Understanding value of variable speed filter

[Gannet]

We bought a house in Plymouth, MA 5 years ago with an older IG vinyl pool, 25K gal. It came with a 2 HP Hayward Superpump (discontinued model), 36 sq ft Nautilus DE filter and we have a Tiger Shark cleaning robot. Pipes are 1.5 inches. 2 skimmers and 4 returns. There are no waterfalls, heaters, spas or any type of special features. Within 24 hours of opening the pool this Spring, the pump died. I realize it might be possible to just replace the motor but we're looking at a new pump to try to improve efficiency. All the research we've done indicates that when we look at the pool pump curves, we want about 50 gpm flow and head is about 45 (although I realize it is dynamic).

If you look at the performance curves on the Pentair Superflow VS pump (which we are considering), then we'd be looking at running the pump at Step 1 speed or 3100 which isn't that much better than the 3450 we've been running it at (not much electricity savings). (Brochure with performance curves http://www.pentairpool.com/pdfs/superflovsDS.pdf)

My question is that the argument for a VS pump seems to be that you can lower your rpms, increase your water turnover time (run pump longer), but still save money and maybe even improve filtration. Except, if in my system, the head is 45 (and head as I understand it is resistance in the system), then wouldn't running the pump at lower speeds effectively stop circulation since there wouldn't be enough flow to overcome the inherent resistance?

I think I may be misunderstanding something fundamental but cannot for the life of me figure out what it is.

 

[woodyp]

I'm wondering if you are correct about the old 2 HP spinning at 3450? A VS pump you can dial waaaaaaaaaay on down there...but it probably won't do the job that low given your head.. Welcome to the forum.

 

[JasonLion]

Dynamic head changes as your change the pump speed. If you are moving 50 GPM on high speed, changing to half speed will move 25 GPM with 1/8 the dynamic head and use 1/4 the electricity. Running the pump twice as long, gives you a net 50% savings in electricity. A variable speed pump lets you go even slower than half speed, resulting in additional electrical savings (and longer pump run time).

 

[Gannet]

I'll admit I'm making an educated guess about the 3450 rpm - the pump itself is discontinued but if you look for replacement motors for the Hayward SP 1615-z-1-M they run at 3450 rpm.

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Thank you JasonLion! Almost sounds too good to be true ;-)