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Pool Filter Sizing

Pool filter sizing
Do you need a new pool filter? This article will help you to choose the correct size of filter for your pool. The first thing to know when it comes to pool filter sizing is that there are three different kinds of filters that you can use on an inground pool. Above ground pools can also use any of these three kinds of pool filters, plus some very small above ground pools also have the option of using a very small paper style filter like you might find in a hot tub. The three types of inground pool filter are:


Cartridge filters

Sand filters

DE filters (diatomaceous earth)



There is no "best pool filter" for every swimming pool. Each filter has their advantages and disadvantages and you need to determine which is the best fit for your needs. Once you have determined which filter style you want then you can accurately determine what size (and make / model) is going to match up well to the size and demands of your pool. Before starting with filter sizing calculations you first need to learn about the potential filtering abilities of each style of filter. The particulate size that each filter can remove from the water is the first consideration.


DE Filters 1 - 3 Microns - Diatomaceous earth filters are used heavily in some areas of North America while other areas have almost no DE filters at all. DE filters have the ability to remove the finest particulate from the water. DE filters can filter debris between one to three microns in size.


Cartridge Filters 5 - 15 Microns - Cartridge filters are the next best pool filter style for removing fine particulate however even under ideal circumstances they are not as effective at removing fine material from the water as well as a DE filter.


Sand Filters - 15 - 30 Microns - Sand filters are the oldest type of pool filter and the most commonly used on residential pools. Sand filters are able to remove debris down to 15 to 30 microns in size which makes them much less effective than DE or cartridge filters.


So how important is filter particulate size for your pool filter? Well, considering that sand filters are the least effective at removing small debris and yet are still the most popular pool filters this should tell you a lot about what "the best pool filter" is for your pool. The truth is that any of these pool filters easily have the ability to keep your water clean and clear. A sand filter and a DE filter will both be able to keep water clear if they are sized properly for your pool, and during the day you would not be able to notice much of a difference with the water...if at all. At night however, with the lights on in the pool, a DE filter would be more effective at removing fine particulate that you can see floating around in the illuminated water at night. So is it important to have the cleanest, clearest water in your pool that is possible, or would you prefer the most cost effective option that will keep your water clear and swim-able? The truth is that the best pool filter is not the one that removes the most debris. Sure this is important, but the maintenance schedule for each filter type is likely even more important than filtering particulate size.





Pool Filter Maintenance

pool filter maintenance A very important part of the process of choosing the best filter for your pool is to understand the differences between each kind of filter so that you can pick the one that suits your needs the best. Once you have considered the pros and cons to the three types of filters you will be able to make an informed decision as to which you want.


Sand Filter Maintenance - Sand filters require "back washing" as the only real maintenance. This is a five minute long procedure that must be completed every week to two weeks depending on your pool system. This process simply reverses the direction that the water is going through your filter and sends the resulting water away through a "waste line" attached to your filter. This eliminates the accumulation of debris from where water normally first enters the filter tank. Without this process your pool pressure would climb steadily and eventually water would not be able to pass through the filter. While this process does not take very long, it does require some effort and it does require to be done on a regular basis. It also requires that you regularly pump off some water in your pool and then you would need to replace it with fresh fill water. If you live in an area with drought or water shortage conditions, then this alone would be a good reason to avoid this filter type. Also, if you have a salt water pool and live in an area which does not allow for draining salt pools to the street, then back washing might be a problem for you. If you want the cheapest pool filter possible that will last a very long time as well as keep the water clean and clear in your pool, plus you don't mind a five minute procedure every few weeks, then a sand filter might be just what you are looking for. Other than changing the sand every seven to ten years there is not much else to caring for sand filters.


Cartridge Filter Maintenance - Cartridge filters require less regular interaction from you, the pool owner, than any other type of filter. While it does not have the small particulate power of a DE filter, a cartridge filter is still around three times better than a sand filter...but around three times as much money as one also. If you can afford a cartridge filter, and you can also afford to replace the rather expensive cartridges every three to five years or so (at best), then a cartridge filter is considered by many world class pool builders to be the best pool filter overall. A DE filter can clean finer debris from the water, marginally, but a cartridge filter can often go an entire swimming season (six months) without needing to have the cartridges cleaned. Most cartridge filters only need to have the filters cleaned once every six months of use or so (regionally, some areas in the USA use small filter cartridges and clean them much more frequently). Pools with a lot of debris and leaves in them might require more filter cleanings but this will still be less overall work than back washing a sand filter or periodically charging a DE filter. Cartridge filters also have a vastly superior flow efficiency than sand filters which allows them to be paired with larger pumps and much higher flow volumes than what a sand filter can accommodate. This makes cartridge filters a much more efficient option than sand filters and more suitable for the powerful variable speed pumps that have recently started to be available on the residential pool market.


DE Filter Maintenance - Diatomaceous earth pool filters are able to filter the finest particulate size from the water of any of the three types of pool filters. For many people this factor alone then answers the question of which is the best pool filter. It is however also worth considering that DE filters require the most time spent on maintenance in order to continually monitor their pressure, charge them with DE, as well as periodically strip them apart to clean the DE grids. DE filters require regular back washing like a sand filter, but they also require you to periodically open the filter and clean the internals just like a cartridge filter. Experienced pool technicians will tell you that stripping a DE filter does not take long, and it does not need to, but the average pool owner will take much longer to perform the same task than someone who does it for a living. You may have heard about some people who eat DE for health benefits. It is important to differentiate between food grade amorphous DE, and pool filtering grade crystalline DE. Crystalline DE as used in pool filters contains up to 70% crystalline silica and can be very dangerous to work with. DE filters require regular charging with DE to keep them operating properly so if you would prefer to avoid storing and using crystalline DE then that is a good reason to choose a cartridge or a sand filter. These reasons are why DE filters are less popular than sand or cartridge filters despite being able to filter finer material from the water.


The amount of filter maintenance that you need to do will depend on the unique conditions that your pool is exposed to. If your pool gets a lot of use, and the people who use it wear lots of sunscreen and makeup and hair products, plus you have a lot of trees (and leaves) on your property near the pool, then you will have a more demanding filtering requirement than someone with no trees, whose pool does not get used very much. In the case for all filters, you know it is time to clean the filter when the pressure of the system goes up seven PSI above the clean operational pressure of the system. You can read more about why keeping your pool pressure down actually saves you a substantial amount of money in this article about how to make your pool more energy efficient.


How To Choose The Right Size Filter

If you want to know how to choose the right size filter for your pool you need to know which of the three filter types you want. The next thing you need to know is the volume of your pool. This information will help you to determine how much water your filter needs to be able to handle each day. This is where the subject of "water turnover" comes into play. A turnover of water in your pool refers to the entire volume of your pool passing through the filtration system. For this example let's assume a pool is 10,000 gallons. One turnover of this pool would be 10,000 gallons, two turnovers would be 20,000 gallons and three turnovers would be 30,000 gallons and so on.


The reason why turnovers are important is because your pool water needs to be filtered every day to keep the water clear and help the chemicals to keep the water safe. The problem is that you can not filter all of the pool water in order...some of the water in the pool will get filtered twice while some will not get filtered at all. There is a mathematical calculation behind this which reveals how much you need to filter the water in your pool in order to effectively get all of it through the filter at least once:


One Turnover = 63% of water volume

Second Turnover = 86% of water volume

Third Turnover = 95% of water volume

Fourth Turnover = 98% of water volume


It is generally accepted that three turnovers per day effectively "filters all" of your pool water. This means that in order to make sure you filter all of your water every day you should be sending three times the volume of your pool through the filter every 24 hours. More filtering would be better, but only marginally so, where as less filtering will leave a relatively large amount of your water each day unfiltered. So what does this mean? Will your water turn green if you only filter your pool volume twice every day? No, probably not. Pool owners tend to run their system much less than three turnovers per day, and largely these pools are "fine". The less you filter the pool water the more you are relying on chemicals to clean the pool. You can try to get away with less filtering but this will have a net effect of increasing the chlorine that you need to use, which in turn will also increase the amount of reactionary chemical adjustments that your pool needs in general. So you can filter your water less than three times every day, however this is less than recommended by industry health and safety standards.


example pool - If a pool has a volume of 10,000 gallons then you should buy a filter that can handle up to 30,000 gallons every 24 hours. 30,000 divided by 24 gives you 1250 gallons required per hour. 1250 divided by 60 gives you 20.83 gallons per minute of flow. In this example a 10,000 gallon pool would need a filter capable of filtering up to 21 gallons per hour. This assumes the pump is running 24 hours per day. If you want to run your pump less than this simply triple the volume of your pool, and divide by the number of hours that you want to run your pump for.


Pool filters all have a maximum designed flow rate. This is the maximum water flow they can handle under normal conditions. Once you know how many gallons per hour you need your filter to handle you can then compare it to the maximum designed flow rate. You can have less flow in a filter and that is not a problem. In fact slower water filters better and more efficiently in all three of the filter styles. If you need 21 gallons per minute capability then just choose any filter that has a higher maximum designed flow rate than 21 GPM. There is nothing wrong with a filter that has a maximum designed flow rate of 80 GPM being used for a pool that needs only 21 GPM, but you never want to exceed the maximum designed flow rate for your filter.


Matching The Pump & Filter Together

You now know that you need to filter three times the volume of your pool every 24 hours, as well as how to convert this number to a gallons per minute number. You also know that you need to check the maximum designed flow rate for your filter to make sure that your filter can handle the GPM that you need. There are two more considerations that you need to factor in before you can decide on a filter model. You need to know how much water your pump can move (in GPM) as well as the size of pipe that your pool is made out of. In total all of this information will get you close to finding the best filter for your pool.


Pump Flow - There are a few different reasons why you might need to know how much water your pump moves through the system. This will completely depend on your unique situation. A single speed pump for example, which most pools still use for the primary filter pump, only has one speed and this speed tends to be a lot of water. If you have a single speed pump you need to know how much water the pump will move to make sure this is not more than the maximum designed flow rate for the filter you are considering. If you have a variable speed pump, like you should, then exceeding the maximum designed flow rate is bad, but you also have the ability to run your pump at lower and safer speeds. A larger concern with a variable speed pump is the minimum flow rate for the filter. Not all filters have a minimum flow rate requirement...but some do. Make sure to double check this specification with the filter you are looking at.


Pipe Size - The other consideration is the pipe size that your plumbing system is made from. Many pools have 1.5" pipe which is barely adequate for modern swimming pools. In reality the next best 2" size is still leaving a lot of room on the table for increased efficiency. If you have 1.5" pipe this means water needs to travel faster through the pipe which means greater turbulence in the line and resistance to flow. 1.5" pipe has a soft limit of 44 GPM beyond which there is a logarithmic efficiency loss that prevents you from moving more water even with a larger pump. If you need more than 44 GPM and you have 1.5" plumbing lines then your system was poorly designed and you should look at upgrading pipe size to 2" or larger on the entire pool if possible. Unless you upgrade the entire plumbing system size it will always be bottle necked by the small diameter pipe you do not upgrade. It is worth noting that the greatest concentration of fittings and flow restrictions in a pool are located on the equipment pad. If you can upgrade the equipment pad to 2" pipe this will improve the flow efficiency of the pool system as a whole even if you still have smaller pipe restrictions underground.


All of this information allows you to understand what your pool filtration schedule should be, and how close you can get your pool to this schedule. If you choose a filter that can handle three times the pool volume daily, as well as have a maximum designed flow rate that is greater than your expected system flow rate, you can choose a good filter. The pipe size and flow rate calculations helps you to make sure that your pool system is designed correctly (many are not, especially older pools) and that your filter can handle the actual flow that your pump creates. If you follow these steps you will get a suitable filter for your pool and it should last for decades before you need to think about getting another one.


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