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.

[mas985]

Assuming you have the 1 1/2 HP Optiflo pump, flow rate on typical AG plumbing is 50 GPM on high which means it would be about 25 GPM on low speed. So marginal. But a heater's operation is dictated not on flow rate but on pressure. If the pressure is too low, the heater will not kick on but because you have an AG pool, the pressure should be fine on low speed.

If you want more than guess, I would need a picture of your equipment setup and also the filter pressure on high speed.

[GaryT]

Get a 40 - 60gal barrel and backwash into it for 1 minute.
Volume will give you your flow rate

[mas985]

Backwash flow rates can be significantly higher than normal flow rates because backwash mode bypasses most of the return side plumbing so that method will not be very accurate.

[bottomcat]

But a heater's operation is dictated not on flow rate but on pressure.

@mas985: Thanks for replying. The reason I said the heater required a minimum flow is because the Service and Installation Manual specifies minimum flow on page 26 here: https://www.hayward-pool.com/pdf/man...stallation.pdf

If you want more than guess, I would need a picture of your equipment setup and also the filter pressure on high speed.

The pressure on high is 15 psi. I'll send pictures tomorrow.

Thanks!