Do You Need Ozone in a Hot Tub? Why I Don’t Use It (and What I’d Use Instead)

Last updated: September 2026

One of the things I get asked about most is whether a hot tub needs an ozonator. Shop-bought tubs often come with one, pool shops love selling them, and a lot of people building their own tub assume it’s a must-have. My answer is no, and I say that as someone who fitted one to my own DIY hot tub and ended up throwing it in the bin.

In this post I’ll explain what ozone actually does, why I don’t fit or specify it, what I’d use instead, and the one thing no gadget changes: you still need a measurable level of chlorine or bromine in the water.

What does an ozonator do in a hot tub?

An ozonator makes ozone (O₃) from air and injects it into the water, usually through a venturi injector in the circulation plumbing. There are two main types:

• Corona discharge (CD): air passes through a high-voltage field, which splits oxygen molecules and lets them recombine as ozone.
• UV ozonators: a UV lamp inside the unit turns oxygen in the air into ozone. (This is different from a UV-C steriliser, which shines UV light straight into the water. More on that below.)

Ozone is a very strong oxidiser. As the bubbles pass through the plumbing, it breaks down body oils, sweat and other organics and kills bacteria it comes into contact with. That’s why ozone tubs can have clearer water, less foam and less “chlorine smell”.

The catch is that ozone doesn’t hang around. It breaks down quickly, so almost all of the work happens in the pipe and the injector, not in the tub where people are sitting. The Pool & Hot Tub Alliance says ozone systems can leave only a small residual of around 0.1 ppm in the water, and still require a primary halogen such as chlorine or bromine. The UK HSE’s spa guidance (HSG282) is just as clear: ozone may be used, but it must be in conjunction with residual disinfection.

So it’s not chemical-free?

No. This is the biggest misconception. Ozone can reduce how much sanitiser the water uses up, because it takes some of the load off. But you still need free chlorine at 3–5 ppm or bromine at 4–6 ppm in the tub. Those are the levels I work to in my hot tub chemical levels guide, and they match the CDC’s advice of at least 3 ppm free chlorine in a hot tub and the HSE’s 3–5 mg/l. Some manufacturers suggest running lower levels with ozone. If you do, follow your tub maker’s instructions exactly and test more often, not less.

Ozonator pros and cons

Pros	Cons
Strong oxidiser that breaks down oils and organics	Doesn’t leave a useful residual, so you still need chlorine or bromine
Can reduce sanitiser use, foam and odour	Extra plumbing: injector, check valve and a Hartford loop to keep water out of the unit
Built into many shop-bought tubs, so there’s nothing to fit	Parts wear out: Leslie’s puts UV ozone bulbs at about 2–3 years and CD chips at 2–5 years, with check valves and tubing replaced yearly
Works quietly in the background once it’s set up	It fails silently, so you often don’t know it’s stopped working
	Ozone gas collects under the cover and can damage the vinyl underside of the cover, and it’s not something you want to breathe

On that last point, ozone is an irritant gas. The HSE’s commercial spa guidance says the air above the water shouldn’t exceed 0.01 ppm of ozone, and that commercial ozone systems normally remove it from the water (deozonise) before it returns to the pool. A home ozonator is much smaller, but it’s one more reason to lift the cover and let the tub breathe before you get in.

How do I know if my ozonator is working?

• Look for a steady stream of fine “champagne” bubbles coming from the ozone injector or the return it feeds, while the circulation pump is running. No bubbles usually means a blocked injector, a failed check valve or a split tube.
• Check the unit’s indicator light, if it has one. On UV ozonators a lit bulb doesn’t prove it’s still making ozone, because the bulbs lose output with age.
• Check the tubing and check valve for water. Water in the ozone line is the classic failure, and it’s what the Hartford loop is there to stop.
• Watch your sanitiser demand. If you suddenly need more chlorine or bromine and get more foam, and nothing else has changed, the ozone may have quietly stopped.
• Check the age. If the bulb or chip is past the maker’s replacement interval, assume it’s tired.

My experience with ozone: straight in the bin

My journey with ozone started when I was building my own DIY hot tub. “Pool Man John” suggested I add an ozonator to my setup. I didn’t buy a cheap one. It was a £200/$250 Balboa unit.

From the start it was a bit of a disaster. Connecting the unit to the spa pack wasn’t a problem. That part was straightforward. It was everything else involved in setting the system up correctly.

You hear how ozone is dangerous if you breathe it in, so that had me a little worried from the outset. First, you need a Hartford loop to stop water getting back to the unit. Then you need a one-way (check) valve. Both were fairly easy to do, although in my opinion they make your control room look a mess.

The real problem was the venturi, which is how the ozone gets drawn into the water. I just could not get it to work. On top of that, the setup made my hot tub leak every time the jets were turned on, and water went everywhere in the control room. The pressure was just too much.

After plugging leaks left, right and centre and trying to live with it for a month or two, I gave up. I cut the unit out, which by then was blocked up and full of “gunk” because it had never worked properly, and threw it straight in the bin. Best thing I did! I piped up the gap and, guess what, no leaks.

Because it had never worked, I didn’t feel I was missing out on anything, and my hot tub stopped leaking. For me, ozone is a bit old school. Of the first 600-odd customers I helped with their hot tub builds, I never specified, designed in or supplied an ozone unit, and I talked everyone who asked about one out of it. Having now helped more than 1,500 people, I haven’t changed my mind. If you want to see how my equipment is laid out today, have a look at my hot tub control room remodel.

What’s a better alternative to ozone?

For me, a much better add-on is UV-C: an ultraviolet steriliser plumbed into the circulation line.

UV light sits between visible light and X-rays on the spectrum, and it’s split into UVA, UVB and UVC. UV-C (around 254 nanometres) is the band that’s good at killing microorganisms. As water flows through the unit’s chamber, the lamp damages the DNA of bacteria and other microorganisms passing through so they can’t reproduce.

How well it works depends on the lamp’s power, the flow rate and how long the water spends in the chamber. The slower the flow and the stronger the lamp, the bigger the dose. That’s why spa UV units are rated for low flows. Blue Lagoon’s 21W spa unit, for example, has a recommended flow of 3 m³/h (about 13 US gpm) and a maximum of 13 m³/h.

UV-C vs ozone at a glance

	Ozone	UV-C
How it works	Oxidises contaminants in the pipe	Inactivates microorganisms in the chamber
Still need chlorine or bromine?	Yes	Yes
Plumbing	Venturi injector, check valve, Hartford loop	Inline chamber on the circulation line
Gas under the cover	Yes	No
Main maintenance	Bulb or chip, check valve, tubing, injector	Replace the lamp and clean the quartz sleeve as the maker says
Easy to tell if it’s working?	Not really	Most units have a viewing window or indicator

One thing both have in common: UV only treats the water inside the unit. The HSE puts it plainly: UV only has a bactericidal action at the point of use, so additional disinfection, such as residual chlorine, is required. What UV does do well is take a load off your sanitiser and keep the water sparkling. Makers claim big reductions in chlorine use, but the level in the tub still has to be in range.

Installing a UV-C unit

Installation is pretty straightforward. Here’s how I do it:

• Plumb it in series on the circulation line, after the filter so the water going through it is clear. On my builds it goes after the heater or spa pack. Always check your unit’s manual for its preferred position and orientation.
• Keep the flow low. Spa UV units are designed for circulation-pump flows, not jet-pump flows. If you don’t have a circulation pump, fit a bypass so only part of the flow goes through the UV unit and your jets still get the flow they need. There’s more on this in how to add a circulation pump to a hot tub.
• Power it with the circulation pump. The units draw very little current, so I like to power them from the circulation pump’s output on the spa pack. That way the lamp only runs when water is flowing through it. This may not be how the manufacturer designed it, but it works well for me. Check the unit’s rating against the pack’s output, and have a qualified electrician do the wiring.
• Leave access for lamp changes. You’ll need room to pull the lamp and quartz sleeve out. Plan it in when you design your control room.

What about a “chemical-free” hot tub?

Lots of people ask me for one. The honest answer is that there’s no such thing as a safe hot tub with nothing in the water. A hot tub is warm, shallow and shared, which is exactly what bacteria like Pseudomonas and Legionella enjoy. UV, ozone and minerals all help, but none of them leaves a residual in the tub.

If you want to avoid chlorine and bromine, the option the HSE lists for spas is silver-stabilised hydrogen peroxide (SSHP), often sold as an “active oxygen” system. HSG282 describes a starting shock dose of 100 ppm for at least 12 hours, then a hydrogen peroxide residual of around 50 ppm (30–70 ppm). Paired with UV, that’s a route some people are happy with. A few things to know:

• It’s still a chemical, and you still test for it, with peroxide test strips rather than chlorine tests.
• Don’t mix systems. Hydrogen peroxide and chlorine or bromine react with each other and cancel each other out. If you switch, drain, clean and refill first, and follow the product’s own instructions.
• “Active oxygen” on a label can mean different things. Some products are hydrogen peroxide, and others are MPS (potassium monopersulfate), which is a non-chlorine shock. MPS oxidises but doesn’t sanitise on its own, so check what you’re buying.

For the full run-down of the options, including salt, bromine and minerals, see are there any alternatives to chlorine for my hot tub?, bromine or chlorine?, saltwater hot tubs and copper-silver ionisation.

Common mistakes

• Stopping testing because “the ozone does it”. It doesn’t. Test chlorine or bromine and pH as normal.
• Letting sanitiser levels drop to nothing. Clear water isn’t the same as safe water.
• Skipping the check valve or Hartford loop on an ozonator, then wondering why water has reached the unit.
• Putting a UV unit on the full jet-pump flow. The water rushes through too fast to get a proper dose, and you can restrict your jets.
• Forgetting to shock. Neither ozone nor UV replaces a regular shock after heavy use. See hot tub shock.
• Blaming the gadget for foam or cloudy water when the real cause is water balance or tired water. See foam in a hot tub and cloudy hot tub water.

Need a UV unit or parts?

You’ll find UV-C systems and replacement lamps at parts4tubs.co.uk in the UK, and hot tub parts for the USA at parts4tubs.com. If you’re still at the planning stage, my DIY hot tub plans include the full plumbing layout, with a circulation line that’s ready for a UV unit.

FAQs

Do I need an ozonator in my hot tub?

No. It’s optional. A properly filtered tub with the right chlorine or bromine level and balanced water is safe and clear without one. If you want an add-on to reduce chemical use, I’d choose UV-C instead.

Can I use ozone instead of chlorine?

No. Ozone doesn’t leave a meaningful residual in the tub, so you still need chlorine (3–5 ppm free chlorine) or bromine (4–6 ppm). The HSE and the Pool & Hot Tub Alliance both say ozone must be used with a residual sanitiser.

Is ozone in a hot tub dangerous?

A correctly working home ozonator puts out very little, but ozone is an irritant gas and it collects under the cover. Take the cover off for a few minutes before getting in, and don’t sit with your face over the ozone return.

How long does a hot tub ozonator last?

It depends on the type. UV ozone bulbs typically last around two to three years, and corona discharge chips a few years more. The check valve and tubing are usually replaced every year.

Is UV better than ozone for a hot tub?

For a DIY build, I think so. It’s simpler to plumb, easier to check, and doesn’t put gas under the cover. Both still need a residual sanitiser.

Can I turn my ozonator off?

Yes. Plenty of people unplug a failed ozonator and carry on with normal chlorine or bromine. If you remove it from the plumbing, cap or pipe the injector port properly so you don’t create an air leak.

Have you had an ozonator that’s worked brilliantly, or one that drove you mad like mine did? Tell me in the comments below. I read them all.

Cheers,
Andi

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Thanks - Andi

 

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