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Hot Tub Water Smells: Chlorine, Musty or Rotten Eggs — and How to Fix Each

Lifted the cover and got a face full of strong chlorine, a musty mouldy whiff, or worst of all a rotten-egg sulphur smell? A hot tub shouldn't smell of anything much. When it does, the water is telling you something — and in almost every case the real problem isn't the water you can see, it's chloramines or biofilm hiding in the pipes and filter where your sanitiser can't reach.

The good news: the chlorine smell is an easy same-day fix. The musty and eggy smells take a bit more work but are still DIY. This guide tells you exactly which smell means what, and the step-by-step fix for each.

The Chlorine Smell — The Counter-Intuitive One

This is the single most misunderstood hot tub smell. People assume a strong "swimming pool" chlorine smell means they've put in too much chlorine and they should add less. The opposite is true.

That smell comes from chloramines (also called combined chlorine) — what's left when free chlorine reacts with sweat, body oils, urine, deodorant and other organic waste from bathers. Once chlorine is bound up as chloramines it's spent: it smells strongly but no longer sanitises. So a sharp chlorine smell actually means your free (active) chlorine is too low.

The fix is breakpoint chlorination — a shock dose that adds enough chlorine to blast through and break apart the chloramines, freeing up active chlorine again. Add chlorine granules at the shock rate (about 20g per 1000 litres) with the pump running and the cover off for the first hour. The smell usually clears within a few hours. A non-chlorine shock (MPS / oxygen shock) does the same job if you prefer.

The Musty / Mouldy Smell — Biofilm in the Pipes

A musty, earthy or "old water" smell that keeps coming back no matter how much you shock the tub is the classic sign of biofilm. Biofilm is a slimy bacterial layer that grows on the inside of the pipes, jets, air channels and filter — anywhere water sits and sanitiser struggles to reach.

It forms from the oils, grease, lotions and dead skin bathers leave behind. Once established it shields the bacteria inside it from chlorine, which is why you find yourself adding more and more sanitiser for less and less effect. Other early signs: cloudy water, a creeping rise in your chlorine demand, and a whiteish scum tide-mark around the waterline.

Shocking the open water won't shift it because the biofilm lives where the shock can't reach. You need a pipe-purge / system-flush product (e.g. a spa flush). Add it, run all the jets and the air blower for 20-30 minutes so it scours the internal pipework, then drain it all away, deep-clean the filter, and refill. This is the step most people skip — and it's the one that actually fixes a recurring musty smell.

The Rotten-Egg / Sulphur Smell — Take It Seriously

A rotten-egg or sulphur smell is biofilm gone further. When severe, anaerobic bacteria in the stagnant parts of the plumbing produce hydrogen sulphide — the eggy gas. It means the water is no longer properly sanitised and bacteria are thriving.

Treat this one as a "don't get in yet" situation. Shock the tub hard, run a system flush through all the jets and air channels, then drain, clean and refill with fresh water. If the smell returns quickly after a fresh fill, the biofilm is entrenched and may need a second, stronger flush — or an engineer to check for dead-leg pipework where water stagnates.

Smell Diagnosis: Quick Reference

Smell	Most Likely Cause	Fix
Sharp chlorine / "swimming pool"	Chloramines — spent chlorine, free chlorine too low	Shock dose (breakpoint chlorination); restore free chlorine to 3-5mg/L
Musty / mouldy / earthy	Biofilm building in the pipes and filter	Flush the plumbing, deep-clean filter, drain & refill
Rotten eggs / sulphur	Advanced biofilm + anaerobic bacteria (hydrogen sulphide)	Hard shock, system flush, drain & refill — don't use until sanitised
Chemical / harsh, with eye sting	Chloramines + low pH; over-acidic water	Balance pH to 7.2-7.6, then shock
Stale / "flat" with cloudy water	High TDS — water been in too long	Drain & refill; see our cloudy & foamy water guide

How to Get Rid of the Smell: Step-by-Step

1. Identify the smell — chlorine (chloramines), musty (biofilm) or eggy (advanced biofilm). This decides whether shocking alone will do it.
2. Test and balance — use 6-in-1 strips. Bring pH to 7.2-7.6 first; chlorine works far better in range (at pH 7.2 most chlorine is the active killing form, at pH 8 most of it is inert).
3. Shock dose — chlorine granules at the shock rate (~20g per 1000 litres), pre-dissolved in warm water, pump running, cover off for the first hour. Clears a chlorine smell within hours.
4. Flush the plumbing (for musty/eggy) — add a spa pipe-flush, run all jets and the blower 20-30 minutes, then drain it away. This is the step that fixes recurring smells.
5. Deep-clean the filter — soak the cartridge in cartridge cleaner overnight; replace if over 12 months old. Biofilm hides in a dirty filter.
6. Clean the cover and scum line — mould often grows on the cover's underside; scrub it and the waterline tide-mark.
7. Drain and refill — for musty/eggy smells, finish with a fresh fill, re-balance and sanitise. The smell should be gone.

Bournemouth-Specific Tips

• Warm summers accelerate biofilm — biofilm grows fastest in warm water with heavy use. Through a hot BCP summer, run a pipe flush at every water change (every 2-3 months) rather than skipping it.
• Hard water + biofilm — Wessex Water's moderately hard supply (~200-250mg/L) lets scale and biofilm bind together inside pipes. A scale inhibitor and a hose pre-filter on refill help keep the plumbing clean and protect the heating element.
• Detergent residue — washing swimwear with detergent or fabric softener feeds both foam and biofilm. Rinse costumes in plain warm water only.

When a Smell Means a Mechanical Problem

Occasionally a smell that won't clear despite a flush and refill points to something the chemicals can't reach:

• Dead-leg pipework — sections of pipe where water barely circulates let biofilm and bacteria entrench. These need a stronger flush or physical access.
• Weak circulation — if the pump runs but water barely moves, sanitiser and flush can't reach the whole system. See our E02 flow-error guide.
• Failed air blower / jets — stagnant air channels harbour smells; see jets not working.

Prevention: Keeping the Water Fresh & Odour-Free

Task	Frequency	Why It Matters
Test free chlorine, pH & alkalinity	2-3 times per week	Low free chlorine is what lets chloramines and biofilm form
Shock dose	Weekly + after heavy use	Breaks chloramines before they build into a smell
Flush the plumbing	Every water change (2-3 months)	Strips biofilm out of the pipes before it takes hold
Deep-clean / replace filter	Soak monthly, replace every 12 months	A dirty filter is a biofilm reservoir
Clean the cover underside	Monthly	Mould on the cover causes a musty smell on its own
Shower before use & rinse swimwear without detergent	Every time	Removes the oils that feed biofilm and chloramines
Full water change	Every 2-3 months (monthly if heavy use)	Resets high TDS and clears stale-water smell

Products You'll Need

All available at B&Q Bournemouth, Amazon, or directly from Lay-Z-Spa.co.uk:

• 6-in-1 test strips — chlorine, bromine, pH, alkalinity, hardness (~£8 for 50)
• Chlorine granules — for dosing and shock (~£8-12 for 1kg)
• Non-chlorine (MPS) shock — alternative oxidiser to break chloramines (~£8-10)
• Spa pipe-flush / system purge — strips biofilm from internal plumbing (~£8-12)
• Filter cartridge cleaner — overnight soak (~£6-9)
• pH Plus / pH Minus — to balance the water before shocking (~£6-8 each)
• Hose pre-filter — reduces minerals on refill (~£20, reusable)

Frequently Asked Questions