Figuring Flow Rate

[bottomcat]

I need some help figuring my flow rate. The reason I need to know is because I want to buy a heater that has a minimum flow rate requirement of 25 GPM and I want to ensure that I can meet that with my pump on low speed. My pump is a two-speed pump with low-speed specifications of 0.25 Label HP, 1.0 Service Factor, and 1725 RPM. I am running about 18 feet of 1.5” PVC on the suction connection, and another 6 feet of 1.5” hose. Also two 1.5” valves and three elbows. The return side is the same as the suction.

The pump is a AO Smith Model BN50, Part Number 0-177803-02. I’d love to know how to calculate the flow rate.

[CarlD]

Hi,'cat. I'm not the expert on pumps and flow rates but I can tell you this: AO Smith makes the motors, not the pumps. But you have a Pentair Optiflow pump in your sig, and it looks like that AO Smith is the 1.5hp model.
From that, the pump folks may be able to help more.

[bottomcat]

Yes, my mistake. It is an OptiFlo pump. The motor says "Century LASAR Pool / Spa Motor," but I determined that it is an AO Smith motor.

Another bit if information in case it helps: when the pump is running on low (pretty much 24/7), the pressure gauge barely registers (it looks to be about 4 psi).

[CarlD]

Mine runs about 5psi at low speed.

[bottomcat]

Okay, so maybe my pressure is not too low. Are you running 1.5" line or something larger?

[bottomcat]

Maybe I shouldn’t be trying to calculate flow rate but just finding the correct performance curve? Is there anyone out there who can provide the curves for the pump I describe in tis thread? The performance curves given here: http://www.pentairpool.com/pdfs/stariteoptifloDS.pdf only go down to ¾ HP but I need the data for ¼ HP. Even with the curve, what formulas should I use to compute the total head?

Appreciate any help I can get, as I won’t buy the heater until I know if it will work with my system.