Pump efficiency

[tcat]

Why does it save $ to run a 2 speed or vs pump? If the goal is running the pump to get a full turnover... if you run on low, you need to run twice as long. Doesn't that use the same amount of electricity? Thinking of buying a new pump as back up, and I keep reading that 2 speed will save money in the long run. Can someone explain how?

 

[woodyp]

Not an expert by any means on this, but I'm assuming that running on low speed draws a lot less than half the amperage than running on full blown does? Educate us BOTH folks!

 

[JasonLion]

First, a quick aside. It is not the goal to provide one turnover per day. A typical pool need about half of that. That doesn't relate to your main question, but I needed to mention that.

Over simplifying a little, energy usage is a function of two main factors, resistance to flow in the plumbing, and the efficiency of the motor. The more resistance to flow caused by the plumbing, the more energy you need to use to push the water through. Similarly, the more efficient the motor, the less energy you need to use. One of the main contributions to resistance to flow is the flow rate. Pushing more water through the same pipe means more resistance to flow.

As it happens, running a standard two speed motor on low speed moves exactly half as many gallons per minute, and requires 1/8th the energy. However, the motor also gets less efficient at the same time, so in practice the actual energy usage is about 1/4 as much per unit time. Since it moves half as much water, you need to run the pump twice as long, so the total energy usage is right around 1/2 of what it would be to move the same amount of water on high speed.

With a variable speed pump, this effect gets larger. You can run at even lower speeds and save even more money, getting the cost down to as low as 1/8 the cost on high speed (under ideal circumstances, in practice not quite that good). However, the big savings come at the beginning, so variable speed pumps don't always pay back the additional up front investment (depending on how much more they cost).

 

[Donldson]

I have a 1 HP 2-speed pump. I am taking this info straight from the label, I am not sure about GPH but I think we still have enough information. On high it runs at 3450 RPM and consumes 8.8 Amps, or 1012 watts at 115 volts. On low it runs at 1725 RPM and consumes 2.6 amps, or 299 watts. I have installed a kill-a-watt on it and it confirmed these numbers.

So to run at half speed the pump uses less than a third of the power to run on high. We run it 24/7 and end up using less energy than we would to run it on high for 8 hours a day.

 

[tcat]

Great info! So by that logic, can I assume that if I have a 1.5hp pump, can I just buy a .75hp pump and run it twice as long, using half the power? Or is a larger pump on low speed, more efficient than a smaller single speed?

 

[JasonLion]

You have the right idea, but the wrong numbers. A two speed pump running on low is 1/8th the HP of the same pump running on high. Purchasing very low HP pumps (that correspond to a regular pump on low speed) is not practical, since you occasionally need high speed for things like priming, backwashing, vacuuming, etc. At the same time, getting a smaller two speed pump is more efficient than getting a larger two speed pump and often a good idea. Of course, replacing a working pump is usually only worth it if your electric rates are fairly high, otherwise you won't pay back the initial investment in any kind of reasonable amount of time.

 

[tcat]

Thanks. I was thinking more of having a back-up for a "quick change" should the need arise (putting the new, more efficient pump in the system with the older pump as back up). Are newer pumps more efficient?

My current pump is Pentair Whisperflo with this motor

About 50' run of 2" pipe to and from pool. Pump is about 8" above water level. Is a new 2 speed 1hp adequate?

 

[JasonLion]

Yes, a 1 HP, or 3/4 HP, will be fine, (unless you have a water feature not mentioned in you signature).

 

[mas985]

You could just replace the old motor with a new high efficiency two speed motor:

http://www.poolsupplyworld.com/Century- ... /B2983.htm

Or downsize the impeller and motor at the same time.

 

[tcat]

I keep seeing suggestions to just replace motor, does the wet side never go bad? It seems for 2-3 hundred more you'd have a full set of spare parts. Is one 2 speed motor much more efficient than another?

 

[JasonLion]

The pump portion can break, but it is rather unusual. It is far more common to have problems with the motor.

 

[mas985]

Is one 2 speed motor much more efficient than another?

In this case, yes. The Century B2900 series uses a run capacitor for the low speed so it gains 20-30% extra efficiency on low speed. However, the current line of two speed pumps don't seem to include this motor for some reason.

http://www.centuryelectricmotor.com/Mot ... 033&id=362


Also, the wet end housing can last for a very long time. About the only thing that can go wrong is that they will sometimes develop a crack but usually after suffering from some sort of trauma. Everything else in the pump is replaceable.

 

[tcat]

Think I'll go with the 1hp:
http://www.poolsupplyworld.com/Century- ... /B2982.htm

No water features, no vacuum (use robot), no solar. 2 drains, 4 returns. I think 1hp should be plenty. So if "high" is 3450rpm, either HP moves the same amount of water? Do I need anything besides the motor? Is this motor included with any particular new pump, should I decide to replace all?

 

[mas985]

If you downsize the motor, you must downsize the impeller too or the motor will be overloaded.

http://www.poolsupplyworld.com/Pentair- ... 073128.htm

And as far as I know, the energy efficient two speeds are not shipping with any pumps yet.

 

[Durk]

I put the B2982 on my WhisperFlo last year. I have a larger pool and a larger DE filter and it does everything perfectly. The power savings are dramatic. I should have done it sooner.

 

[tcat]

Thanks guys. Just pulled the trigger on the B2982, the 073128 impeller, and a GoKit32 seal kit Looks pretty easy to change out, I have lots of room around my pump. What do you do about a water proof switch, or is it on the motor?

edit: All working well. One comment, I replaced the existing impeller with 073128, but I believe the impeller that was on the old 1.5hp single speed was the same as the new one I put on the 1hp 2 speed. Looked to be same size, and looking at Whisperflo 1.5hp pump info it does call out 073128, so probably didn't need to change it. I run the pump on low 95% anyway, so I doubt impeller is going to have much effect. Still can't believe low id 50% of the flow, seems like about 10%. A backwash on high looks as fast as my old 1.5hp backwash (like a fire-hose), on slow it just "flows". Need to figure out a way to measure it... Nice running 8 hours at less than half the electrical cost!