Frankensteined Pump - Should I change the impeller while installing new VS motor?

[Chuck_Davis]

I'm installing my new EcoTech VS 1.5 HP motor on my Hayward SuperPump, and ran into a bit of confusion.

The pool was installed somewhere in 1997-2000 by the original owner of the house, who was also a pool builder. It appears that he Frankensteined the pump assembly.

The pump body is a Hayward SuperPump, Model No. SP2621X25, which is supposedly a 2.5 HP model. (Although I suspect that the same pump body is used for various HP ratings.)

The motor is an A.O. Smith 1.5 HP, 2-speed unit with a Service Factor of 1.3.

The impeller is a Hayward SP2610-C, which is rated at 1.0 HP.

The piping is 1.5 inch. The pump/filter/heat pump are 4-5 feet above water level. There is 22 lbs. of back pressure (!) from the filter (Tagelus TA-60, Zeolite that was just cleaned, newish multiport valve). There is a 30 foot run to/from the heat pump, which adds even more head when the heat pump is in-circuit during the summer.

With the heat pump bypassed in winter, I could run the pool, water feature and (just barely) Liquidator with the motor on low speed. With the heat pump in-circuit, I had to run the motor on high speed.

As part of installing the new EcoTech VS motor (1.5 HP, Service Factor 1.6), should I change out the impeller for a 1.5 HP rated impeller? I'm looking for maximum energy efficiency while keeping all the pool systems running properly.

 

[JamesW]

I don't think that the bigger impeller would give you any benefit. You should be able to get more than enough flow from that impeller at less than full rpm.

 

[JasonLion]

Leaving in the smaller impeller is both fine, and will be (just slightly) more energy efficient. Using a larger motor with a smaller impeller is always fine. You never want to use a motor with an impeller designed for a larger motor. Using your existing impeller will give you the same maximum water flow that had before, and let you go to lower effective speeds, which are more efficient.

 

[Chuck_Davis]

I scurried down to the pool store before they closed and grabbed a 1.5 HP impeller just to have options. Then I dove back into Google.

This paper caught my eye (http://www.pump-zone.com/topics/pumps/pumps/variable-speed-or-impeller-trim), and especially one of the conclusions: Efficiency decreases with a reduction in speed. However, this reduction in efficiency with speed is small, while the reduction in efficiency by trimming the impeller can be substantial.

Is it a correct interpretation of this paper that for a given flow and head I will use less net energy with the larger impeller and the motor running at a lower speed than with the smaller impeller and the motor running at a higher speed?