Efficiency: Full Rated vs Up Rated

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So ignoring the filter pressure and just using the other information you gave me, I estimated the following operating point for your existing pump: 83 GPM @ 67' of head, filter pressure is estimated at 23 PSI with a clean filter.

If you swap out the impeller for a WFDS-24, the operating point becomes:
72 GPM @ 50' of head (low speed is half the flow rate)

If you swap out the impeller for a WFE-2, the operating point becomes:
65 GPM @ 42' of head (low speed is half the flow rate)

So even the WFE-2 impeller could provide enough flow rate for the heater on low speed. BTW, you could keep your old motor and just replace the impeller and see how that works before springing for the two speed motor. Your old motor should have been drawing around 2200 watts but just changing the impeller should drop that to around 1200 watts. Low speed with the WFE-2 impeller will run around 270 watts.
 
First, thanks again for taking the time to help me understand, and to calculate those numbers so that I can be confident in the decision.

Once you mentioned using a smaller impeller on my current pump/motor, things really clicked!

I didn't put it all together, that the motor would only put out as much HP as required by the impeller @ the functioning RPM.

Based on what you've mentioned, it sounds like the difference of going with the 1/2 HP vs 3/4 HP will be negligible at half-speed (32.5 vs 36 GPM) yet the difference in power would make a lot more sense to go with the 1/2 HP. And I think you mentioned the curve for the half-speed is relatively flat, which means that even if my numbers were slightly off, the GPM would still fall pretty close to your estimate.

When you mention replacing only the impeller and testing the original motor, you mean the impeller AND diffuser, correct? I'm not sure why they want the diffuser to be changed as well (it doesn't look like it would vary much between size) but according to Pentair, I'd have to order both.

Is there a good source you recommend for purchasing pumps online? I have yet to find anyone selling pumps at a good price locally.

Thanks again, this has been a eye-opening thread!
 
Pump affinity laws apply for half speed so the head curve shifts down and to the left but by different amounts. Each point on the full speed head curve will shift left in GPM by 1/2 and down in head by 1/4. So the curve looks flatter but that is just due to the head scaling vs GPM scaling.

The Whisperflo series uses the same diffuser for the WFE-2 up through the WFE-6 so only the impeller needs to be changed.
 
Hi Mark,
Okay, I've got the B980 2-Speed pump ordered.

There is one other area where I've become stuck, despite searching for about 2 hours on the 'net...

I'm also replacing the motor on my 2 HP Whisperflo, which runs the jets on one side of the spa. This was a rebuilt pump motor, and the face plate is sprayed over. I am not 100% certain whether it's a 2.5 when the service factor is worked in. I figured I'd just open it up and look at the impeller part number, to determine the impeller's size, and buy a new motor based on that. To my surprise, there is no part number on the impeller!

Is there some way to get a list of the impeller vein widths, relative to the impeller HP ratings? I noticed that my 1.5 HP Whisperflo appears to be about 6mm, and the jet pump, which should be 2-to-2.5 HP, appears to be about 8mm. I'm wondering if the vein width, in MM, happens to correlate to the part-number for the basic single-speed pump line?

Thank you in advance!
 
The difference between the up rated a full rated motor is fairly small. The full rated has a 2.6 THP and the up rated is 2.2 THP. So getting the larger motor would work fine either way.

The diameter of the impeller is related to the maximum head of the pump but both pumps seem to have similar maximum head so that won't help. However, if you really want to know what impeller you have, then my suggestion would be to call Pentair and see if they can give you some help.
 
Thanks again Mark!

I wish I could better understand pump head to be able to grasp what you mean, but I think I have some fundamental misunderstanding of it. For example, I keep thinking of head as the amount of effort a pump must make to get the water to its destination. And based on that, I always mistakenly thought it was about the size/length/orientation of the pipes. I still haven't figured out how the pump's HP and the impeller factor in to change the head. I'll keep looking for an article I understand so that I don't have to bother you in the future ;-).

If I am interpreting your reply correctly, it sounds like I could get away with the same impeller for both the WFE-8 and WFE-28.

With the WFE-8 being the full-rated pump marketed as 2.5 HP:
2 * 1.30 = 2.60 SHP

And the WFE-28 being the up-rated pump marketed as 2.0 HP:
2 * 1.10 = 2.20 SHP

To confirm, I just attempted to search/match their impeller part numbers, and it appears they are currently using a different impeller for the 2.2 vs 2.6 HP pumps, with a 1.5 HP

I'm still not sure if the motor I had was the 2.6 or 2.2, but if I keep the impeller, which could be the 2.5 HP model, do you feel that I'll be okay? This is only for spa jets that typically run less than an hour at a time, only once or twice per week, I don't know if this is important.

Thanks again!
 
What I was trying to say was that a 2.6 THP (WFE-8) motor will work with either impeller. Yes, the WFE-8 and 28 have different impellers but they also have different THP motors to match. However, the WFE-8 motor will also work with the WFE-28 impeller but not the other way around. You can always use a bigger motor than necessary but never use a smaller motor than necessary. A larger motor on a smaller impeller will be underloaded but that is ok as an induction motor has a very large operating range. An overloaded motor will quickly burn out.

Also, the load generated by the impeller is called braking HP (BHP) and is dependent on head, flow rate and pumping efficiency (friction and recirculation losses). For most residential pool pumps, BHP is linearly related to GPM. So higher GPM results in higher BHP which drives the motor harder and in turn consumes more energy.

Anyway, pumped101.com is a great source to learn about pumps. I summarize some of that information in the Hydraulics101 sticky in my signature.
 
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