Pumps are the heart of your pool’s filtration system, but sometimes their cycle isn’t just water and debris—they also, for better or worse, engage in their own life cycle. If yours has gone kaput (or you’re just ready for a fresh unit), no need for a tearful eulogy. This is a great opportunity to find a pump specifically tailored for your pool that will offer maximum efficiency—and save in ongoing costs.
By determining your pool’s maximum and minimum flow rate, you’ll be able to choose the pump with the ideal flow rate for you—meaning you’ll set yourself up to never unnecessarily waste time, energy, and money. Sounds good, doesn’t it?
So You Want to Check the Horsepower and Be Done, Huh?
It might be tempting to cut to a quick answer, but it’s just that—a detraction from your pool’s potential. No, horsepower alone is not enough—at least not anymore. Pumps are a lot more efficient than they used to be, which is a good thing for many reasons (especially your wallet). However, if you simply go by horsepower ratings, you run the risk of oversizing your pump, and thus adding unnecessary strain to your overall system.
Already have a pump, and wondering if it’s right? Again, horsepower alone won’t do it—especially if your pump was already in place when you bought your house. Builders are known to embrace the bigger-is-better mentality to land a sale, which often results in a larger horsepower than the pool needs to run efficiently.
So while horsepower is an essential consideration when looking for the right pump, let’s table it for a second. It’s much more accurate to check the maximum and minimum flow rate of your pool—and simple, too. Hang in there and you’ll see.
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How to Measure the Minimum Flow Rate
You might already know a thing or two about turnover time, which is essential information to determine how long to run your pool pump every day.
In case you’re new here, just know that the sweet spot should typically be around eight to ten hours. If you have a heavily-used pool, you’ll want to reduce that turnover rate—but that’s a rare (and less efficient) exception.
To determine your minimum flow rate, all you need to understand is your pool size—that is, the number of gallons in your pool—and your desired turnover rate in hours.
Not sure how many gallons of water your pool contains? It’s simple geometry anyone can do. You just have to multiply the length, width, and depth of your pool in feet, and then multiply that figure by 7.5 to convert the number to gallons. Prefer formulas? Follow the one below:
[Pool Length in Feet] x [Pool Width in Feet] x [Pool Depth in Feet] x 7.5 = [Volume of Your Pool in Gallons]
Now that you have your pool size and the turnover rate, find your values in the chart below to determine your minimum flow rate. Relax—no algebra needed for this step.
If you think better in graphs, you can also follow the one below.
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So You Got the Minimum Flow Rate. Now What’s Your Maximum?
We’re all working within our physical limitations. Pools are no different. The size of both your pool’s plumbing and equipment will determine your maximum flow rate.
Calculating the Maximum Flow Rate of Your Pipes
First thing’s first: check your pipes. The width of your plumbing will determine how many gallons can actually pass through per minute. Two-inch pipes, for example, will only allow about 73 gallons per minute. Install a powerful pump that’s designed to push through 120 gallons per minute and you have excessive strain on your system—which will lead to expensive wear and tear.
Once you measure the width of your pipes, check the chart below to determine your maximum flow. For example, 3.0 inch pipes will have a maximum flow of 160 GPM (gallons per minute) and 2.5 inch pipes will have a maximum flow of 120 GPM, while 2.0 inch pipes will have a max flow of 73 GPM, and 1.5 inch pipes will have a max of just 42 GPM.
Are your intake lines a different width than your pipes leading back to your pool? Measure the maximum flow of the smallest pipes. This number will apply to your overall system.
Have other exciting add ons, like a spa or a waterfall? Check the diagram below in which all pipes are consistently two inches wide. This system can run in either spa or pool mode, so you’ll want to calculate each of these flow rates separately.
In this case, there is only one pipe connecting to the spa, so you can expect a maximum flow rate of 73 GPM. Since there are two pipes connected to the pool, you want to double this number to 146 GPM. However, you want to choose the lowest maximum flow rate of the two as your overall maximum flow rate, so your final number would be 73 GPM.
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Calculating the Maximum Flow Rate of Your Pool Filter
Not quite done yet. Your pool filter also has the potential to slow things down—and each has its own maximum flow rate. Exceed that number, and you’ll have a damaged filter and a strained system. That’s best avoided. Check your filter’s max below.
Say you have a sand filter with a tank diameter of 30 inches. Your maximum flow rate is 100 GPM. However, let’s pretend that the maximum flow rate of your pipes is just 73 GPM. When you have two different maximum flow rates for your pool plumbing and your pool filter, the lowest value of the two will be your final number. In this case, your maximum flow rate would be 73 GPM.
Now Use Your Minimum Flow Rate and Maximum Flow Rate to Calculate Your Ideal Flow Rate
None of this is rocket science, but this part is especially straightforward. Your ideal flow rate lies somewhere between your minimum flow rate and your maximum flow rate. If you’re a visual learner, draw a chart like the one below.
Within that sweet spot of your ideal flow rate, you have options based on the requirements of your pool’s filtration system. If you just have a pool, it’s much more efficient to choose a pump that operates on the lower end of your ideal flow rate—and save money in energy costs in the long run. If you have a pool and a spa, you’ll want enough power to push water through the spa jets, so a pool pump that operates on the higher end of your ideal flow rate is ideal. This will cost you more in energy bills, but that could be solved if you get a variable speed pump, meaning you can run it at a lower speed when water is running to the pool and a higher speed when water is running to the spa.
What is Feet of Head, and Why Should You Care?
Feet of head is the total resistance to flow in a pool system, and ultimately what your pump must overcome. Imagine you’re on a mile-long run against the wind—you have to work a lot harder than on a still day. For pools, the greater the resistance to flow, the lower the flow rate—and the more powerful your pump needs to be in order to operate efficiently.
Generally, it’s fairly safe to assume that an in-ground pool has about 50-60 feet of head, and an above-ground pool has about 30 feet of head.
However, if you want to double-check your pool’s specific needs, you can measure your own precise feet of head. In order to calculate the total number, you’ll need to add the vacuum pressure feet of head and the clean filter pressure feet of head, or follow the equation below:
[Vacuum Pressure FOH] + [Clean Filter Pressure FOH] = [Total Feet of Head]
First, use a vacuum gauge to measure the vacuum pressure in inches of mercury (Hg) on the suction side of the system as marked in the diagram below. Multiply that number by 1.13 for your vacuum pressure feet of head.
Then, use your filter’s pressure gauge to measure the clean filter pressure in pounds per square inch (PSI) and multiply that by 2.31.
Add the two together, and you have your total feet of head—and the toughest part is over.
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How to Consider Feet of Head When Choosing Horsepower
Because your feet of head is the degree to which your pool system will resist the flow of water, you’ll need to choose a horsepower that can overcome that resistance and land at your ideal flow rate. Remember, this determines how much power is going through your pump—and sorry builders, but bigger is not always better. Who wants a bigger energy bill than they need?
Find your ideal horsepower through the chart below. Check where your total feet of head (located on the y-axis) and your ideal flow rate (located on the x-axis) intersect. The colorful curved lines traveling across these axes denote horsepower.
For example, if you have 60 feet of head and your ideal flow rate is around 50 GPM, you’re best off with a pump with one horsepower, as it’ll give you a flow of about 52 GPM.
There, You’ve Done the Work. Now You Can Reap the Reward.
Congratulations. You’ve approached this in a thorough and pragmatic manner, and now you’ll have a super-efficient pool system—plus a lower operating cost in energy saved, replacement needs avoided, and maintenance visits prevented. Now your future looks especially bright—just don’t forget to reapply sunscreen every few hours.
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