Figuring Flow Rate

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.

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.

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).

Mine runs about 5psi at low speed.

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

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.

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.

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

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.

Quote:

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Originally Posted by mas985

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

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@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

Quote:

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Originally Posted by mas985

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

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

Thanks!

You can put a flow meter in your system. Try www.bestbuypoolsupply.com. Look under "in ground solar parts" and scroll down to the bottom of the page. About $60.00. Mark may want to comment about their accuracy.

'rameses, I considered getting one. I might use Gary's trick, but just disconnect the return line to fill the bucket.

Any modification to the return side plumbing will have a dramatic affect on the flow rate so again, would not be very accurate.

Okay, I'll drop that idea. I suppose the resistance of the return fitting would change the flow rate.

Quote:

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Originally Posted by mas985

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

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Mark, here are the pictures (I think… never done this before):
http://s23.postimg.org/s99ojope3/pump_end.jpg
http://s7.postimg.org/771ppvzaj/pool_end.jpg
http://s14.postimg.org/5fuvu6w01/pump_to_pool.jpg
http://s9.postimg.org/attodep5b/motor_label.jpg
http://s4.postimg.org/6fmrrta31/pressure_on_high.jpg
http://s10.postimg.org/6722s2smx/pressure_on_low.jpg

I figure somewhere around 60 GPM on high speed and 30 GPM on low speed. But, adding a heater will add head loss and will reduce flow rate. If I add the head loss of a typical heater, it would drop the flow rate down to about 56 GPM on high and 28 GPM on low. So it should still be ok but it will depend on the heater head loss which isn't always published.

Thanks Mark. I just read a little more of the installation manual. Nothing there about head loss, but the heater is adjustable for pressure, so I'm hoping I'll be OK. Also I noticed there is a paragraph about two-speed pumps:

"TWO-SPEED PUMP:
In a few cases the pressure from a two-speed pump is below the 1- pound minimum required to operate the water pressure switch on the heater. This is apparent when the pressure switch cannot be further adjusted. In these cases the pump must be run at high speed to operate the heater. If the pump and piping arrangement are such that the required 1-pound minimum pressure cannot be obtained, do not attempt to operate the heater. Correct the installation."

I assume by "pound" they mean "PSI" so given that I am at 4 psi on low, I should be OK I guess.

Thank you again to all for your help! Pool Forum is a great resource!