First off, a 1.5 HP pump on a 18" sand filter is WAY TOO BIG. It can channel the sand bed making filtering ineffective. Even a 30" tank is too small.

A 1/2 HP pump is sufficient for this application. I don't see a VS pump being worth your while for such a small body of water. Water moving at a slower rate through a sand filter traps dirt easier. You should consider a large cartridge filter instead. You'll wind up "dialing" down the VS pump anyway to regulate flow rates.

Also, the flow through the heater cannot exceed about 75 GPM without an external bypass. You'll wind up "dialing" down the VS pump anyway to regulate flow rates.

We install a lot of VS pumps when they are practical for the application.

Thanks for the advice. I am going to change my selection to a cartridge filter. The original pump is a 1.5 hp speck. The reason for changing to a VS is to keep the water moving at a lower rate over the winter at a lower electrical cost. Does this change your opinion on a vs for my circumstances?

Thanks

I would go with the largest filter. Oversizing for a filter is fine. We use the largest DE or cartridge filters on all pools and on stand alone spas. Model depends on application and need for servicing.

I understand your idea to run over the winter. It's such a small body that a very small pump would give you an energy savings as well. A VS pump's upfront cost may have a long pay back over a conventional larger pump in your application. Believe me, I'm all for the VS pumps, just may not worth the while with your set up.

I use Jandy VS pumps and they have a on-line calculator to see what your energy savings would be, based on your current electric rate, for VS pumps. You can plug in a conventional pump and compare to a VS. Might be worth a look to see how much a savings you would incur. If it's worthwhile to you, go for it.

Once again, thanks for the great advice. I will check out the online calculator at Jandy.