Why would this have been designed this way? and will this be alright if I put in the 3/4 inch valve?

I doubt it was designed that way. More likely reason is the one inch valve is what was on hand. Pressure losses for most valves are published. I think you will find the difference in pressure loss between the 3/4 inch valve and the 1 inch is nominal.

 

Thanks!

 

That doesn't make any sense whatsoever to me. If everything else is 1" why step it down before and after just to go back to 1"? If it was me I would cut all the 3/4" out that I could and replace it with 1". You are already in there and have it all dug up, it's just a little extra work.

 

Hello!

The line that feeds into the irrigation valve manifold is 1 inch, then it was stepped down to 3/4 inch, so the manifold and irrigation lines as well as everything after this are all 3/4 - the original valves were 1 inch - the lines don't ever step back to 1 inch - it was just the valves that went up to 1 inch, but there was an adapter to fit the 1 inch valve into the 3/4 inch lines.

all my lines look to be 3/4 except the line going into the manifold.

 

The laterals are already 3/4” so performance wise there should be no real difference in a 1 inch valve versus the 3/4 unless the valves specs indicate a significant pressure loss difference. But I agree that 1” mainline step down to 3/4” is pretty bobo!

 

I misread what the OP posted about the lines on the other side of the valves are going out to 3/4", still not the best way to go but make a little more sense now

 

3/4 valve is fine