Expected Pump Power Usage with a Downgraded Impeller

[haz]

I replaced the impeller on my Jandy Zodiac FloPro FHPM 2.0-2 with a 0.75 HP impeller to try to get better energy efficiency. I was under the impression from what I'd read here that replacing a 2.0 HP impeller w/ a 0.75 HP impeller was equivalent for installing a 0.75 HP pump. I tested amps to see what kind of energy savings I was getting with the smaller impeller.

Results with clamp amp meter: 240 volts.
Amps:
0.75 HP impeller: 3.0 in low-speed, 5.25 in high-speed
2.0 HP impeller: 3.2 in low-speed, 8.8 in high-speed

Seems like less savings than I would expect if I swapped a 2.0 HP pump for a 0.75 HP one. Especially less than 10% difference in low-speed.

Is this level of energy savings what I should expect dropping from 2.0 to 0.75 HP? If not, should I consider getting a variable speed pump or replacing my pump with a lower HP 2-speed pump?

 

[JohnT]

I think what you're seeing is the effect of the actual load on your pump.
8.8A indicates a very lightly loaded 2HP motor. You could see as much as 7A on a 3/4HP pump. Your numbers are probably reasonable, though you'd have to consider the SF of your pump and how your plumbing is configured.

 

[Bama Rambler]

It sounds like you have a lot of head loss in that system? What was/is your filter pressure before the swap and after?

Can you post a pic of the layout?

 

[mas985]

When downsizing an impeller, you really can't use amps to measure power. What happens is the load on the motor drops and so without a power factor measurement, the volts * amps may give you an incorrect power measurement. Volts * Amps is a measure of total power which includes both real power and reactive or imaginary power. But you are not charged for the reactive part, only the real part. The best way to measure the power is with the power company's power meter on the side of the house. If you have a digital readout version, it should be pretty easy. Just take a reading with the pump on vs off.

Having said that though, a 40% reduction is not unreasonable for that change.

 

[haz]

Here are some pics of the pool equipment. A somewhat complicated setup with a regular 2.0 HP 2-speed pump, a 3.0 HP pump for the water feature, a solar heater, and a gas heater (not in picture).

In terms of filter pressure with the downgraded 0.75 HP impeller - I think it may have been 8 PSI, but I'm not sure. Currently w/ the original 2.0 HP impeller installed, it was 12 PSI before I raised it to about 16-17 PSI when I increased the pressure to allow the solar heater to be primed. In low-speed mode, the filter is around 4-5 PSI, which is around the lowest it reads.

I'm getting the message that I have a lightly loaded motor and that I might have a lot of head loss. My system is new and should be put together fairly efficiently. I know the builder had to put/increase the force of a few Jandy check valves to prevent backflow, but I would hope that the system is fairly efficient / low head loss.

I'll check to see if I can see on the house meter the power usage change w/ the pump on and off.

Now my questions is: I thought that going from a 2.0 to a 0.75 HP motor might decrease my amperage by over 50% while just decreasing my flow rate by say 25% or so. I assumed the flow rate difference in high speed would be similar to that in low speed. So if I'm getting say 25% less flow with a 0.75 HP impeller but only having power drop from 3.2 to 3.0 amps, it doesn't see to be that efficient of a proposition.

In high speed, if my flow rate drops around 25% or so and the amperage does drop 40%, that is worth it. But, at least in the warmer months with the solar off more and the pump running in low speed (it switches to high speed when solar is needed), the inefficiency of just dropping from 3.2 to 3.0 amps in low-speed while decreasing flow by ~25% doesn't seem worth it.

 

[JamesW]

The 3/4 HP takes a different diffuser than the 2 HP. Did you change out the diffuser?

Also, as mas985 said, you need to measure real power, not amps, to get an accurate comparison.

 

[mas985]

Going from a 2 HP impeller to a 3/4 HP impeller on the same plumbing will reduce flow rate by about 15%-20% depending on the plumbing. A 20% reduction in flow rate results in about a 35% reduction in head loss which is close to what you are seeing with pressure.

 

[haz]

Yes, I changed the diffuser for the one for the 0.75 and 1 HP pumps when I downgraded.

So basically I need to see if I can read an actual kW difference between the impellers to see the difference. But it seems that in theory everyone agrees that a downgraded impeller should function similar to a motor with a HP equal to the downgraded impeller, so that the lower HP impeller, if it provides enough power for my system should be better. So I need to confirm that the only 6% less amp reading on low speed from the 2.0 to 0.75 HP impeller is not accurate, no?