SPRINKLER TALK FORUM - You Got Questions, We've Got Answers

For 18 GPM (depending on length of pipe runs) 1.25" mainline and laterals are sufficient. I'm assuming they also designed your mainline at 1.5"?

1.25" poly pipe will lose approx. 2psi per 100' of pipe at 18gpm. 1.5" poly will lose roughly 1psi per 100' of pipe at 18gpm. If nothing else, I would run 1.5" mainline and 1.25" laterals.

You need to change your thought process on pipe sizing. Larger diameter pipe isn't for more volume, it's for reducing the amt. of pressure lost as the water travels through the pipe.

I would measure the length of pipe it would take to feed the zone farthest from the water source. Using the values above, you can determine the amt. of pressure lost for your "weakest" zone.

Not a mistake. More than likely you'll need to adjust the pressure switch to a 65psi cutout once the system is installed
As for the 1.5" pipe, if the pro thought shaving a .5psi loss was worth the added expense of larger pipe, then so be it. Not knowing the exact specs of your well, its hard to say one way or another if 1.5" is necessary. If 18gpm is max, I would think 1.25" would be more than enough.

A rule of thumb I like to use:
<15gpm= 1" pipe
16-24gpm= 1.25" pipe
25-35gpm= 1.5" pipe

There are exceptions like long runs, and low operating pressures where upsizing pipes might be recquired, but usually not until you reach the upper limits of the above flow rates. 18 gpm falls rather low on the 1.25" scale.

The dollar cost of 1-1/2 pipe is not so much more than that of 1-1/4 pipe, so it can be an easy choice for a mainline. As for laterals, they might be all 1-inch on an 18 gpm system, provided the mainline is located so as to pass through the middle of zones, so that two diverging pipes comes from a zone valve, each one carrying 9 gpm.

On well-water systems, more attention can be paid to minimize pressure losses in the plumbing, because the supply pressure will max out at the cut-off pressure setting. (actually a few psi below that cutoff setting, so as to avoid cycling)

$30 difference between 1.25" and 1.5" per 100'. Not to mention the added expense of fittings, which on some can be as much as $2 more per fitting. I just can't see where it justifies saving 1psi. But, again, I haven't seen the property nor have I seen the drawings.

Better that he overdesigned than underdesigned.

I think it's the all-PVC systems that some pros don't bother with 1-1/4 pipe, probably like some poly-pipe guys don't ever use 3/4-inch poly, because they don't want to carry the extra inventory in pipe and fittings.

Understandable.