Upgrading From Sand To A Cartridge Filter
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Let's talk about swimming pool filters and specifically what you need to know in order to upgrade your old, existing sand filter to a (fancy) brand new cartridge filter. The first thing you need to know is that it is absolutely, undoubtedly an upgrade to switch from sand filters to cartridge filters for your swimming pool filtration system. You can find advice from some people saying that sand filters are better, but I think it should be pretty easy to dispel this with some basic facts about filtration and flow rate which I will cover in more detail below. In short, cartridge filters have better flow ratings, less restriction to flow, remove finer debris from the water and when sized correctly will take even less maintenance than a sand filter does. Sound good to you?
So what, if anything is bad about cartridge filters for pool filtration? In a word; cost. Cartridge pool filters are expensive and quite a lot more expensive than the more entry level sand filter options. Further, you must replace the cartridge elements every three to five years, also at significant cost. To me there is value in this equation, but if you manage to ruin the filter elements in your cartridge filter early from improper care, cleaning or maintenance then you certainly tip the scales to being quite expensive. I definitely endorse cartridge filters but I also endorse caring for them properly to avoid extra (early) replacement filters. Clean a cartridge filter with a degreaser product and rinse clean with a garden hose or filter comb. Never use a pressure washer to clean cartridge filters. This drives the dirt and debris deep into the paper material where is becomes lodged and can not be removed.
Are cartridge filters better than sand filters? - You bet Bub. In the shortest form I like to describe cartridge filters as being three times as good as a sand filter...but also three times as much money. If you are willing to pay more to have cleaner water, less floating visible debris (especially at night) and you want to avoid backwashing and wasting water in the pool then a cartridge filter is for you. They filter three times finer debris than sand filters, and when sized well to the pool volume you can go three to six months between needing to open and clean the filter elements. One of the most important features of cartridge filters is the flow rate. An average sand filter might have a maximum designed flow rate of only 50 or 65 GPM. Where even medium sized cartridge filters are capable of handling 120 GPM or more. Without a multiport valve, and without a tank of sand to push water through to filter, the cartridge filter has substantially better hydraulic design with less resistance to flow.
Cartridge filter sizing - In some areas it is common to see 100, 150 and 200 square foot cartridge filters used on pools that are 10k, 15k or 25k gallons. For me this is far too small. This would require regular cleaning of the filters far more than I would prefer to do. Yes, even small cartridge filters like these can be used on larger pools because even a small cartridge filter like this can handle 75, 100 or 120 GPM or more. What they can not do is go very long before they start to fill with debris and you note the system pressure on the filter gauge is rising. Ideally I like to use a larger is better approach to cartridge filters. A 16x32' pool with a modest 15k gallon volume could be filtered adequately using a 300sf cartridge filter for sure. However if this were my pool I certainly would upgrade to a 400sf model, maybe even a 500sf (about as large as they come) could be used if there was enough budget to allow for the upgrade. The larger the filter the longer you can go in between filter cleaning procedures.
Additional plumbing considerations - One of the most important things you need to do when upgrading from a sand filter to a cartridge filter is to add a provision in the plumbing system to allow you to vacuum water to waste. With a sand filter you have this feature built into the multiport valve. Since cartridge filters have no multiport valve they lack the ability to direct the water flow anywhere except through the filter. For light debris and vacuuming this is fine but for heavy debris loads and spring cleanings you would be way better off to pump that debris straight to waste instead of loading up your cartridge filter with a years worth of debris all at once. A 3 way diverter valve installed in between the pump and the new cartridge filter is the ideal way to pump to waste with a cartridge filter. Just connect a lay flat how to the new diverter valve and unroll when you want to vacuum with the filter bypassed.
Use a flow meter to monitor flow rates - When you install a cartridge filter you are likely reducing the resistance to flow of your system. This means you will pump more water than you used to even with the same pump as before. This can be a good thing, but it can also be a safety concern. Swimming pools require a maximum suction line water velocity of 6 feet per second and if you increase your flow too much you could now have too much suction line water velocity. Using a flow meter will allow you to ensure that you stay within safety limits but also it will allow you to optimize your flow and filtration schedule and maximize the savings you experience with your variable speed pump. For more information you can read this article about pool filter sizing and also this one about How pipe size affects flow rates.
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