Need to replace pump

[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