Sandfilter Troubleshooter

- when Sand-filters go wrong . . you need to find out what's wrong quickly and rectify it even more quickly because without a filter you've got a problem that can, andalmost certainly will, rapidly escalate to even more expensive problems

Warning: - Due to the possibility of residual pressure in the filter itself, or back-pressure from the pool, take these simple steps before opening the filter

  • Switch the pump off
  • Open the air-relief valve (or loosen the pump lid)
  • Close all the valves

Cloudy water possible causes -

  • The filter is full - 
  • Cure: - Backwash and rinse the filter 
  • The media is old, tired and needs replacement -
  • Cure: - replace the media but think about using Zeolite instead of sand
  • Some of the media has escaped from the filter leaving insufficient to catch all the suspended dirt, some of which returns to the pool via the Jets. Particularly noticeable when vacuuming the pool
  • Cure: - Well, it’s not simply a matter of topping up the media because there is always an underlying reason WHY the media has escaped

It could be any of the following: -

The pump is too powerful for the filter and, during a backwash, some of the media is pumped out with the waste water

  • Pool builders sometimes offer a bigger pump as a 'free upgrade' when trying to outsell their competitors and, consequently, many pools have pumps that are far too big for the filter
  • Check online with the manufacturers for the recommended pump size for your filter model

The media has eroded due to turbulence when back-washing

  • The tiny fragments of media are likely to be caught by the remaining media and then flushed out of the filter during the next backwash, with a gradual reduction of the total media within the filter

Sand is coming into the pool at the return jets

  • One or more of the collectors in the lateral assembly is worn, cracked or broken - allowing sand to be pushed through and into the pool or
  • The breather (pressure relief) tube in the filter has broken
  • Cure: - empty the filter and examine the collectors and breather tube; replace as require
  • You may as well replace the media whilst you're at it!

Filter need frequent back-washes possible causes -

  • Insufficient time allowed for each backwash
  • Cure: - Backwash until the water in the sight-glass runs clear
  • Insufficient flow through the filter
  • Cure: - First check that the pump strainer basket and impellers are not clogged as either of these will severely reduce flow - and backwashing will ineffective

Tip:- Run the system on Filtration and observe how much water is returned to the pool at the Jets. You can do this just by feeling the force of the water with your hand.  Then stop the pump and turn the Rotary Valve to 'Circulate'. Now run the pump again and decide if the force of the flow has increased

  • If there is an increase - the filter is the culprit
  • If there is no increase - the pump is the culprit
  • Incorrect water balance
  • Cure: - maintain a pH of 7.4, or as close as possible, and maintain free Chlorine at 1-3ppm
  • This will stop algae from growing in the filter and also prevent calcification of the filter bed
  • Check and adjust Total Alkalinity and Calcium Hardness levels


  • The filter is calcified
  • Calcification is caused by high pH of the water so check and adjust pH as required; aim for a pH of 7.4


Leaking filter

The course of action to take depends on where the leak is coming from: -

 From the seal between lid and filter

  • Cure: - Remove the lid and examine the rubber sealing ring ('O'-ring gasket)
  • If you spot any damage at all, and this includes any breakdown of the rubber (it will leave black stains on your hands), replace the sealing ring
  • Clean the seating and sealing ring thoroughly, smear them both lightly with silicone grease or Vaseline, replace the lid

Note: - Some older types of filter lid can become distorted over time, particularly if the pump is over-sized

  • In this case, replace the lid with a new one and check that the pump is the correct size for the filter

 From one of the compression fittings to the Rotary Valve

  • Cure: - Tighten the clamp-ring of the offending item, but be careful not to over-tighten because they can split
  • Filter connectors are sealed by squeezing a rubber 'O'-ring between 2 flat flanges
  • Eventually, the 'O'-ring can flatten, reducing its ability to seal the joint
  • Replace defective or damaged 'O'-rings as required

Tip: -

  • Usually 'O'-rings are retained in a groove on one of the flanges and the flattening occurs on the side of the ring that is pressed up against the other flange
  • In a emergency you could try removing the 'O'-ring and putting it back other-side-up
  • You'll still need to replace it at a later stage though

 From a crack or split in the filter body

  • Cure: - Do not attempt to repair a cracked filter - replace it with a new one
  • Pool filters are 'pressure vessels'; meaning that they are under pressure during the filter sequence
  • We have seen filters that have literally been blown apart by pressure from the pump

If your repair fails you could have a catastrophe on your hands. This could include: -

  • A flooded pump-room - causing further damage to the pump and/or other electrical items
  • An empty pool - if the water is not being returned to the pool because it is escaping from a split filter
  • If the water table is high your pool could float up out of the ground and you'll never get it back down again
  • Subsidence - if the water is flowing near the pool, or even worse near the foundations of the house, subsoil can be washed away, causing subsidence

 From the pressure gauge or drain plug screw-thread

  • Cure: - Remove the offending item and wrap 20 turns of PTFE tape around the male thread before re-fitting

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Ken Walker - MyPoolGuru©