Why Does My Hot Tub Smell? What Each Smell Means and How to Fix It

As the evenings get colder, most of us soak a little less often and the cover stays on for longer between uses. That's often when owners first notice it: you lift the cover after a few days and the spa doesn't smell quite right. Each type of smell points to a different cause, so here's how to read it and put it right.

Start with a test, not a dose

Before you add anything, test the water. Our 3-in-1 Test Strips (£5.99 for 50) read free chlorine, pH and total alkalinity in one dip. Dip a strip, take it straight out, hold it level for 15 seconds and compare it with the chart on the bottle. Aim for:

• Total alkalinity: 80 to 120 ppm
• pH: 7.2 to 7.6
• Free chlorine: 1.0 to 1.5 ppm

Correct alkalinity first, then pH, then chlorine.

What each smell usually means

A sharp "swimming pool" chlorine smell

This surprises a lot of people, because a strong chlorine smell usually means there's too little working chlorine, not too much. Once chlorine has dealt with sweat, skin oils and cosmetics, the used-up chlorine stays in the water, and that's what you can smell. Adding more chlorine on top rarely fixes it, so measure every dose rather than adding an extra scoop.

Instead, add Non-Chlorine Spa Shock (1kg, £25) with the jets running. It oxidises the organic waste your filter can't catch and gets your chlorine working again without raising the chlorine level. Leave the cover off for a while afterwards, then retest. If anyone has sore eyes or itchy skin as well, our guide to itchy skin or sore eyes after a hot tub goes into more detail.

A musty, stale or earthy smell

This is organic build-up: body oils, skin cells and residue that the sanitiser and filter haven't kept up with.

1. Bring chlorine back into the 1.0 to 1.5 ppm range.
2. Shock the water with Non-Chlorine Spa Shock.
3. Take the filter out and rinse it well between the pleats.
4. For a deep clean, soak it overnight in Canadian Spa Hot Tub Filter Cleaner (1 litre, £20) mixed in a bucket of warm water, then rinse it and let it dry completely before refitting.

Check the filter's age too. Portable and inflatable spa filters need replacing every 4 to 6 weeks, and acrylic hot tub and swim spa filters every 6 to 12 months. Our guide to cloudy or musty hot tub water explains how shocking deals with this kind of smell.

A smell that keeps coming back, or a whiff when the jets start

If the water smells fresh after treatment but goes stale again within days, or you get a burst of smell when the jets first come on, the cause is often biofilm. That's a slimy layer of oils, lotions and skin residue that builds up inside the pipework, where shock can't fully reach it.

The fix is a pipe clean followed by fresh water. Add Spa Flush (£18.99) to the old water before you drain, following the label. It breaks through biofilm and fatty deposits so the water pressure can push them out of the plumbing. Badly fouled pipework may need more than one purge. Then drain, clean the filter and refill.

Old water is part of the same picture. Most owners change the water every 4 to 6 months, depending on how much the spa is used. With our chlorine, pH stays stable once you've adjusted it after the first fill from the tap. If your pH keeps going up and down, there's usually too much detergent in the water and a full change is the best fix. Our guide to how often to drain and refill a hot tub walks you through it.

A damp or mouldy smell from the cover

Sometimes the water is fine and the smell is coming from the underside of the cover, which spends the colder months sitting in warm, humid air. If you can see mould there, wipe it off with mild soap and water and let the cover dry in the open air. If it keeps coming back, check your sanitiser is in range.

Leave the cover open for 30 minutes after adding chemicals so the fumes don't attack the underside. If the cover has become noticeably heavier, water may have got into the foam core. A waterlogged cover can smell and insulates poorly, so it may be time to look at a replacement.

A perfumed or soapy smell

This comes from what's carried into the water, such as body lotion, hair products and washing powder left in swimwear, and it often comes with foam. Ask everyone to shower before they get in, and rinse swimwear in clean water rather than washing it in detergent or fabric conditioner. A Spa Ball (£15.30) sitting in the filter compartment absorbs surface oils and helps reduce foam and odours. When it sinks, it's full and needs replacing.

A burning, hot plastic or electrical smell

This one isn't a water problem. Stop using the spa. If you can do it safely and with dry hands, switch it off at the isolator. Don't open the equipment bay or control box. Call a qualified electrician if you can see scorching or melting at the socket or isolator, and call us for anything inside the spa. Our guide to a hot tub that keeps tripping the RCD explains which job belongs to whom.

A simple routine that keeps smells away

• Test two to three times a week, and the day after a busy soak.
• Shock weekly, or after heavy use, even if the water looks clear.
• Rinse the filter regularly and deep clean it at every water change.
• Shower before you get in, and keep detergent out of swimwear.
• Change the water every 4 to 6 months, with a pipe clean before you drain.

Hot tub smell questions

Is it safe to use the hot tub if it smells?

Test first. If the readings are out of range, or you can't get a chlorine reading at all, keep everyone out until the water is balanced and testing correctly. Never get in if you smell burning.

Why does my hot tub smell worse in winter?

Usually because it's used less often and the cover stays on for longer, so problems build up unnoticed between soaks. Test as often in winter as you do in summer.

If you've tested, treated and the smell is still there, call Canadian Spa Company on 01293 824094 and we'll help you work through your readings.