Are Pool Sand Filters Good?
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If you own a swimming pool and you are looking to replace or possibly upgrade your filter then you almost certainly will end up looking into sand filters to see if this might be an option for your pool. Silica sand filters after all are the oldest and most common type of pool filter used today for residential swimming pools. If I had to summarize the popularity of sand filter I would say it is because they work and they are the cheapest filter option that does a decent job of keeping the pool clean. Perhaps you could add in that they are also fairly low maintenance as compared to other types of filters but this is a subject that warrants further discussion.
What is the perfect filter for one pool might be the worst possible option for another pool. This is the first concept that you need to understand when buying pool equipment. You are looking for the perfect filter for your specific needs. This is one of the common ways that pool owners can end up getting bad advice about the pool or pool equipment. Any advice that you take in, at absolute minimum, you should qualify the type of pool that person has and the geographic area in which they are located as these factors will have evrything to do with the type of advice that they give, and more importantly whether it is relevant for your situation or not. If pool owners did this before accepting advice from others it would help to head off a lot of potential problems.
Are sand filters any good? - Absolutely. Sand filters have been the mainstay of the pool and spa industry for decades. This is because they work, are very reliable, and do not take a lot of day to day maintenance. However as our world changes so do our requirements from things like pool equipment. Sand filters in particular have a few unique features that make them unsuitable for use in some areas. At 15 to 30 microns in size sand filters are the least effective of the three main filter types at collecting and straining out fine debris. This would be most noticeable in the pool at night with the lights on when it is much easier to see physical debris within the water. Cheap, works well enough, lasts a long time and does not take too much fussing with...what's not to like? Actually, there is an answer to that question and the answer is flow dynamics.
Sand filter flow rates - The single greatest deficiency with sand filters relates to their efficiency of water flow. Specifically sand filters are a large resistance to flow and if you have a sand filter it is very likely it is the single greatest resistance to flow on your entire pool system. The filter tank of sand takes a lot of energy to push water through, but more importantly the multiport valve that most sand filters have is a massive flow restriction. A diverter valve is different than on or off valves. A diverter valve sends water this way or that way, and usually there are five or seven or more different "ways" the water can flow. The flow of this water is controlled through a dial valve on the filter head. Here are some of the common filtration settings on a multiport valve and what they mean:
Filter = Normal water filtration. Water is pushed through the sand under pressure.
Backwash = Reversing the water flow from the filter setting and diverting this out through the waste line
Waste = Instead of going through the sand the water comes into the filter head and straight out through the waste line
Rinse = After backwashing or use of the waste line there may debris remaining within the filter head. Rinse clears out this debris
Winter(ize) = All ports open
Closed = All ports closed
Recirculate (whirlpool) = Bypass the filter media but still using the normal in and out ports used for the filter setting
Sand Filters Versus Cartridge Filters
The fact that sand filters have limited flow abilities is evidenced directly in the maximum design flow rate of each model. Using some examples of small, medium and large sand filters a small sand filter might only be capable of 35 GPM, where a medium one might be 50 GPM. Sand filters that can handle 75 or 80 GPM would be huge with as much as 600 pounds of sand in them, but this is the size of filter you would need if you wanted that much flow. If you are wondering why all this business about flow is important then you should know that the flow rate of your filter is your limiting factor for efficiency of your pool filtration system. You want your pool filtration system to experience very little resistance to flow as this indicates your potential for efficient operation.
On the other end of the spectrum you want a high potential flow rate as this indicates the high end capabilities of your filtration system. So in this way sand filters very much come up short. They are cheap to buy and yes they definitely do work, but they leave a lot on the table in terms of the potential for system efficiency as well as potential for total system flow rate. They also require regular backwashing which is a concern for some pool owners who have limitations on water consumption or water discharge.
Why does flow rate matter in swimming pools? - There are a lot of reasons why the potential flow rate in your pool matters but one way in specific is that you should be aiming to filter the entire volume of your pool three times every 24 hours. This equals approximately 95% of the entire volume of the pool being filtered at least one time. Could you filter less? Yes, many people do. This means you will filter less than all of your pool water very day but that is okay because we use chlorine in swimming pools as a chemical sanitizer speicifically because it can build and hold a residual value in the water. Anything in terms of bacteria and organic debris that you fail to filter out of your pool will be dealt with by the chlorine. Of course most pool owners these days want the least chemical exposure possible and if that is the case then the single best thing you can possibly do would be to filter your water more. Every iota of debris that is removed via mechanical filtration is one less that you will need to add chemicals to treat. The only catch is that running your pump that much costs a lot of money. Or at least it used to...not any more!
$4300 savings on filtration cost breakdown
With variable speed pumps it is now cheaper to run your pump on a 24 hour schedule than it used to be to run your pump on a reduced schedule like only eight hours per day. Lower speeds for longer hours has a massive impact on the amount of electricity you use. In fact you end up pumping more water daily and spending far less in doing so. A part of maximizing this energy saving process is providing your pool with an efficient path to filter the water. The less twists and turns in the pipe the better. The less length of pipes needed the better. The larger the pipe diameter size the better. All of these can help to improve the overall system efficiency, and if an efficient pool and spa system is your goal then a sand filter might not be for you.
Cartridge filters versus sand filters
Cartridge filters can filter down to around 5 microns optimally which is a substantial improvement over sand filters (but still shy of the filtering potential of D.E. filters) and cartridge filters also lack a multiport diverter valve. As a result cartridge filters have a much higher flow efficiency as the water does not struggle to pass through the filter head and filter media as much as with sand filters.
Where small, medium and large sand filters might allow for a maximum of 35, 50 or 75 GPM cartridge filters perform much better. Even the smallest 100sf cartridge filters can still be upwards of 75 GPM. Medium cartridge filters up and over 120 GPM and large cartridge filters into the 150+ GPM range. These numbers are vastly superior to the potential for flow from sand filters.
Sand filters are great for entry level pools or budget installations. Even higher end systems can have sand filters with minimal downsides, especially in areas where water conservation or salt water discharge are not issues. Some areas are on water restrictions which makes backwashing a problem for these pools. Also many areas now prohibit draining of salt water pools so sand filters in these areas are specifically problematic. If you have water to spare, are not too picky about flow rates or efficiency of flow, and are on a tight budget, then a sand filter might be the best option for you. If you live in an area with water restrictions or efficiency and cost of operation is a priority for your pool then a cartridge filter is certainly going to be the superior option.
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Swimming Pool Steve is an award winning, second generation swimming pool and spa specialist from Ontario Canada. With over 10,000,000 views on the Swimming Pool Steve YouTube Channel, winner of the 2018 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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