Correct. The rule mostly came about in the past when most pumps were single speed high head pumps that were way too big and most plumbing was way too small.For instance, if we had the 3 way valve on pump outlet, going to waste, it would be very little return side head loss, but that would not cause any issues. only if suction side is high would there be issues.
If a pump is below the surface of the pool far enough, you can have a situation where the pressure in the suction is never a vacuum and is always positive.as an aside, i suppose you could have negative head loss? like the the outlet of pump was connected to pipe going straight down 5 feet, then you would have close to -5 head loss on return?
Ok. Looks like the heater head loss will apply.