When Should You Replace Filter Sand?
Silica sand filters are cost effective, they work well, and they do not take a huge amount of ongoing maintenance day to day. There might be better options for pool filters that waste less water, are more flow efficient and are capable of removing finer physical debris from the water than sand filters but that does nothing to diminish the popularity of sand filters. You actually could make a very good case for upgrading to a more efficient filtration system that has much less resistance to flow than a giant tank of sand that you are pushing water through.
A well designed and efficient plumbing system would be one that is easy for the filtration pump to draw and push water through. If you had a system that was incredibly hard to push water through this would result in a less efficient plumbing system. This is actually kind of what a sand filter is, relatively speaking, and this is why an upgrade to something with a much better flow design should always be on the table if you currently own a sand filter. Before you consider spending any money on a filter service you should weigh the comparison of cost for servicing the filter you have now versus replacing with a new model. There could be enough advantage for you to upgrade already, and the money spent on service to the old filter might be better off going towards the purchase price of a newer, more reliable and more efficient filter system.
The single biggest problem with pool sand filters is the multiport valve they use to select filtration settings such as backwash, rinse, winter, close, recirculate and filter. Convenient for the pool operator, but definitely a huge flow restriction when compared to pretty much every other valve, fitting and piece of equipment within your filtration system. The maximum designed flow rate, a very important number relating to the volume of water your filter can handle before the potential exists for damage to internal components, is specifically low in sand filters versus any other filter type. Take a look at this comparison between a sand filter and a cartridge filter, where both of these filters are of suitable size for a 20x40' swimming pool.
Hayward S270T - 350 lbs sand filter
Maximum design flow rate = 74 GPM
Hayward C4030 - 425sf cartridge filter
Maximum design flow rate = 150 GPM
Both of these filters are very large, but not the largest available for either option. Both of these filters will have no problem keeping an extra deep 40,000 gallon 20x40' pool clean and clear, but as you can see those flow rates are majorly different. This is especially important since average one horsepower pool pumps can easily move 80+ GPM and new variable speed pool pumps in the 2 to 3 horsepower range like the EcoStar, TriStar 950 and IntelliFlo are easily capable of 150+ GPM so long as the plumbing system can handle this much flow. If you have a sand filter, and you do not have a flow meter telling you how many GPM is moving through your system, then you are almost certainly overdriving your sand filter. This is very important to know.
How does a sand pool filter work? - A sand filter is a pressure vessel, essentially a tank filled with sand. Water is applied under pressure from the pump and forced through graded sand which removes all but the smallest physical debris before the water is collected clean on the other side of the sand and continues on through your plumbing system and back to your pool. Backwashing, a term synonymous with pool sand filters, is exactly as it sounds in that the water is sent backwards through the filter from the direction it normally travels. This forces all the physical debris that is sitting in the first few layers of sand to wash out through the waste pipe or drain on your pool filter. When the pump applies water pressure the entire filter tank swells in size, a quite unsettling feeling when you are standing next to one while doing this as it really shows you how much pressure and lower exists in these systems. The filter weighs hundreds of pounds and feels as solid as can be, despite being made from resin. But when pressure applies from the pump the entire tank swells like an inflating balloon. Definitely never take the potential for danger lightly with a pool filter.
A quick recap here, a sand filter is a pressure vessel and swells and contracts every time you turn on and off the pump, and it is rated for a certain maximum flow rating that many pool owners accidentally exceed for the entire operating life of the filter. So now we can start to talk about the main point of this article you are reading, which is when should you replace your filter sand? As an experienced pool technician I can easily tell you that most of the people who change their filter sand, or think that they need to change their filter sand, actually do not need to do this. Worse than this they specifically should avoid changing the sand in their filter.
How Often Should You Change Filter Sand?
If you want to follow the book about how often you should change your filter sand the answer will be either every 3 to 5 years, or every 5 to 7 years, depending on who you ask. But is this really good advice? I don't think so. These established values for sand filter changes were developed and agreed to in a different age of pool equipment. Pumps were smaller and filters were built to last. Every pool pump these days is absolutely gigantic, and pool filters, much like everything else these days is built to work, but not to last. Parts are made using the cheapest, thinnest possible materials, and then these cheaply made filters are attached to pumps that provide way too much flow. The end result is a filter that works, but if you attempt to change the sand every 3 to 5 years you are almost certainly going to encounter a problem.
How long does filter sand last? - Pool filter sand does wear over time much like play sand versus a sharp sand or brick sand. Once the edges of the sand have worn down the filter will have less filtering ability compared to what it was when it was new. Additionally a pool filter is the garbage can for every last disgusting thing that finds its way into your pool. After a few years time this becomes a bacterial breeding grounds. Sand filter changes every 5 to 7 years is how I was trained many years ago. However it was not exactly linear, it was told to me as every 5 to 7 years, or when there is a problem. This is important, because most pool owners think sand changing is one of the first things you should look to when the pool is cloudy or green. Filter sand is probably the last thing you should look at, percentage wise. Chances are extremely high that your problem with water quality is either a chemical imbalance of some kind, or you simply are not running the filter enough. It should be running 24/7 any time you are having water quality issues.
As a service technician I have regularly removed sand filters from service that were still operating on original sand after 25 years. There is no rule that says you must change your sand. Only a recommendation that if you have a problem with your water you might need to change it. If you don't have a problem in the first place then I am of the opinion that you should leave your filter alone. If your pool is green and you think maybe it is the filter sand, it is probably a chemistry issue in well over 90% of cases.
Why you should NOT change your filter sand - Pool filters used to be made to last a half century or so, with some models even being stainless steel and strong enough to withstand decades of use, or even a direct strike with a vehicle...almost. Pool filters manufactured today are mass produced with resin and ever-cheaper materials, gaskets and moving parts. The end result is a product that works, but does not last. Add to this the fact that the filter is constantly exposed to chemicals, changing pressures, reversing pressures and physical debris and you might start to see why opening yours to change the sand might be a bad idea. While I used to perform filter sand changes regularly at the beginning of my career this is something I would rarely recommend these days. The chances of damaging something inside the filter during the sand removal process is very high. Even when you're careful, many times the internal lateral assembly parts are hanging on by a thread and even the lightest disturbance causes them to fail immediately in your hand, or fail in a way that you do not notice until the next day when all the sand from your filter is now on the floor of your pool.
When a filter sand replacement service call goes well it is a quick, albeit gross job. When a filter sand change does not go well I spend the time cleaning the old sand out, bringing in hundreds of pounds of new sand, put it in the filter, clean up and start up the system with a lengthy backwash to prevent the pool from going cloudy from the new sand. But then the next day I get a call from you in a panic because the floor of your pool is covered with hundreds of pounds of sand. The standpipe or laterals broke. You might be able to order replacement parts. If they make them still, and if they will not take longer than 48 hours to get (special order parts is six weeks) you can buy new sand, replace the broken part and begin cleaning hundreds of pounds of sand out of the pool. Remember one part already broke inside the filter, so unless you replaced everything than another failure seems likely. This is why you should strongly consider not replacing the sand in your filter.
A new cartridge filter is a lot more money than a sand filter, but they are a much better filter in terms of what they can remove from the water, as well as being vastly better from a hydraulic efficiency standpoint. On paper the increase in system efficiency will probably pay for the difference in price between a sand filter and a cartridge filter over the course of its service life. If you factor in that you might pay a few hundred to have your filter sand changed by a professional, and at best you have an old and possibly failing filter, for sure that few hundred would be better allocated towards purchasing a new, more efficient cartridge filter.
Channeling sand - If the sand in your filter solidifies from age or adverse chemical situations then this would be one of the only reasons I would explore changing the sand in your filter, as this could happen to a fairly young filter that still has some life left in it. Calcium hardness and a scaling state according to the LSI can cause your water to calcify and solidify your sand in your filter, such that the water no longer filters nice and evenly, but has worked channels into the sand creating pathways of low resistance to flow through. The end result is that water is going through the filter but no water is actually being filtered.
Bacterial buildup - The stuff inside a sand filter gets pretty gross after a while. A cartridge filter gets disgusting after a half season of operation so you can only imagine what a sand filter is like since it never gets cleaned out in the same thorough way that a cartridge filter does regularly. Instead of changing the sand to deal with the bacterial buildup in your sand filter you can also look towards a chemical treatment and cleaning to regenerate your sand media. This is a maintenance item that you would 100% try before you can even start to consider replacing the sand in your filter. If you have not been using a sand filter degreaser every year in your filter, then you need to try this. If you do have a poorly performing sand filter this is going to make a huge, positive difference. Use it once every year at minimum.
Sand Filter Cleaner
I would say that regular use of a filter cleaner product is the only long term maintenance that sand filters need other than backwashing of course. Do not look quickly to changing the sand in your filter as the answer to water quality issues. Filter your water 24/7 and ensure you are within tolerance for your water chemistry as that is almost always the source of water quality and clarity problems. Sand filters wear out but the effect would be a slow and steady decline over years and years. Decades. Bacterial buildup and channeling is a concern but those are both best treated with proper water balancing as well as a filter cleaning treatment, not sand changes. Finally, with the chance that you overdrive your filter being high, or at least operate it at the very maximum operating range as a guarantee, and with how cheaply things are made these days, changing the sand in your filter is as likely to cause a problem as it is to fix one. Perhaps even more so.
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