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How Do You Backwash A Pool Filter?

how do you backwash a pool filter
Sand filters are the most common pool filter for residential swimming pools. They are cost effective to buy and economical to operate over a long period of time. They do a good enough job of removing contaminants and physical debris from your pool water, however they do require periodic maintenance that other pool filter types (cartridge, DE) do not. Since a sand filter is exactly what it sounds like, a giant pressurized tank of sand use to filter debris from the water, it stands to reason that sometimes you need to clean that debris out of there. Unlike other filter types you can not just open your filter to clean the media with a sand filter. Instead you must perform a backwash cycle.


A backwash also sounds just like what it is. Your filter is filled with sand and pressurized water is pushed through it. On one side of the sand (relatively) clean water emerges, but on the other side organic and physical debris develops. Most of the larger debris will be caught within a few inches of the surface of the sand in your filter. As this layer accumulates it gets harder and harder for new water to pass through and be filtered also. When you backwash you are reversing the direction of travel for the water going through the filter and then rerouting this water out to waste. To waste simply means a pipe intended for draining, or wasting, the water. Some people will have a permanent rigid pipe backwash connection where others, most pools probably, have the traditional blue lay-flat hose that you roll out any time that you need to backwash your pool. This leads into the next logical question which is "how do you know when you need to backwash?"





When to backwash your sand filter - In order to know exactly when you should backwash your sand filter you need to know an important piece of information that is unique to only your pool. You need to know your clean filter operating pressure. You probably are aware that there is a pressure gauge on the filter. The function of this gauge in a primary sense is so you will know when to backwash. When your filter is just cleaned, which is to say it has just had an extended backwash cycle and been returned to filter mode. This is when you consider your filter "clean" and so the clean operational pressure is simply the unique system pressure that your pool runs at when your filtration pump is running. This assumes a single speed pump and valve orientations which do not change. If you have a variable speed pump you would note your clean operational pressure at whichever speed is the maximum normal speed that you run your pump at.



According to filter manufacturers you should backwash your sand filter when the pressure if your system rises between 7 to 10 PSI higher than your normal clean operational system pressure


Almost all swimming pool equipment is designed for operation between 0 to 30 PSI maximum. A pressure higher than 30 PSI means that you should immediately turn off the pump and investigate the cause of the high system pressure further. In the guidelines for a 7 to 10 PSI increase on your filter, this presumes that you are at a lower value than 30 PSI. If your filter pressure starts at 25 when clean then you must backwash when your pressure rises only 5 PSI because you are never to exceed 30 PSI for safety reasons.


1) Turn off pump - Any time that you turn the filter dial you must turn off your pump first. Many people mistakenly think that it is okay to change between filter settings while the pump is running but this is definitely not something you want to do. This puts undue pressure on a weak-link component, the spoke gasket, located within the multiport valve. Changing filter settings with the pump running can cause the gasket to pinch and jam, and more commonly can cause water to escape the spoke gasket and begin leaking out through the backwash line when the filter is running normally in filter mode.


2) Turn multiport valve dial in one direction only - While it might seem intuitive to rotate the dial in the opposite direction sometimes as this is closer to your target setting, however this also can cause binding and failure of the spoke gasket and should be avoided. Always rotate the filter valve in a clockwise direction.


3) Backwash Setting - Turn the multiport valve dial to backwash and then turn the pump back on to start pumping the water backwards through the filter and out of the waste port. Be sure that you are prepared for this as some pools require that you roll out a backwash hose before doing this step. The amount of time that you backwash your filter will depend on how dirty the filter is. The sight glass on the waste port of the filter serves this function, to be able to see when the water coming from the waste line looks clear. This can be anywhere from one to five minutes as a general range. If you can see dirty water in the sight glass still then try backwashing longer. Some sight glasses are permanently stained with algae and scum so be sure to note if yours was clear before starting your backwash.


4) Rinse Setting - Once the sight glass shows clear water, shut off the pump again and turn the dial to the rinse setting. Not all sand filters have a rinse setting however most do and if yours does you should definitely use it. During the backwash there is a lot of physical debris, slime and gunk that will come out of the filter. Some of this stuff is still inside the multiport valve and if you go directly from backwash to filter again then a good amount of the debris will end up back in the pool. Also a concern is that some of this debris will jam up the spoke gasket and either cause it to displace, or to not seal properly which again could cause water to start leaking out of your backwash line. Once on the rinse setting, turn your pump back on and rinse for one minute, or up to half of the amount of time that you backwashed the pool.


5) Filter Setting - Turn off the pump again and return the dial valve to the filter setting. Start the pump and this time note the operational pressure for your filter. Since you have just completed a backwash the number you see now should be your clean operational pressure. Just be aware that any changes to motor RPM or any valves will change the pool filter pressure, and you want to make an apples to apples comparison when determining your clean operational pressure and comparing it to your dirty filter. Since the clean operational pressure for a pool should more or less be a static number, I often like to write that number in pen or marker right on the wall next to the filter so you always know it, and anyone else who takes over the pool care for you could step in day one with the info they need to monitor the state of the filter.



That is all there is to backwashing your sand filter. A simple and easy process that takes only a few minutes time but is still often done incorrectly. You must have a functional pressure gauge so you know when it is time to backwash the filter. Turning off your pump before moving the filter settings, using a rinse after the backwash, and letting your sight glass tell you when the backwash process is complete are all great tips for new pool owners to follow.


In addition to problems you can have with water leaking out of the waste line when in filter mode there are a few more considerations and questions that many new sand filter owners will have at this stage:


Broken Pressure Gauge - If you do not have a pressure gauge you should get one. More commonly you have one, but it might give you some funny numbers sometimes and you are just not all that sure how much you should trust it. The quick test for all pressure gauges is simple. You would turn off the pump and remove the gauge from the filter and in doing so the gauge will read zero. If the gauge reads any number other than zero when you remove it from your filter then it is broken, unfixable, and should be replaced. When you replace the pressure gauge in the filter you will need to reapply new thread sealant, most commonly white teflon tape. The gas tape, brightly colored, is too thick for plastic flange equipment and while it can work, it is also much more likely to result in cracking of the plastic flange on your filter. Use white teflon tape and a light touch when installing your pressure gauge. You do not need to bury the threads all of the way into the depth of the flange in order to achieve a seal.


Filter Pressure Rises Quickly - A sand filter that lowers in pressure after a backwash, but then rises in pressure again within hours or only a day or two indicates a problem with the filter media that is either related to dead algae blocking up the filter (did you just treat for algae recently?) or a buildup of biofilms and scum on the sand. A treatment with a filter degreaser would be a good first step to try to resolve this. Failing this you might need to consider replacing the sand in your filter but before you do you should read this article about how often should you change the sand in a pool filter.


Red & Green Arrows - Some pool filter gauges come with red and green indicators built in to the gauge. With these simply set the green indicator to your clean system operational pressure and when the pressure line hits the red mark this means it is time to backwash your filter.


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