Water Feature Pump Question

[Dabby]

We have a 1.5 hp that runs a "weeping wall" type waterfall and two bubblers on the tanning ledge. The pool builder insisted that we needed this much power to run the features. He apparently likes the sound of Niagara Falls and 6 foot high gushers on his tanning ledge. We, however, prefer to have more of a gentle babbling brook effect.

There are two valves that control the amount of water that goes to the two features. I have to shut these down almost all the way to get the amount of flow we desire. My concern is that I am causing too much back pressure on the pump. Is this really a problem or am I worrying for no reason? If I am flirting with disaster, is there anything I can do other than replace the pump with a less powerful one?

 

[duraleigh]

You are not causing any harm. Your psi goes up but it does not put any extra strain on the motor.

 

[mas985]

If I am flirting with disaster, is there anything I can do other than replace the pump with a less powerful one?

I agree with Dave. The only downside is that the water feature pump is using a lot more energy than it needs to. A simple solution would be to downsize the impeller to a 1 HP or even a 3/4 HP. It depends on how much you are actually reducing the flow but it sounds like the 3/4 HP would be more than enough.

 

[Dabby]

Thanks for putting my mind at ease. I pushed for the 3/4 HP pump from the beginning, but I let myself be swayed by the builder. If I'm not doing any harm, I may just leave it the way it is. We don't use the water features very often. What all is involved with downsizing the impeller? About what would the cost be?

 

[Dabby]

I have another question. I just looked up how to change the impeller. I'm showing my ignorance here, but how does changing the impeller save energy? Is that not determined by the pump motor?

 

[bobodaclown]

Think about it like an 8 cylinder vs 4 cylinder (electic motor/different HP). Driving the same size vehicle (impeller). (Think a Pinto, I believe they came with 4 cylinder engines but people added a 8 cylinder engine.)

The rpm of the electic motors are fixed 3450 (3600) RPM. If you down grade the impeller lighter load, the larger motor will work easier. If you put in a smaller motor but leave the higher rated impeller you're putting too much load on the motor shorting it's life.

There's a whole different conversation about figuring out total horse power that's what really matters when it comes to electric motors. As long as the total horse power are the same you can down size the motor. .75 HP motor * 1.5 Service Factor = Total HP of 1.125
or a motor could be rated at 1HP * 1.0 Service Factor = Total HP 1.0

In that case a .75 (3/4) HP motor could be used.

Changing out the impeller is much cheaper than changing out the motor. And you'll get some savings.

 

[mas985]

It is the impeller that determines the load and power draw from an induction motor. Downsizing an impeller reduces the power draw from the motor without having to change out the motor. The motor label HP and service factor is a rating which indicates the maximum load that the motor can support. A motor can support a lower load than it's rating but not a load that exceeds it's rating so downsizing an impeller is not an issue.

To change an impeller, you need to purchase a new impeller (smaller) and a new shaft seal. Then separate the motor from the pump wet end (usually 4 bolts), remove the old impeller and shaft seal, then replace with new. The hardest part can be removing the impeller because sometimes they get stuck. The shaft moves freely so you will need to put a wrench on the shaft in the motor end cap and may need a strap wrench for the impeller end.

 

[Dabby]

Do you know if we're talking a noticable savings in energy or just a few cents?

 

[mas985]

I can only give you a ball park but if you restricted the flow rate of the 1.5 HP to be the same as the 3/4 HP (without restriction), then the 1.5 HP would be using about 400 watts more in energy than the 3/4 HP. The savings is then dependent on how long your run the pump and what you pay for electricity.