Is it standard for them to install a GPM check valve after the pump, or is that something I should expect to do myself? Seems like a good idea to have so that I can decide what RPM to run it at for proper operation of the heater and just 3 returns. So let's say I want to turn over the pool twice, and the heater has a minimum of 40 GPM so say I figure out what RPM it takes to do 45 GPM and turn it over twice? I.E. 13,000 gallons twice is 26k/45gpm is 578 minutes i.e. let's call it 10 hours. Then when I get solar, I'll want those 10 hours to be from around 10am to 8pm?
I'm not familiar with a GPM check valve, so I cant really answer that. I don't believe I have one, nor do I think I could come close to 120 GPM. If you really want to figure out everything, you can calculate the resistance/head which includes total length of pvc pipes, angles, equipment, any vertical rise, etc. Once you plug all of that in and calculate the head, you can look at the pump charts to see if you really need to be worried about exceeding the max GPM of your equipment. I'm assuming you will be fine here.
As for "turnover", as Marty said, that is a myth and not really important at all. I run my pump to do 3 things:
1. Generate chlorine from my SWCG
2. Skim debris off the top
3. Pump the water through the filter to remove any particles in the water
Most people put way too much emphasis on #3 and think that the filter will kill algae and that their water must pass through the filter x number of times a day. The chlorine in the water is what kills and prevents algae, not the filter. If you focus on #1 and #2 above, #3 is generally covered.
If helpful, I also have a 13,000 gallon pool and run my pump 24/7 as follows:
20 hours a day @ 1400 rpms (9 gpm)
4 hours a day @ 2400 rpms (41 gpm)
I just did the math and that means I turnover my pool 1.6 times a day. I've never had algae because I keep up my chlorine, and I've never noticed any cloudiness or anything (even at night with the lights on) that would tell me I need to be filtering more often. Hope that helps.