Pump usage suggestions

[Lou Cifer]

Hey there,
I have 4 Mag pumps a Mag18, Mag12, Mag7 and a Mag2. I just want to hear some suggestions/opinions on how to get the best out of them.
For the last couple months I've been using the 18 for the return, the 12 for the chiller and the 2 for the skimmer.
I've noticed that the temp has gone up a few degrees especially since summer is here now.
Today I removed the 18 and replaced it with the 7 to see if there would be a difference in temp.
How would you guys use them? Should I leave the 7 as the return and use the 18 externally as a closed loop? Is the 7 too small to use as a return?
Any and all input will be appreciated.
Thanks

[sjm817]

Mag7 is a better choice for the return than the 18. What size plumbing are you using on the 7 and 18?

[Lou Cifer]

I believe it's a 3/4" return.

[sjm817]

3/4" will severely restrict the flow on that Mag18 anyway. You need a minimum of 1.5" to get full flow from it. Even the Mag7 is restricted with 3/4". It should have 1" for full flow.

[Lou Cifer]

I'll double check to be sure.