Ask the General: How to Backwash a Sand Filter in My Fiberglass Pool?

Dear General;How Do I Backwash the Sand Filter in My Fiberglass Pool

I love my fiberglass pool, but here’s my problem. I don’t understand how to backwash a sand filter. I keep my pool clean and add all the chemicals I’m supposed to, when I’m supposed to. What is backwashing, and WHY do I have to do it to my sand filter?


Bad Backwasher in Bainbridge


Dear Bad Backwasher:

Funny you should mention bad back(washing), as I’m writing this with my feet propped up and an ice pack on my back. I’d like to tell you that I pulled something doing a triple Lindy off the high board into the pool, but the truth is, I hurt my back bending over to tie my shoe. The General’s not as young and spry as he used to be. My point is, much like tying a shoe, the simplest things can cause a lot of pain. Which brings us to your learning how to backwash a sand filter.

If you want to talk about pain (which clearly I do because I’m in it), let’s talk about the pain of paying for extra chemicals to clear up a pool that got cloudy due to poor circulation. Why might you have a clogged filter? Failure to backwash your sand filter. You can religiously add chlorine to the chlorinator, check its settings, sweep down the walls of the pool, empty skimmer baskets, vacuum dirt and debris from main drains, do water tests, maintain the correct water level and clean the steps going into the pool, but if you forget about the cleaning the filter, your pool water is going to get cloudy. Nobody wants that.

Pool filters

There are three available types of pool filters: sand, diatomaceous earth (DE) and cartridge. Sand filters are the oldest form of swimming pool filtration. They are particularly good at catching microscopic particles like algae and phosphates and need to be backwashed about once a week. Sand filters require very little maintenance, but the sand must be replaced about once every 2-4 years.

What is backwashing?

Backwashing simply means running water backwards through the filter to lift the dirt off the top layer of sand. Your pool pump moves the water into the top of the filter and then evenly distributes it across the surface area of the sand bed. Then, it pushes the water to the bottom of the filter. By the end of the process, the water has washed the debris out of your filter, readying it to effectively filter your pool water again.

How to backwash your sand filter

Backwash process

Things to watch out for when you’re backwashing

  • Sand in your pool.  If you notice sand in your pool during or after backwashing, you might have cracked of laterals or piping inside the filter or you might also have too much sand in the filter. The space from the top of the filter to the beginning of the sand line is called “free board.” This space is essential to allow you to backwash without having sand come out of the filter.
  • Too much of a good thing. While backwashing is essential, it is possible to do too much of a good thing. Turn the hose off as soon as your water runs clear. Prolonged backwashing of your sand filter can cause “channeling.” This is a situation in which channels develop down the sides inside the filter. When this happens, water will be able to bypass the sand altogether and this will essentially defeat the whole purpose of your sand filter.

As with all aspects of your pool’s operation and maintenance, I recommend reading the instructions on how to backwash a sand filter that are typically found on the filter, as each model varies slightly. Your filter and your pump are the backbones of your pool. It’s important to pay equal attention to both of them. A failure of either will cause you pain (and, of course, money.)

Speaking of pain, it’s time for me to grab a float, walk SLOWLY AND CAREFULLY into my pool, relax, and rehabilitate my own back.

Until next time–

The General


Who is The General?

The man, the myth, the legend….we just call him The General. His organized, systematic approach to pool installations over the years had his crew members calling him “The General” and it stuck. The General has over 30 years’ experience in the pool and spa industry, working for one of Pool and Spa News’ Top 50 Pool Builders.”

Over that time, he designed, sold, project-managed and installed over a thousand inground swimming pools. As a pool owner himself, he’s the perfect authority to give you the inside scoop, with amazing tips and tricks to make pool ownership a breeze!

About Thursday Pools

Thursday Pools designs and manufactures durable and elegantly crafted fiberglass pools. We are ISO 9001 certified, which means that fiberglass pool shells are made with the highest standards and from the best quality material available. We are also ISO 14001 certified, which means we are committed to environmental stewardship.

Our one-piece, inground fiberglass swimming pool manufacturing facility is based out of Fortville, Indiana. At Thursday Pools, we aspire to be the world’s most respected fiberglass pool manufacturer and to help our customers create a lifetime of memories with their family and friends. Our innovations, commitment to quality and beautiful designs set us apart. Thursday Pools is the creator of the beach entry (or zero entry) fiberglass pool (Patent US 10,358,837 and 10,472,839). View our exclusive designs for the Grace Beach Entry design and the Sandal Beach Entry Design.

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