I have an old 40 x 20 (deep), in ground pool. I estimate it at +/- 40,000 gallons. It has a 1.5 inch main drain line and a 1.5 inch skimmer line on the suction side. On the return side, it has just one 1.5 inch return (I am going to convert an unused 1 inch pressure cleaner line to an additional return line, but, for now there is just the one). For many years the pool has run fine using a 1 hp, single speed (3450 RPM) pump and a 48 sq. ft. DE filter. Recently, the old filter needed replacing, so, I replaced it with a 60 sq. ft. DE filter (Pentair FNSP60). Since I was replacing the filter, I decided to upgrade the entire system. I replaced the pump with a Pentair Intelliflow VS (011018). LADWP has a $500 rebate on that pump, so, it was a perfect time to upgrade. I, also, added a Hayward separation tank. I installed a vacuum gauge on the suction side of the pump and a pressure gauge on the pressure side. The equipment is, approximately, 50 feet from the pool and 18+ inches higher than the pool water level.
Initially, I set the maximum speed at 2500 RPM (for prime/startup) and run speed at 2000 RPM. I am being cautious about speeds because of the size and age of the plumbing. It required a hose forcing water through the skimmer to get prime. After priming, the pump dropped to the 2000 RPM setting and ran fine, for a while. I left for a few hours and when I returned, the pump was running, but, very little water was flowing back to the pool. I turned it off for a minute then, restarted it. It went to prime mode, primed fine and dropped back to the 2000 RPM setting, at which point it was running fine. I left again and, when I returned, same problem. I bumped up the run speed to 2200 RPM and restarted, but, the problem still persists. For reference, the vacuum gauge reads 14 at prime speed of 2500 RPM and 10 at 2200 RPM. The pressure gauge reads, about, 10, either way. Each time the water discharge drops, as above, the vacuum gauge shows 2, while the pressure gauge remains the same! The basket remains full of water and there are a few small air bubbles coming from the return line, very little air in the filter. It appears to me that the pump is losing suction after running for a while, although, it maintains enough flow to keep running. What would cause that and how do I fix it? Any suggestions would be greatly appreciated.