What Size Pump Do I Need For My Pool?
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What size of pool pump do I need for my pool? You would think this question would be very straight forward and easy to answer but this would be very far from the truth of the matter. Not only is pump (and motor) sizing not intuitive, but I would go so far as to say it is made purposefully confusing which makes it extremely challenging to know what you are buying as a home owner. By the end of this page you will have a much better idea of what you should be looking at when shopping for a new pump.
At the very top of the list of things you need to know about pump shopping and sizing a pump motor for your system is that the horsepower rating is not useful. You would think that all pumps rated a 1.5 horsepower would be similar, but they are not similar at all. In fact they are not even 1.5hp motors at all. This is due to something called service factor. A 1.5hp pump with a service factor of 0.85 and a 1.5hp pump with a service factor of 1.3 is an example. While both show 1.5hp right on the label, in practice they operate differently. Multiply the rated HP by the service factor to reveal the thp (total horsepower).
1.5 rated horsepower x 0.85 service factor = 1.27 total horsepower
1.5 rated horsepower x 1.3 service factor = 1.95 total horsepower
So as you can see we already have a huge gap in the actual performance of both of these "1.5 horsepower motors" as the service factor for the electric motor means that the total horsepower is actually quite different. Unfortunately this is only one of the things that you need to know about in order to make an informed decision about what size of pump you need for your pool filtration system.
WEF Pump Ratings - Another consideration here is that just because a pump has a certain horsepower motor does not mean anything about how much water it will actually move under certain conditions. In fact, would not measuring the actual flow rates be more useful than horsepower ratings which are essentially useless? The answer is yes. The pump industry has recently moved to WEF ratings which is weighted energy factor. A WEF rating measures how much water a pump can move (in thousands of gallons) for 1000 Watts of power consumed. So a WEF rating of 9.3 would be superior to a WEF rating of 8.0. In this case pump #1 moved 9300 gallons of water for 1kW of power consumption, where pump #2 moved less, only 8000 gallons for the same amount of power consumption. This is not all you need to know in order to choose the right pump for your pool...but it sure helps being able to actually compare WEF ratings.
High head versus medium head pumps - Finally, if we compare HP, service factor, THP and the WEF ratings we still do not know a critically important piece of information. We do not know how much water the pump is actually going to move. Which is sort of the whole point of this exercise. In short, some pumps are made to work best pumping against low resistance and some pumps are made to perform well when pumping against higher resistance to flow. This is super important when choosing a pump. It will be difficult for you to know exactly what your specific (and unique) resistance to flow is for your pool, but if we make a reasonable guess it will drastically help you to find the right pump for your pool. The plumbing system resistance to flow is measured in feet of head. A very small and efficient pool system with the pump and filter located directly adjacent to the pool might have 20 feet of head resistance. An average inground pool with a pump room within 30' or so of the pool likely has a resistance to flow of 30 to 50' of head. A large pool, a pool with equipment 50' or more from the pool, or a pool with lots of features that require flow...something like this might be 60 or 80' of head resistance. Using these ballpark numbers you can now look at the most important numbers. Make a reasonable estimate as to what you think your resistance to flow might be, somewhere between 20' to 80' of head for the vast majority of pools. Now, look up the performance curve for the pump you want to buy. One axis is resistance to flow (in head) and the other axis is GPM. This is the critical piece of information that you need to find the right pump size for your pool.
Now you are much better armed with information to shop for a new pool pump. You know that the horsepower rating is misleading and you know to look at the service factor and multiply this number by the horsepower rating to learn the total horsepower potential. For example, the Hayward Super Pump and Pentair Superflo both advertise as 1.5hp pumps, but in actuality the Hayward is 1.65thp and the Pentair is 2.2thp and this makes a big difference to the system it is being installed on. In general people tend to oversize pool pumps thinking bigger is better, which pretty much the opposite is true. Most pools would be well served by a 3/4 horsepower high head, high efficiency pump. So by the time you are at 1.5ho or larger this is already huge. Most people do not need 2hp or 3hp pool pumps despite how common they are on the market. You should really have 2.5" pipe size or larger for pumps this large, and most people still have 1.5" or 2" pipe size on their system. As a final note on pump sizing, and probably the single thing you need to look at most closely is the maximum design flow rate for your filter. Make an estimate to your resistance to flow and then look up the performance curve for your pump to see how many GPM it will generate. You then compare this to the maximum flow rate your filter can handle. You do not want your pump to exceed the maximum flow rate that your filter can safely handle.
Using this new information you can now read these variable speed pool pump reviews with more confidence. In particular be sure to review the chart I made at the bottom of the page which compares the actual flow rates for both 30' and 60' of head resistance across all of the most popular brands and models of pool pumps. From this comparison list you will see the night and day performance difference for flow rates between these different pumps. There is a right pump for each application and you need to find the one that is a best fit for your pool. For more information about pump sizing and how to pick the right pump for your pool you can continue with this article: What size pool pump should I get? If you are still overwhelmed and need help you can also Ask Steve for help picking the right equipment for your pool.
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Swimming Pool Steve is an award winning, second generation swimming pool specialist from Ontario Canada and one of the most trusted voices in the swimming pool industry. With over 20,000,000 views on the Swimming Pool Steve YouTube Channel, winner of the Pleatco Pool & Spa Industry Leadership award and author of hundreds of pool and spa articles both online and in print. Steve is committed to helping pool and spa owners as well as pool and spa industry workers learn more about the technical side of building, renovating, repairing and maintaining all types of swimming pools and spas. Follow Swimming Pool Steve on Facebook, Twitter and YouTube.
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