by Rob Cox June 24, 2014
To Replace or Repair your pool filter - that is the first question.
If your pool filter has been struggling the last few seasons, and you are tired of the extra work and chemical use it creates for you, it's time to do something.
Let me ask you some other questions to explore the question of whether you should replace and old filter tank, or if you could renovate it?
If your filter once performed well, keeping the water clean and clear, without many problems with cloudy water, but now is struggling ~ the pool filter media (DE grids, Cartridges or Filter Sand) could be in need of replacement. Typically, sand lasts about 5 years, cartridges about 2 years and DE Grids can last over 10 years. They will last longer and perform better if they are routinely cleaned. Replacing the filter media (sand, cartridge, grids) can be done at a much cheaper cost than replacing the entire filter, even if you also replace other parts, like the backwash valve.
Pool filter media can be rejuvenated by using a pool filter cleaner, to remove oils, 'gunk' and mineral deposits, which clog filter sand, or cartridge and grid fabric, making filtering much less efficient. It's a good idea to use a filter cleaner at the end of each swim season, or anytime that filter efficiency decreases to a noticeable point.
For sand and DE filters, a multiport or push-pull valve is attached, to allow for backwashing. The valve directs water in the right direction, but if internal parts, gaskets or o-rings become damaged, a portion of the water can return to the pool un-filtered.Generally, you will have slightly lower than normal filter pressure if this is taking place.
Water can also bypass the filter inside the filter tank. For sand filters, if there is not enough sand, or if an overpowered pump pushes the water through well-worn channels. Torn cartridge filters, or misplaced or missing parts inside of a cartridge filter can cause water to bypass the cartridge. The same for torn DE grids, or cracked manifolds - which would also blow back some DE powder into the pool.
Some pools - those that have repeated algae blooms, or a green pool every spring, very high hardness levels, or pools that don't get a lot of replacement water, from the hose or rain - over time the water can become choked with so many dissolved solids that it makes the filter's job more difficult. This also slows water sanitation, as the good-guy chlorine molecules fight microscopic traffic jams while trying to disinfect. A pool with 'thick water' can make it seem like the filter is not doing its job. Many pool guys, myself included, find fresh water much easier to manage than water that has not been changed in many years.
Plastered pools may be drained about every 5 years anyway, to do a light acid wash, or make some surface repairs. For vinyl and fiberglass pools, emptying the pool completely can be problematic, but 1/3 to 1/2 of the water can be safely replaced without any issues in most cases.
Many pool owners make the mistake of replacing a pool pump with a larger or more powerful pump which is too big for their filter. I call this the V-8 mentality, guys want the biggest engine. If water is pushed too fast through the filter media, dirt gets pushed right through, especially on a sand filter or cartridge filters, and to a lesser degree on DE filters. Cartridge filters and DE grids can be damaged if the Design Flow Rate of the filter is exceeded.
How to Read a Pump Curve: If you suspect that your pump may be too big for your filter, check the flow curve chart for your particular pump model, and find the line that represents your pump's horsepower. On the vertical axis are listed increments of resistance, in feet of head. Every pool has a different amount of resistance in the system, the combination of pipe size and length, pipe fittings, valves, and other equipment like filters, heaters and salt systems that the pump has to push water through. An average pool would have about 30 feet of head, but if your pool has long pipe runs or lots of bends and turns and a heater, or attached spa, you can use 40 feet of head, or more.
Move across the chart to find where the estimated resistance, in feet of head, intersects with the gpm, on the horizontal axis. At that point is the flow rate that this pump model, and hp will produce, at the chosen level of system resistance.
Next, check your filter label, or owner's manual or look on our pool filter pages, to find your pool filter's Design Rate of Flow. All pool filters are designed with a maximum do-not-exceed flow rate. Each filter type and size will have a different design rate of flow, for instance - here's the Pentair Tagelus sand filter chart that shows the maximum flow rate that should be pushed through these particular filters.
A right-size pool pump would not exceed these flow rates, and the water would travel through the filter media at a speed that allows it to trap the dirt in the sand bed, cartridge or DE cake.
Many pool owners make the mistake of going too small with a replacement pool filter. I call these 'marginal filters'; they can be adequate in most conditions, but with hot weather, an algae bloom, lots of storm debris, or spring clean-up, they struggle trying to clean adverse water conditions.
Cartridge filters can be great filters, when they are large enough. My rule of thumb for sizing cartridge filters is to use 100 sq ft of filter surface area for each 10000 gallons of pool water. Sand filters, likewise are great filters - when they are big enough, but when they're too small, they create more work for you and more chemical expense. For Sand filters, buy 1-2 sizes larger than the brochure says you need for your pool size. For most inground pools of over 20-40,000 gallons, I would recommend a 30" sand filter, which uses 600 lbs of filter sand. For pools that are 10-20,000 gallons, buy the 24" sand filter tank size, with about 350 lbs of sand.
Same with DE pool filters - if you can buy 1-2 sizes larger, you'll never regret it - with fewer pool water issues, you can run the pump less, and use fewer chemicals.There is also less risk of damage to the pool filter from a too large pool pump.