Sand Filters Explained: How They Work, Backwashing, Sand Changes and Glass Media

Last updated: September 2026

Sand filters are the workhorse of the pool world. They sit in the corner of the plant room doing their job for years with very little fuss, and they’re a popular choice for swimming pools, spools, plunge pools and, with the right setup, even hot tubs.

They’re simple once you understand them, but the multiport valve on top confuses a lot of people. In this guide I’ll walk you through how a sand filter works, what every valve position does, how and when to backwash, how often to change the sand, and whether glass or zeolite media are worth the upgrade.

Here’s my video walk-through of sand filters, which covers everything below:

How a sand filter works

A sand filter is a pressure tank, usually fibreglass or moulded plastic, filled with filter sand. At the bottom is a set of slotted plastic “laterals” connected to a central standpipe. The multiport valve sits on the top (or side) of the tank and directs the water.

In normal filtering, the pump pushes water into the valve, which sends it into the top of the tank. It spreads out, runs down through the sand bed, and the sand traps dirt and debris. The clean water passes through the slots in the laterals, up the standpipe, back through the valve and returns to the pool. That’s the flow direction Hayward’s owner’s manual describes for a top-mount filter.

As the filter gets dirtier, it resists flow more, so the pressure on the gauge rises. That rising pressure is your signal to clean it. Interestingly, a slightly dirty sand bed filters better than a freshly backwashed one, because the trapped dirt helps catch finer particles. That’s one reason not to backwash more than you need to.

The multiport valve: every position explained

The multiport valve is arguably the most important part of the filter after the sand itself. It gives you control over what the water does. Most have six or seven positions:

Position	What it does	When to use it
Filter	Water goes down through the sand and back to the pool	Normal running, and normal vacuuming
Backwash	Reverses the flow up through the sand and out to waste	When the pressure has risen
Rinse	Normal flow direction, but out to waste	Straight after a backwash, to resettle the bed
Waste	Bypasses the filter and sends water straight to waste	Lowering the water level, or vacuuming heavy debris
Recirculate	Bypasses the filter and returns water to the pool	Mixing in chemicals, or if the filter is damaged
Closed	Shuts off flow from the pump to the filter	Working on the pump (never run the pump on Closed)
Winter	Sits between two positions to let the valve drain	Winterising, where fitted

Positions are from the Hayward Pro Series manual. Other brands are very similar, but check yours.

How to backwash a sand filter, step by step

1. Check the pressure. Note the reading when the filter is clean (after the first backwash on new sand). Hayward says to backwash when the gauge is 8–10 psi (0.55–0.69 bar) above that. Trouble Free Pool uses a rise of about 25% as its trigger. Either way, let the gauge decide, not the calendar.
2. Switch the pump off. Moving the valve with the pump running damages the valve’s internal gasket.
3. Set the valve to Backwash, make sure the waste line is open and going somewhere sensible, and switch the pump back on.
4. Watch the sight glass. That little clear window on the valve. The water will run dirty, then clear. When it’s clear, you’re done. That’s usually about two minutes.
5. Pump off, set to Rinse, pump on for 30 seconds to a minute until it runs clear. This resettles the sand and stops a puff of dirt going back into the pool.
6. Pump off, back to Filter, pump on. Note the new pressure and top up the water level.

Backwash water carries dirt and sanitiser, so check with your local water company or council where it can go. Don’t send heavily chlorinated water onto plants or into a watercourse.

How often should you backwash?

A lot of guides say once a week in summer and every two weeks in cooler months. That’s a reasonable starting point for a busy pool, but I’d rather you used the pressure gauge. Pool use and your surroundings make a big difference. A pool with lots of swimmers, surrounded by trees or in a windy spot will need backwashing more often than a covered pool in a sheltered garden.

Changing the sand

Sand doesn’t last forever. Over time the grains round off and stop catching dirt as well, and the bed can clump or form channels that water bypasses. Typically you’ll change it every 3–5 years. Signs it’s time:

• The pressure climbs back up soon after a backwash.
• The water won’t clear even with good chemistry.
• You find clumps or “mud balls” when you open the tank.
• Sand is getting back into the pool (check the laterals for cracks while you’re in there).

Use the grade and quantity on the filter’s label. Hayward specifies No. 20 silica sand (0.45–0.55mm), which is typical. Half-fill the tank with water first so the sand cushions as it goes in and doesn’t crack a lateral. Between changes, Trouble Free Pool suggests an annual deep clean: open the tank, run a hose through the bed and gently break up any clumps, taking care not to damage the laterals.

Alternatives to sand: glass and zeolite

You don’t have to fill a sand filter with sand. These media go in the same tank. They’re not cartridges. I’ve compared cartridges and sand filters in Cartridge or sand filter?

Media	Pros	Cons
Silica sand	Cheapest, proven, easy to get	Coarsest filtration (about 20–40 microns), changed every 3–5 years
Glass media	Finer filtration, less backwashing, longer life, resists bacterial biofilm	More expensive upfront
Zeolite	Filters finely and removes ammonium by ion exchange, which cuts chloramine formation	More expensive than sand, and needs occasional regeneration with a salt soak

Glass media

Crushed and processed glass is the most popular upgrade. It costs more than sand upfront, but it lasts longer and needs less backwashing, which saves water and effort over time. The best-known brand, Dryden Aqua’s AFM, is claimed to filter down to around 1 micron, cut backwash water by up to 50% compared with sand, and keep performing for up to 10 years. Those are the manufacturer’s figures, but the general direction (finer, longer-lasting, less backwashing) is widely reported.

Zeolite

Zeolite is a volcanic mineral. As well as filtering, it removes ammonium from the water by ion exchange, which reduces the chloramines that cause that “chlorine” smell and red eyes. That makes it worth a look for pools with heavy bather loads. Its capacity can be restored by soaking the filter overnight in a salt solution. It costs more than sand. How long it lasts depends on the product, so follow the supplier’s guidance.

Which should you choose?

• On a budget, with proven results: sand.
• For clearer water, less backwashing and a long life: glass is an excellent upgrade.
• For busy pools with chloramine problems: zeolite is worth considering.

Sizing a sand filter

The filter has to match the pump and the pool. Every filter has a maximum flow rating. Make sure your pump’s flow at your system’s head is within it, and that the filter can turn the whole pool over in a sensible number of hours. An undersized filter runs at high pressure, filters badly and needs constant backwashing. An oversized pump can also push dirt straight through the bed. If you’re changing the pump, read my pool pump upgrade tips and the shocking truth about pool pumps.

Can you use a sand filter on a hot tub?

You can, but it needs its own dedicated pump, because a spa pack’s small circulation pump won’t push enough flow through a sand bed. Luckily, many DIY builds have a separate filtration pump anyway, so it’s not always an issue. The bigger problem is that every backwash sends warm, treated water to waste, and in a hot tub that’s a big share of the volume. That’s why I recommend a cartridge filter for most DIY hot tubs. There’s more in Can I use a sand filter on my DIY hot tub?

For spools, cocktail pools and plunge pools, sand (or glass) is a great choice. See my guides to building a DIY cocktail pool or spool and a DIY plunge pool.

Troubleshooting a sand filter

Problem	What to check
Sand coming back into the pool	A cracked lateral or standpipe, or sand that’s too fine. Some fine sand after a sand change is normal until you’ve rinsed properly.
Water leaking from the waste line in Filter mode	A worn spider gasket inside the multiport valve, often from turning it with the pump running.
High pressure straight after backwashing	Clumped or calcified sand, or a restriction on the return side (a closed valve, a blocked heater).
Cloudy water even though the filter is running	Channelled sand, too few hours of filtration, or chemistry out of range. Test the water.
Low pressure and weak returns	A full skimmer or pump basket, or an air leak on the suction side.

Planning a pool or spool build?

Getting the filter, pump and pipework sized together is exactly what my plans do. My plunge pool and cocktail pool plans (for example the 14ft x 7ft plunge pool plans) come with the full plumbing layout and a parts list, and the hot tub plans do the same for tubs. If you’ve got a custom shape in mind, design it in the free Tub & Pool Designer Pro software and I can turn it into architectural CAD and a 3D plumbing design.

For filter sand, glass media and valve spares, have a look at parts4tubs.co.uk in the UK and parts4tubs.com in the USA.

FAQs

How often should I backwash my sand filter?

When the pressure gauge reads about 8–10 psi (0.55–0.7 bar) above its clean reading. For a busy pool in summer that might be weekly. For a quiet, covered pool it could be much less often.

How long should I backwash for?

Until the sight glass runs clear, which is usually around two minutes. Then rinse for 30–60 seconds.

How often do you change the sand in a pool filter?

Typically every 3–5 years. Change it sooner if the pressure rises quickly after backwashing or the water won’t clear.

Is glass media better than sand?

It filters finer, needs less backwashing and lasts longer, but costs more upfront. For most pools it’s a worthwhile upgrade, though sand still works perfectly well.

Why is there sand in my pool?

Usually a cracked lateral or standpipe inside the filter, or sand that’s the wrong grade. Open the tank and inspect the laterals.

Can I leave my multiport valve on Recirculate?

Only for short periods. Recirculate bypasses the filter completely, so the water isn’t being cleaned.

Got a question about your sand filter? Pop it in the comments below. I read them all.

Cheers,
Andi

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Hi, Andi here. I own Buildahottub.com and also write all of the articles and info pages on the site. Some years back now, I built my own hot tub but struggled to find the information I needed. So, once my tub was complete, I started this website to help others in their own pursuit of hot tub and plunge pools DIY building information.

Fast forward to 2025, I've helped over 1400+ DIY customers just like you all over the world build hot tubs and pools. Have a good look around the site, there are lots of resources here. Please do get in touch if I can help you. - Cheers, Andi