arrangement may be part of the problem. If you feed a zone with 3/4" pipe, you tee off with 1/2" pipe. If it is 1" you tee off with 3/4" or 1/2". Short runs with plenty of pressure and flow would not be noticable. Long runs and a lower flow you want to step down in pipe. Someone may have changed with 3/4" all around with 3/4" sprinklers trying to get more water to the sprinklers. To make sense of it you have a 3/4" pipe and then teeing off to the sprinkler and continuing to the next sprinkler. One 3/4" pipe cannot supply enough water to two 3/4" pipes.

For the zones with the furthest sprinklers, the sprinklers closest to the zone valve, change a few of them to 1/2 inlet sprinklers. This will allow more water to the one further down the line that might improve the performance of the furthest sprinklers and still get coverage that you want.