Sand filters have been around for a long time. In fact, they are the swimming pool’s oldest form of filtration. As water flows into the filter, it passes through the sand. The dirt and other particles become trapped in the sand and, clean filtered water flows back into the pool.

But, as tried and tested as they may be, sand filters need to be correctly installed and do need maintenance. So, be sure that your filter is the right size for your pool, and that the sand is changed every two years.

Otherwise, here are the main causes for an ineffective filter, and they show that you have to tread a fine balance to make sure they work properly.

• • Your pool filter has no pressure.

Run your pump, and if the filter pressure gauge reads zero or close to zero, something is amiss. Give the gauge face a flick to see if it pops up. If it doesn’t, inspect the gauge dial or face for warping.

• • Your pool filter has too much pressure.

If your pressure is higher than normal, this usually means a dirty pool filter. Backwash your filter.

• • Your pool filter pressure rises too slowly.

Pool water sometimes bypasses the filter and return to the pool unfiltered. This can cause the pressure to rise slowly. It probably means problems with the filter valve or damaged filter parts, so you’ll need to replace the filter.

• • Your pool filter pressure rises too rapidly.

In this case, the filter may be clogged with oils or minerals, so you’ll need to use a filter cleaner specifically made for your filter type, or replace the filter itself.

• • Your pool filter is leaking out the backwash line.

Is there a trickle of water draining out of the backwash line? Open up the multiport valve and inspect the gasket, which may be twisted or broken. You probably need parts replacement.