Hi Robert! There is some madness to pump selection. For your main circulator feeding the HX the following would apply. 1) Find out how many gpm we need on the HX to obtain the desired performance. 2) Figure out pressure drop in HX and system piping at the gpm picked in step 1 from the circulator to HX back to tank. 3) Look at pump curves, mark a point showing the gpm and ft of pressure drop. 4) Curves above this point will provide adequate performance, but, eliminate any pumps that are at the extremes of their curves ie you don't want to be running a monster pump at near stall conditions or making pump run out to the extreme limits of its capacity. 5) If you still have several pumps left at this point, check the power use and efficiency and go for the least power/most efficient. The hardest thing is finding the pressure drop. You will need to use a Hazen-Williams headloss calculator. There are online ones. You also need to have pipe measurements and fitting counts. For common pump sizes there will be models available from each brand. Do you like chevy or ford basically at that point! OH, if you want to implement an outdoor reset mixing type setup right from the start using the HX pump, you will need a VS pump. That will reduce your selection of available pumps.