My pool filter pressure goes down, not up, what gives?

In a recent experience we had an 18k gallon inground guinite pool with a Hayward S-200 sand filter we helped the customer troubleshoot their sand pool filter to try and find the answer to an uncommon problem.

Note, this particular case is specific to sand (or Zeo-Sand) pool filters. Typically this only happens in a sand pool filter, we explain why below.

sand pool filter diagram how it works

“After running it for a while the pressure actually drifts down instead of up. The sand filter is trapping dirt because the water is cloudy during backwash and then clears up. I keep reading the pressure should go up as the sand gets dirtier. We do have cloudy water problems so I know it’s not working as best as it could.”

Pool pro:
Interesting, and we have seen this happen when out on swimming pool inspections, you must have developed water channels inside of the sand of that filter. Allows most of the water to travel through those channels without being filtered there by not increasing the pressure.

If you haven’t replaced the sand in the filter in the last five years it is recommended to go ahead and do so.

“I really appreciate the response.  I’ll go ahead and replace the sand.  I added zeo-sand about 5 years ago so I’ll stick with the zeo-sand unless you think it’s a bad idea.”

Pool Pro:
Zeo pool filter sand should be okay but you may want to consider picking up a new pressure gauge and when you change the sand it’s always advised to update the gaskets and O-rings in the process, and do not forget to check the condition of the laterals at the bottom of the filter, use changing the sand as an opportunity for a comprehensive inspection of the pool filter and its parts.

“Understood on everything, thanks so much for your help!”

Need some help replacing your pool filter sand?

Check out this video and more tips below!


How to change the sand in a pool filter

  1. Turn off the pool pump. Make sure that the pool filter pump won’t be able to turn back on while you are working on it. This will stop water from entering the system while you clean it, as well as cutting power to the area while dealing with water.
  2. Remove the filter drain plug and let the filter tank fully drain. Locate the drain plug near the bottom of your filter tank and unscrew it completely.
    1. If your pool filter is in a pump house or anywhere else you don’t want water draining, quickly attach a hose over the drain plug after you remove it. This will let the water drain elsewhere.
    2. Keep the drain plug and o-ring somewhere safe until they are ready to be reattached.
      Sand pool filter multiport
  3. Take the collar off the base of the multiport valve. Locate the collar around the edge of the multiport valve, near the top of your filter tank. Use a screwdriver to loosen the bolts on either side of the collar until you can remove them completely. Pull apart the two sides of the collar to remove them.
    1. The collar on the multiport valve is used to clamp it in place, so may also be referred to and appear as a “clamp.”
  4. Unscrew the unions holding the pipes to the multiport valve. Any pipes connected to the valve will make removing it much more difficult. Carefully unscrew the unions near any pipe attached to the valve and disconnect the pipes.
    1. If the pipes attached to your valve do not have unions to allow them to be removed, you will have to use a saw to cut the pipe instead. As you’ll need to replace the sand in your pool filter regularly, install union fittings on the pipe this time so that you can easily connect and disconnect the pipes in future.
  5. Remove the multiport valve by twisting slightly and pulling upwards. Firmly grip the top of the multiport valve and begin wiggling it as you lift the valve off of the tank. Carefully remove the valve so as to prevent damaging it or anything inside the tank.
    1. Cover the standpipe with tape or a plastic bag, or drape a rag over it. This will prevent any sand from getting into the pipe and working its way into your pool while removing or adding sand.
  6. Begin to use a shop vac to remove the old sand. As you remove sand, be careful to avoid directly touching or hitting any components or laterals at the base of the tank. These can be fragile and difficult to replace.
    1. If you do not have a shop vac, you can use a large cup or scoop to remove the sand from the bottom of the tank. This will take longer, and as the sand has been filtering everything in the pool, can be unsanitary. If you choose to do this, always wear gloves and be careful.
  7. Rinse the tank thoroughly with clean water. Once you have removed the bulk of the sand from the tank, use a garden hose to thoroughly clean it both inside and outside.

Make sure any fittings on the tank are cleaned fully before reattaching anything to them.

Hayward sand pool filters


  1. Reattach the drain plug to the base of the tank. Before adding any new sand to the tank, you’ll need to reattach the drain plug. Screw it back in place at the base of the tank, making sure to tighten it in order to prevent leaks.
  2. Pour new pool silica sand or Zeo Sand into the tank. Position one corner of a bag of pool-grade silica filter sand over the mouth of the tank. Make sure the stand pipe is covered as mentioned above.
    putting new sand in pool filter
  3. Make a small cut in the corner of the bag to allow sand to slowly fall into the tank. Work slowly and carefully to prevent any sand from spilling. Repeat until you have added the amount of sand needed for your filter.
    1. If you are worried about damaging the components on the inside of the tank, add a little bit of water to the bottom before pouring in the new sand. This will cushion the sand as it is added but may mis-align the standpipe.
    2. As you add sand, make sure the standpipe in the center of the tank stays right in the middle and at the right height. It will need to reattach to the base of the multiport valve when you are finished, so you can line the two up to check if you’re unsure.
    3. The amount of sand needed will vary between pool filters. Check for stickers along the side of the tank or in your tank’s manual to find out how much your tank needs to operate.
      How to change your pool filter sand
  4. Fill the tank with water up to the level of the sand. Use a garden hose to add water to the tank until it just begins to cover the sand. This will give the tank enough water to clean the sand and get the pump functional before you start filtering your pool water.
  5. Lubricate the O-ring on the valve with a multipurpose lubricant. Locate the rubber “O-ring” around the top of the multiport valve where it will form a seal with the mouth of the tank. Apply a small amount of lube to your finger and rub it around the O-ring. This will make reattaching the valve to the tank easier, as well as conditioning the rubber seal.
    1. If the O-ring is damaged, you might need to replace it rather than just lubricating it. These should be available from hardware and pool supply stores in the size you need.
  6. Reattach the multiport valve. Remove the tape or plug from the top of the standpipe and position the multiport valve over it. Carefully connect the opening in the valve to the top of the pipe, and firmly push the valve into the top of the tank. Wiggle it around slightly as you push it into place to make sure it is secure.
  7. Screw the collar and pipes back in place. Position the collar around the edge of the multiport valve and use the two bolts to tighten it into place. As you do so, alternate between bolts to ensure the pressure is equally distributed around the collar. Reattach the pipes and screw the unions on as tightly as you can to prevent leaks.
  8. Rinse the filter for 1 minute. Turn the handle on the top of the multiport valve to the “rinse” position, and let the pump run for another minute. This will clean the water further, so use the sight glass or hose to make sure the water inside the tank is clear.

Always make sure you have turned the pump off before switching positions on your multiport valve.

A helpful reference guide we found on WikiHow

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