You are quite correct about the friction loss... however it is one big loop about 800' long. I can calculate friction loss numbers (same as fire hose I would expect). My intent was only to send a very small amount of water through the loop - like a garden hose trickle. It can remain in the line longer and absorb more heat with no problems. I just need to be sure that water is in fact flowing through the loop at all.

I don't have filter pressure readings for each valve position because I don't have this all hooked up yet. It is only circulating (skimmer->pump->filter->return).

My intent is to somehow have a visual indicator that there is water flowing through the loop. I just don't know what to put in.

Thanks!
Josh