Run the largest drain that you can! My 1.5 was ripped out and replaced with a 2 inch due to trapped air (flushing). I could have just reduced the flow (or gated the drain) but I wanted to get my moneys worth out of the Iwaki MD70. This pump is pushing about 900 gph through 1 " flex with a 15 ft head and is running my skimmer in the basement. The RC pump calc was not accurate (too low) on this pump with a high head.