Need to replace pump

[mas985]

Actually, for 2-speed pumps, the GPM drops to half of the original value since GPM is proportional to RPM which also drops by half. The turnover doubles but the current draw is much less than half so the overall efficiency goes up. Also, head is proportional to square of the GPM so head is 1/4 of the original head.

However, there is only a minor cost advantage, ~10%, of a 1HP at low speed compared to a 3/4 HP at full speed.

I ran a few head calcs for a similar pool (I can't be exact without knowing all the pumbing) and using Northstar pump curves:

3/4 HP = 73 GPM @ 50' head - 231 kwh / month for 1 turnover
1 HP = 82 GPM @ 63 ' head - 300 kwh / month for 1 turnover
1 HP @ 1/2 speed = 41 GPM @ 16' head - 205 kwh / month for 1 turnover

[PoolDesignnet]

Hi,

It's not half....

If you look at the curve they show for the low speed it stops at 15 ft. of head. I had them fax me a curve for 2.5 hp last week for the pool I was working on. According to their chart... it has 0.0 gpm at 22.45 ft of head.

[mas985]

One thing to remember is that head loss (pipe friction loss) is dependent on the flow rate. So you cannot use the same head loss on the pump curves for both speeds or even changing HP of a pump. For a 2-speed pump, the low speed head loss is 1/4 of the high speed head loss. Those with 2-speed pump notice a PSI change on the filter of a factor of four between high speed and low speed. For example, a pool with 50' of head (18 psi) at high speed will have 12.5' (4.5 psi) at low speed . If you look at the GPM at each curve you will notice that the low speed GPM is half of the high speed GPM at those head loss points. For the the Whisperflow WFDS4, at high speed 50' of head is 90 GPM and at low speed for 12.5' the flow is 45 GPM.

Whisperflow WFDS4 Pump curve:
http://www-1.pentairwater.com/jwcs/w...rFlo_curve.jpg

Check out these references for more information how head and GPM changes with pump speed.

http://www.mcnallyinstitute.com/02-html/2-01.html
http://edis.ifas.ufl.edu/AE004