What Size Pool Pump Should I Get?
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If you want to know what size pool pump you should get the first information that you need to know is that most pool owners tend to oversize their filtration pump. This is not an advantage. It costs you more to buy, more to operate, and worst of all this can create a serious safety issue with your pool. Did I mention that it can also destroy your other pool equipment if you install a pump that is too powerful for your pool system? Yeah...oversizing your pump is definitely not what you want to do with your swimming pool and unfortunately that is exactly what the majority of pool owners shopping for their own pool pump are going to do. The information contained on this page will help you to make an informed decision about which pump (and which pump size) you buy for your pool.
There is no sense to sugar coat this. You need to be aware that buying pool pumps is confusing. Very confusing. It is extremely hard to make a fair comparison between different models of pool pumps, even ones which appear to be essentially the same size. In short what you need to know is that pump motor sizes and horsepower ratings are insufficient to determine the suitability of a pump. The hydraulic design and performance of one pump to the next is night and day different sometimes even when both pumps share the same horsepower rating. This is why pump motor size is not actually the right metric to compare and choose a pool filtration pump from. Instead it is better to look at flow rates, resistance to flow and actual power consumption.
Pump sizing and how to choose the right pump for your pool is something that is a deep technical subject that I could talk endlessly about. In order to provide you with the most information in the least amount of time you need to know that the rated horsepower for a pool pump is not suitable to compare pumps. Instead look at resistance to flow, pump power consumption and actual flow rates for that particular pump. This is actually quite a hard comparison to make, which is why you are here in the first place reading this, but we have already covered the most important part about how you should be more or less ignoring pump horsepower ratings. What you really want to be concerned with is how much water a given pump will move in your specific system. This is an unknowable number to you without taking a total dynamic head measurement for your pool, and even this involved step would only give you an estimate of a snapshot of the filtration system as it exists in that moment.
Resistance to flow - If you had the exact same size pump on a pool with low resistance to flow it would pump more water then when installed on a system with high resistance to flow. This is why pump motor sizing is not enough. You need to know the resistance to flow of your specific system (measured in feet of head) but you can also make a reasonable estimate. You only really need to know whether your pool has low, medium or high resistance to flow. Even this loose description is enough to help you narrow down on the right pool pump. The scale for resistance to flow is in feet of head measured from zero to about 100 feet of head. A small pool with minimal equipment which is located near the pool might have 15' to 25' of head resistance. A medium sized pool with equipment 20' to 30' away from the pool edge might have 30' to 50' of head resistance. Complicated pools with lots of pipes, features and peripheral devices installed might have a higher resistance to flow of 60' to 80' of head. Using this loose description you should be able to make an estimate as to whether your pool represents low, medium or high resistance to flow. Choose how many feet of head you estimate your pool to be. Another tip would be if you know the existing PSI that your pool filter runs at. Take this number and multiply it by three to give you an estimate of resistance to flow in your system. If you have 15 PSI on your filter normally, then a rough estimate to resistance to flow in your pool might be 45' of head.
You can measure TDH by measuring vacuum on the suction side and pressure on the pressure side of the pump winterization ports. Multiple the suction by 1.13 and pressure by 2.31. Add these together and this is the resistance to flow (in feet of head) for your swimming pool...at least a momentary picture of the system as it is currently running.
Actual flow rates - Using the number value you have assigned to your pool resistance to flow, we can now look up the exact flow rate for each and every pump you are considering to buy. Every pump manufacturer makes available a pump performance curve which shows you precisely how much flow you will get for a given amount of resistance to flow. You now have a GREAT point of comparison to make between pumps. I have made a table comparing the popular models of pool pump flow rates based on 30' and 60' resistance to flow. When you look at this table you can clearly see how two pumps with the same 1.5 horsepower rating perform night and day differently in terms of the actual flow they generate. This is the missing link that most pool owners do not have. To see the performance of multiple popular brands of pump you will find these at the bottom of this page: Variable Speed Pool Pump Reviews
Maximum safe flow - There is definitely more to choosing the right pool pump for your pool. So far we have only looked at determining flow rates and learning how to actually compare the performance of one pump to the next. You still need to know what the maximum flow rate your system can handle is. You do not want to exceed this for both safety as well as for protecting your pool equipment. Check your pool filter. It will have a maximum design flow rate which you need to know (and not exceed). If the pool pump you are looking at will generate flow which exceeds the maximum design flow rate for your filter, this means the pump is too powerful for your system.
A critical point of safety is that no suction line in a swimming pool should have more than six feet per second of suction line water velocity. In 1.5" PVC pipe this is about 38 GPM and in 2" PVC pipe this is about 65 GPM. 2.5" PVC is about 90 GPM. This is extremely important information. Looking at the suction lines of your pool you need to see how many you have, and which size they are. Look at your pump suction. How many pipes meet together just in front of the pump suction? If you have one pipe then you have one suction line. If you have two pipes then you have two suction lines and so on. If you have one suction line and it is 1.5" in size then you should choose a pump that will not exceed 38 GPM on your system. If you have one 2" pipe you want to choose a pump which will not exceed 65 GPM. The more suction lines you have and the larger they are in size, the more pump power (and flow) you can safely have. As you can see there are a lot of moving parts here when selecting a pool pump (the right pool pump) size for your pool. If you want help selecting the best pump for your pool you can always reach out and Ask Steve directly.
To learn more about pool pump sizing including WEF ratings and motor service factor you can read this additional article: Picking the right pump 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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