What is the best pump size for my pool

[Krickett]

Sorry to be such a pest but I want to do things right as I am replacing the filter and pump. I've posted a few questions concerning filter size but the response I got was based on the pump which I've decided to also replace so I want to make sure the pump is the right size. Originally, the pump was a 2hp. I replaced the motor 2 yrs ago with a 1.5 hp but I'm wondering if that is still too big. What is the best sized pump for these measurements?

Size: IG pool with spa (about 21000 gallons), size is kind of a figure 8 shape of a rough estimate guess of 20-25' wide x 35-40' long
Input: 1 skimmer, 1 spa drain and 1 main drain, 2" plumbing
Output: 3 pool returns, 4 spa returns, 1 pressure side cleaner outlet, 1.5" plumbing

 

[JohnT]

A 21,000 gallon pool would work well with a 1HP pump. But you have a spa and a cleaner port to consider. I'd look into a Whisperflo 2-speed pump. If your current pump delivers enough water to the spa to suit you, go with the 1.5HP 2-speed. If you'd prefer to have more water in the spa, then consider the 2HP 2-speed.

 

[Krickett]

What's the purpose/advantage of a 2-speed pump? Also, I've been researching into the performance charts of various pumps and they show feet of water vs GPM and depending on the pump for the same point on the graph it has shown any where from a 1/2 hp to a 1 1/2 hp. Do I need to consider any of these factors and if so how do I best determine those?

 

[JohnT]

What's the purpose/advantage of a 2-speed pump?

It would allow you to run on high for cleaning, spa use or just clearing up a mess, and then to switch it to low for normal circulation to save electricity. It's like having two pumps.

Also, I've been researching into the performance charts of various pumps and they show feet of water vs GPM and depending on the pump for the same point on the graph it has shown any where from a 1/2 hp to a 1 1/2 hp. Do I need to consider any of these factors and if so how do I best determine those?

Analyzing the head and flow curves is important when doing a new install or selecting new equipment, but since you've already got real-world operating experience with the pool, you can rely on that. The complicating factor in your setup is the spa and cleaner. You need a bigger pump for satisfactory spa operation. That means you either have to oversize your pump to manage the spa or go with a flexible pump to meet both requirements.

 

[Krickett]

Thanks for the info. Do you know why different pumps have different hp ratings for the same parameters? How would that choice affect the filter size I need to chose?

 

[duraleigh]

Look at pumps that produce enough GPM to turn over all the water in your pool 2-3 times in a 24 hour period.
The HP issue is quite confusing. What is important is that the pump GPM (that you can get from the manufacturer) should be smaller than the filter GPM (which you can also get from the manufacturer)

A filter larger than the capacity of your pump will result in a smoother overall system and save you time and money with less backwashes/cleanings. The filter cannot be too big (within reason) but it can certainly be too small.