Buying A New Pool Pump Checklist
Before you purchase your next pool pump it would benefit you greatly to consider the information contained in this article. There are more than a few ways in which you can end up making a poor choice for your new pump such that it will possibly damage other pool equipment that you have, and most certainly can end up costing you more money than a well sized pump would. Not just a few dollars either.
As part of my swimming pool consulting service people retain me to audit their energy consumption when looking at buying a new variable speed pool pump. This is probably the single most commonly requested consulting service that I receive. It is a rewarding process for me as the average savings between a large single speed pump and a new variable speed pump with a programming schedule created that matches the specific needs of the pool is somewhere between $5000 to $10,000 savings in electricity over an 84 month service cycle evaluation.
In my experience pool owners are usually pretty excited to learn about how much money can you save with a variable speed pool pump. However the majority of pool owners out there know only about the potential for savings with a variable speed pump, and know little about the technical considerations for choosing the right variable speed pump for the right pool. Variable speed pumps for the most part are massively powerful. If sized only for pool filtration applications you probably would never need more than 1/2 horsepower or 3/4 horsepower, and every residential pool could meet their filtration requirements with a 1 horsepower pump, assuming the plumbing system was efficiently designed, which most are not.
If you understand that 1/2 to 1 horsepower is more than enough for any residential pool filtration system you might then be able to appreciate that there are only one or two variable speed pump options that are 1 horsepower or less. There are at least 40 variable speed pumps which are between 1.5 to 3 horsepower. Let that sink in. 1/2 to 1 horsepower is big enough for any pool, but of the 42 pump options on the market only one or two of those are within that size. Insanity. Without properly considering the technical aspects of how to size and install a pool pump correctly, almost every pool owner shopping for a new pump on their own will end up with the wrong one for the job...or at least you can estimate that they will probably end up with one a lot bigger than the one they need.
This is the "Tim the Toolman" approach, as I like to call it, in that pool owners always want to buy a bigger pump with more horsepower because it is assumed that this is somehow better. In short the horsepower rating is just an indication of how much it costs to run your pump. A 3 horsepower pump costs three times as much to run as a 1 horsepower pump...kind of. Pump power sizing is actually specifically complicated and confusing. Motor manufacturers play with "service factor" ratings which allow them to call a pump a certain horsepower size when technically that is not entirely accurate. Up rated (max rated) and full rated motors are terminology that allow two different sized motors to carry the same horsepower rating on the name plate. A 1.5 horsepower max rated motor is actually a 1 horsepower full rated motor with a few extra copper windings, essentially making the motor operate in sprint mode. This is a very watered down technical description however this is a specifically deceptive power rating and naming process invented by motor manufacturers. Fortunately the swimming pool industry has just adopted a new form of measuring the potential of pool pumps. It used to be that you would take the nameplate horsepower rating and multiply that by the service factor rating to get the total horsepower rating for a pump. Single speed pool pumps are disastrously inefficient and now that variable speed pumps have been required by law (est. July 19, 2021) a new system of labeling pumps based on efficiency potential has come with it.
WEF rating for pool pumps - The WEF rating (weighted energy factor) for pool pumps has been implemented to allow for more easy comparison of energy efficiency with electric motors. Measured in kgal/kWh (thousands of gallons per kilowatt hour or electricity consumed) a higher number indicates a more efficient pool pump compared to a lower WEF rating which indicates a less efficient pool pump. Since power consumption for variable speed pumps varies greatly over the total RPM range of the motors, a single HP rating, or service factor rating like previously used simply lacked the depth to compare these variable speed pumps in any kind of meaningful way. If you want to see the complex math behind WEF ratings you can read this article about the uniform test method for the measurement of energy efficiency of dedicated-purpose pool pumps.
WEF pump rating
Example pump #1 = WEF rating 8.7
Example pump #2 = WEF rating 11.9
In this example pump #2 is more energy efficient overall. Under various loads and RPM settings which are then averaged out pump #1 was able to pump 8700 gallons per kWh of power consumption. One kilowatt hour is 1000 Watts consumed over the course of one hour of time, and this is the measurement you use to pay for your electricity bill. The nationwide average per kWh of electricity is just over $0.13 which is an average from about $0.10 to $0.30 per kWh depending on where you live and the time of day you are consuming this power. Most places have peak power rates for prime times of day, with lower costs per kWh during lower times of power use, like at night. Pump #2 in this example is more efficient when averaged over various tested speeds, achieving 11,900 gallons of water per kWh of power consumed. Understanding WEF ratings for pool pumps in going to be very important of you want to make an informed purchase decision for your new pool pump.
Pipe Sizes For New Pool Pumps
Now that you understand how many variable speed pumps are very large and very powerful, most of them actually, you might start to appreciate how careful you must be as a pool owner to avoid buying something that is too large. Fortunately for you this is information you can endeavor to find, but it is knowing in the first place that you should be evaluating your system before buying the new pump that is the problem. If you had a 2 HP motor before then logic would seem to dictate that a replacement 2 HP motor should be the right choice. It might be, or it might be that the pool system was never designed well to begin with and you have been using the wrong pump for your application this whole time. On a national scale, to give you a sense of how serious of a concern this is, I would estimate that most inground residential swimming pools have improperly sized, hydraulically poor (possibly dangerous) and specifically inefficient filtration systems. Most of them.
First, as illustrated in this article on pipe size for swimming pools, residential swimming pools almost all have very small plumbing systems. 1.25", 1.5" and 2" are the most common pipe sizes found in residential pools and even the largest of the common sizes is still extremely small and limiting to a pool pump because of how powerful they are. Following the established guidelines for safety of 6 feet per second of water velocity for suction lines in pools and 8 feet per second of water velocity for pressure side lines, which also correspond with the limits of hydraulic efficiency in PVC pipes, you can easily see how pool plumbing is chronically undersized.
1.5" pipes = 38 GPM (6 fps)
38 x 60 minutes = 2280 GPH
2280 x 24 hours = 54,720 Gallons per day
1.5" @3 turnovers max pool volume = 18,240 Gal.
2" pipes = 63 GPM (6 fps)
63 x 60 minutes = 3780 GPH
3780 x 24 hours = 90,720 Gallons per day
2" @3 turnovers max pool volume = 30,240 Gal.
So this information can conclude that pools with 1.5" plumbing systems should all be smaller than 18,000 gallons (they aren't) and all pools up to 30,000 gallons should have 2" pipes. Pools over 30,000 gallons (18x36' / 20x40' / 20x44' / 30x50') all require plumbing lines larger than 2" in size in order to operate within the limitations of efficient flow. Let me cut through the fog for you here and tell you that all of this boils down to pool plumbing being inherently inefficient. When you design for efficiency of a pool system the plumbing will be 2", 2.5", 3" and 4" for common "every day" residential pools, and larger if you want any fancy effects or water features.
How pipe size relates to buying a new pool pump - Pool pumps are huge, pool pipes are undersized chronically, and the end result is that pumps that are too large get installed on pipes that are too small every single day. In the above example we looked at a 1.5" pipe moving 38 gallons per minute as the top end of the range for efficient (laminar) flow of water. If you install a 3 horsepower variable speed pump on that system it will attempt to draw up to and over 100 gallons per minute through that pipe. While this might be technically possible, though tremendously inefficient, when the pump is so oversized for the pipes it will starve for water in a process called cavitation. This process damages the pump impeller and has a hallmark sound like there are rocks tumbling around inside of your pump.
Horsepower ratings are confusing, but working within the traditional rating system I tend to tell pool owners that 1.5 horsepower is about the maximum I would look at for any 1.5" plumbing systems. If you want a 2 HP, 2.5 HP, 2.7 HP or 3 HP pool pump then you must have 2" pipes as the minimum size. Having multiple pipes can potentially compensate for smaller pipes, usually at a ratio of 3:1 (three 1.5" pipes to replace one 2" pipe) however this would also include that the trunk lines, suction manifold and return manifolds are also efficiently designed with 2" plumbing before feeding to multiple 1.5" lines. In general avoid all larger pump models with 1.5" plumbing systems.
Maximum Design Filter Flow Rate For New Pool Pumps
In the example above we looked at a 1.5" pipe pool system that moves 38 GPM and a 2" pool system that moves 63 GPM efficiently. The only problem with this information is that up until very recently you had absolutely no way to control the rate of flow of water in your pool system. Also there was no way to measure how much water was moving in your pool system so it really didn't matter that we could not control the flow rates. With variable speed pumps and flow meters for pools we now have the ability to control the flow of water and to monitor the rate of water movement through your system.
When running at full speed you will be able to get up to and over 100 GPM with pool plumbing systems. 1.5" systems tend to top out around 90 GPM and 2" systems tend to top out around 120 GPM however these are only soft limits relative to pump power. If you hook up a jet engine to a 1.5" pipe you can bet you are going to get more than 90 GPM! It is very important to understand the efficient flow range that you want to operate your new pump at but you also need to be aware of how much flow these pumps might actually produce in your pool system. When you are buying a new pool pump you definitely want to evaluate your plumbing system to determine how much flow potential your system might have, but in most pool systems the largest limiting factor will be the filter.
Maximum flow rate for pool filters - All pool equipment will have a maximum flow rating that it can handle beyond which damage to the equipment is likely to occur. If you are going to have a problem like this with too much flow it is most likely going to be from your filter. Every pool filter will have the maximum designed flow rate printed right on the nameplate, as well as being available from the manufacturer (maximum flow rates for most pool filters) so you should not have any trouble finding this information for your filter. Sand filters with a multiport valve are the worst for maximum designed flow rate by quite a bit. Both DE and cartridge filters have higher maximum flow rates than similarly sized sand filters. If you have a sand filter you should be specifically concerned that you could be overdriving your filter.
Sand filters often have 40 to 75 GPM maximum flow rates and even a modest 1 horsepower pump on a single 1.5" pipe system might be able to exceed this value on a low-head system. If you have a pump larger than this you could double your maximum flow rates...but not for long as eventually the filter will start blowing sand back to the pool through a broken standpipe or lateral.
How to prevent from overdriving your filter - If you are worried your pump and flow rates are too much for your filter how are you supposed to deal with this? With a single speed pump there is no solution. You will replace your pump with a smaller one, or you will replace your filter sooner than you could have because it will be worn out internally. Now that pools use variable speed pumps you now, for the first time, have the ability to actually do something about overdriving your filter. You can learn how much flow your filter is able to handle, and then buy and install a flow meter for your pool. When setting up your variable speed pump schedule simply learn what your maximum motor RPM is to achieve but not exceed your filter maximum designed flow rate. Also be sure to change your priming cycle speed to be no higher than this value. Most variable speed pool pumps have the ability to change the RPM associated with the priming cycle and this would be important to ensure you never overdrive your filter.
How To Size A Pool Pump?
I am very sorry that this is such a long and drawn out process to choose a new pool pump for your pool, but there is a lot of education about plumbing systems that needs to happen before we can proceed. You needed to know that pool pumps are way oversized for what pool systems actually need, and you need to know that pipes are chronically undersized on almost all residential pools. It is also helpful to understand that three times your pool volume per day is the minimum filtration goal that you should be able to achieve with your pool. When you start to put all of this information together you are almost done correctly sizing your pump to your filter and also to your pool.
Most of the process of sizing your pump to your pool system correctly is just not buying the Tim the Toolman crazily-oversized pump. Next best is to realize how filters can be a major flow restriction, and upgrading to a cartridge filter from a sand filter is a great idea. You might have started looking for a new variable speed pool pump to buy, but now you might be considering a new variable speed pump as well as new cartridge filter. This is definitely something that I recommend to pool owners who have the budget to make the switch, especially if you are thinking that your current filter might be nearing the end of the service life you can expect to get from it. If you have a sand filter of any size other than gigantic (600 lbs. of sand or more) then you probably have to buy a very small variable speed pump, or upgrade your filter to something that handles more flow.
Pool pump flow curves - The traditional method of sizing pool pumps (before we could control and monitor flow rates) was to take a snapshot measurement of the total resistance to flow in an operating pool plumbing system. Even though the pump speed was linear, or dual at best before variable speed pumps, this was still a limited measurement as it does not account for changing system conditions, valve zones and filter states ranging from clean to dirty. For a snapshot of the total dynamic head of a pool system you would measure the pressure in PSI of the system and multiply this by 2.71, as well as the suction vacuum measured in inches of mercury and multiplied by 1.13, and the sum of these two totals is the total resistance to flow of the system measured in feet of head. Most residential pools from small and plain to large and fancy will probably fall within the range of 20 to 50 feet of head. Look at the published pump flow curve for the pump you are looking to buy and compare the flow rate at your specific feet of head resistance. This is how you would "know" how much a pump would move on any given pool plumbing system.
When you are buying a variable speed pump the process is the same that you can look up the amount of flow your system might expect to have, but it would probably be more useful to just use this as a tool to create a more apples to apples comparison between pumps. How much water does pump A move at 40 feet of head versus how much water pump B moves at the same head resistance. New variable speed pool pump flow curves include more information than older single speed flow curves did, and you can usually see flow curves for different RPM example speeds, or sometimes you can see efficiency ranges which look like a U or circle overtop of the flow curve. Designing hydraulic systems for EOC (end of curve) is where you extrapolate a situation where there is no resistance to flow in the plumbing system and the pump achieves the maximum possible flow rate, designated as the far right side of the descending flow curve. For a given system head resistance you should choose a pump that will meet this value on the far right side of the curve. For residential pools the takeaway information from this section is to pick a variable speed pump that is the right size within reason. Just because you can control pump RPM does not mean that you can put any size pump on any system without penalty.
The New Pool Pump Checklist:
- Check to see if you have 115 Volts or 230 Volts for your pump
- Check your pipe sizes (and number of pipes)
- Check your filter maximum design flow rate (especially sand filters)
- Multiply your pool volume by three and then divide by 24 to get GPM average
- Buy smaller horsepower as opposed to larger (1.5HP or smaller on 1.5" pipes)
- Consider upgrading your filter / improving your suction & return manifold design
- Install a flow meter to monitor flow rates and establish maximum flow RPM
- Program pump to achieve three turnovers of the pool volume daily
If you have considered this information then you are more ready than 99% of pool owners to make an informed decision about buying a new pump for your pool. Genuinely this is just the tip of the iceberg when it comes to properly sizing a pool pump for your pool. What is significant is that every single swimming pool system is different and what might work well for one pool would be a disaster of a choice for another one. It takes a fair bit of consideration to plan for all of the peripheral items that a pool system has and then size the pump appropriately. Fortunately there is a lot of overlap with pool pumps and so long as you do not oversize your pump to your pipes and filter, then you will probably be able to control maximum speed and flow rates with your flow meter and the difference in energy consumption will be negligible versus any other variable speed pump in a similar class.
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