I would like to see how many gallons per minute (gpm) I can get out of my 2 HP pump (2 inch line) and 140 gpm rated filter. In normal operation I tend to max out at 80 gpm, however when I "backwash" or "rinse" I get about 120 gpm flowing though the plumbing. Not sure why normal operation lowers the gpm to 80.

My first thought is that the sand is calcified, but wouldn't that slow down the "rinse" operation also? I regularly break up my sand to keep it loose. So i think i have ruled this out.

My second thought is that there may be an obstruction in one or more of the three return lines, although i don't have a good way of checking them. If you have a good way of seeing if the plumbing is clear let me know.

A third possibility is that the attached heater may obstructing the flow. There is no by-pass which is something i am looking at correcting.

Any ideas on other possible scenarios or if 80 gpm is going to be my best flow rate? What i have read is that a 2 inch inlet could potentially yield a 200 gpm flow (probably not with a 2 hp motor though).

Pool Info: Indoor commercial 180,000 gallon pool. Three total filters with two heaters. Sand is Zeolight brand very old (5 or more years).