I have 1" inside diameter loop pipes running out to my pond and back, and I'm building a flush cart for them. I have this 1.5 HP Hayward pump with 1.5" intake/discharge in hand: link to product here. The docs say that I must use at least a 1.5" suction pipe, which I can do. However, since my loops are 1" I must reduce to that size on the discharge side of the pump. I understand that I would lose efficiency, which shouldn't matter in this application. What other problems could this cause? Thanks in advance for any advice you may share.
That isn't really the right way of looking at things. You need a flushing flowrate (2 ft/s air, 6ft/s dirt) for ground loop. Your pump is then sized based on that. A 1.5hp pump is fine for your purposes - 1" connections aside. Please note, it is possible to build an unpurgable ground loop.
