Pump sizing query

I've been shopping for a two speed pump and have looked at some online pool schools to try to get the right size. For about 27000 gal and 8hr turnover, I figure I need about 56gpm minimum flow rate. My max plumbing flow is 73gpm and best flow rate for my filter would be about 72gpm. Thus the ideal flow rating should be 72/73gpm. Have I done that right? I've been looking at the Whisperflo 2hp uprated and 2hp full rated but haven't been able to find what their flow rates are. Can anyone help guide me here, or could I drop back to a 1.5hp? I don't think I can swing the extra dinero for Intelliflo. I'm assuming a TDH in the 60s. Thanks! Hal

Pump flow rates are determined by the amount of head loss in your plumbing. For a Whisperflow, a TDH of 60 will give you over 110 GPM for a 2 HP. If you only ned 56 GPM, then you only need a 1/2 HP or 3/4 HP. Here are the pump head curves:

http://www.poolplaza.com/WhisperFlo-techspecs.shtml

The lower the HP, the lower your TDH and GPM will be.

However, since you have a spa, you may need the higher HP for the jets, assuming the pump runs both. If so, then I would go with a 2 HP 2 speed pump to get the high HP for the jets and the low speed for general circulation.

Another option, if you have extra cash to burn, is the Intelliflow 4x160. This will give you 4 settable speeds to customize the exact flow rates you need.

I've found a really good price on a Hayward MaxFlo 1.5hp twospeed. Do you think there would be much difference between 2hp versus 1.5hp when running the spa? Hal

It is hard to say if there will be a significant difference. You really have to compare the braking HP of each pump which is the service factor times the label HP. If they are close, there probably will be minimal difference. Your GE pump should have a service factor listed on the label. Usually older pumps have lower SF than newer pumps.

The MaxFlo pump is speced as an uprated pump which means that a 1.5 HP uprated pump is really a 1.0 HP full rated pump. So it is likely to be much less powerful than your current pump. In addition, the MaxFlo pumps, despite it's name, tend to be low head and low flow pumps. At 50 feet of head, this pump will only output about 50 GPM which is much too low for a spa.

I would stick with one of the high efficiency pumps such as the Hayward Northstar, Pentair Whisperflow or Sta-Rite MaxEPro. Even though these cost more money, over the life of the pump you will more than pay for the cost difference in electricity savings.

Thanks, Mark. That's a great answer. In fact, I read some literature about the MaxFlo that showed me the low flow rate you mentioned. Now I'm back to leaning toward the Whisperflo 2hp 2sp (#011524). I called a Pentair dealer and he said that on low speed at 60 ft of head, it would flow between 50 and 60 gpm, and about 120gpm on high. However, isn't the high speed flow rate irrelevent since my 2inch pipe only flows 73gpm max?Thanks....Hal
PS: This pump is an uprated 2hp.

On low speed you won't have 60 ft of head. The dealer probably meant that with 60 feet of head at high speed at low speed it would deliver 50 GPM. Pump affinity laws tell us that:

The head on low speed will be 1/4 of the head at high speed

The GPM at low speed will be 1/2 of the GPM at high speed

So if you have 60 feet of head currently with a 2HP uprated pump, then according to the pump head curve shown here, the Whisperflow should perform as follows:

High speed head = 60 ft.
High speed flow = 100 GPM
Low speed head = 15 ft.
Low speed flow = 50 GPM

The flow rate of the pipes are recommendations not actual limits. Although very inefficient, you can go way above those values without too much problem. However, the pipes will wear out faster but it should happen over a fairly long period of time.