Can I Use a Sand Filter on My DIY Hot Tub?

Last updated: September 2026

Can I use a sand filter on my DIY hot tub? It’s one of the questions I get asked a lot, often from people who already have a sand filter from a pool, or who’ve been told by a pool shop that it’s “the same thing”. I’ll let you into a secret: the answer is yes, but only if you plumb it the right way.

In this post I’ll explain when a sand filter works on a hot tub, when it doesn’t, how to size one, and why I still fit a cartridge filter to most DIY tubs.

Yes, you can, with a dedicated pump

Let’s start with the answer. You can use a sand filter on your DIY hot tub, but your plumbing needs a separate filtration loop with its own pump. You’d normally have that kind of setup anyway if you’re heating with an air source heat pump, gas, propane or any other external heater rather than the spa pack’s own heater. There’s more on that sort of layout in How many pumps do you need? and How to add a circulation pump to a hot tub.

What you can’t do is put the sand filter on the jet plumbing. It all comes down to flow rates.

Why a sand filter can’t go on the jets

A hot tub jet needs a lot of water. Waterway’s gunite jet instructions rate their jet internals at 10 GPM each. So a 16-jet tub needs around 160 GPM (about 36m³/h) at the jets.

Now compare that with a sand filter. Hayward’s 24in ProSeries sand filter, which is a big residential pool filter, has a design flow rate of 62 GPM. Push much more than that through it and you’re over its rating. The pressure climbs, the filter works badly, and the jets are starved.

Try to put a dual-speed jet pump through a sand filter on high speed and you just won’t get the flow you need for the jets. There’s too much resistance. I’ve covered this from the pool side in Why pool and hot tub plumbing should stay separate.

Why not use the spa pack’s circulation pump?

The small circulation pumps used with spa packs are designed to push a modest flow through a cartridge and a heater. A sand filter needs decent flow to filter properly, and even more to lift the sand bed during a backwash. A little circ pump won’t do either well.

So the setup that works is a proper filtration pump, sized for the sand filter, on its own loop: suction from the skimmer (and drain), through the pump, through the sand filter, then the heater and sanitiser, and back through the tub’s wall returns. The jet pump and jets stay on their own plumbing. If you’re buying the filtration pump, have a read of Pool pump upgrade: top tips. On this loop, a variable-speed pump makes a lot of sense, as I explain in Can I use a variable speed pump in a hot tub?

How a sand filter works

A sand filter is a pressure tank filled with graded filter sand. Water enters at the top, runs down through the sand bed, where the dirt is trapped, and is collected by slotted “laterals” at the bottom before going back to the tub. A multiport valve on top lets you switch between filter, backwash, rinse and a few other settings.

As the sand fills with dirt, the pressure on the gauge rises. That’s your cue to clean it by backwashing, which reverses the flow up through the sand and sends the dirt out to waste. I’ve explained every valve position, and when to change the sand, in Sand filters explained.

How to backwash

1. Switch the pump off. Never move the multiport valve with the pump running.
2. Set the valve to Backwash, make sure the waste line is open, and switch the pump on.
3. Run it until the sight glass runs clear. Hayward’s ProSeries manual says about two minutes or less.
4. Pump off, set to Rinse, and run for half a minute to a minute.
5. Pump off, back to Filter, pump on, then top the water up.

Backwash when the pressure gauge reads 8–10 psi (0.55–0.69 bar) above its clean reading, which is Hayward’s figure, rather than on a fixed weekly schedule. On a hot tub, remember you’ll be topping up with cold water, then reheating it and rebalancing the chemicals after every backwash.

How to size a sand filter for a hot tub

1. Work out your volume. Length × width × average water depth. A typical DIY tub holds 2,000–3,000 litres (530–790 US gallons).
2. Know your pump’s flow. Use the pump curve at your system’s head, not the headline figure. My guide to correctly sizing a hot tub pump explains how.
3. Pick a filter rated for at least that flow. Every sand filter has a design flow rate on its label or spec sheet. Your pump’s flow must not be higher than it. Going a size bigger does no harm.
4. Check the turnover. In a small tub it’s easy. A 2,500-litre (660-gallon) tub on a loop moving 30 GPM is turned over roughly every 22 minutes.

Use the sand grade on the filter’s label. Hayward specifies No. 20 silica sand (0.45–0.55mm), which is typical. Glass media is a popular upgrade that filters finer and needs less backwashing.

Sand vs cartridge on a hot tub

	Sand filter	Cartridge filter
Typical filtration	20–40 microns	10–20 microns
Cleaning	Backwash and rinse, water to waste	Remove and hose off
Pump needed	A dedicated filtration pump	A circ pump or a dual-speed pump on low
Extra kit	Multiport valve and a waste line	Just the housing
Media life	Sand every 3–5 years	Cartridge every 1–2 years
Best for	Spools, plunge pools, pools	Most DIY hot tubs

Sand filters are simple, durable and cheap to run on a pool. But on a hot tub, a cartridge filters finer (Trouble Free Pool puts sand at 20–40 microns), needs no backwashing and works happily off a circulation pump. That’s why almost all my customers fit one. The full comparison is in Cartridge or sand filter for a hot tub?

When a sand filter makes sense

• You’re building something bigger than a hot tub. For a spool, cocktail pool or plunge pool, the backwash water is a small share of the volume, and sand copes well with leaves and garden debris.
• You already have a separate filtration and heating loop, for example with an air source heat pump. See the guide to air source heat pumps for hot tubs.
• You’d rather not buy and clean cartridges, and you have somewhere sensible for the backwash water to go.

Common mistakes

• Plumbing the sand filter into the jet loop.
• Running it off the spa pack’s small circulation pump.
• Choosing a pump whose flow is higher than the filter’s design flow rate.
• Moving the multiport valve with the pump running, which damages the valve’s gasket.
• Backwashing on a timetable instead of by the pressure gauge.
• Forgetting that backwash water carries sanitiser. Check where you’re allowed to discharge it.

Troubleshooting

• Weak jets after adding a sand filter: it’s on the jet loop. Move it to a separate filtration loop.
• Heater flow errors: the sand filter needs backwashing, or the pump is too small to push water through it. See Hot tub FLO error.
• Cloudy water: the filter needs a backwash, isn’t running for long enough, or the chemistry is out. See cloudy water in a hot tub.
• Sand in the tub: a cracked lateral or the wrong grade of sand. There’s more in How to remove sand from a hot tub.

Planning your own build?

The filter, pumps and heater all need sizing together, and it’s much easier on paper than in a finished control room. My DIY hot tub plans include the full plumbing layout, with separate jet and filtration loops, and a parts list. Building a plunge pool or spool instead? There are plunge pool and cocktail pool plans too.

Filters, cartridges and filter sand are available from parts4tubs.co.uk in the UK and parts4tubs.com in the USA.

FAQs

Can you use a pool sand filter on a hot tub?

Yes, on a separate filtration and heating loop with its own pump. Don’t put it on the jet plumbing, because it can’t pass the flow the jets need.

Is a sand filter or cartridge filter better for a hot tub?

For most hot tubs, a cartridge. It filters finer, needs no backwashing and works with a small circulation pump. Sand suits bigger volumes like spools and plunge pools.

What size sand filter do I need for a hot tub?

One whose design flow rate is at least the flow of the pump driving it. The tub’s small volume means turnover is rarely the limit. The pump’s flow is.

How often should I backwash a hot tub sand filter?

When the pressure gauge reads about 8–10 psi (0.55–0.69 bar) above its clean reading. Then top up, reheat and rebalance the water.

Can I run my hot tub jets through a sand filter?

No. A 16-jet tub needs around 160 GPM, and even Hayward’s large 24in residential sand filter is rated at 62 GPM.

Thinking about a sand filter for your build? Drop your question 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