Real-life case: the hot tub that smelled of rotten eggs
- The situation: a strong rotten-egg (sulphur) smell on opening the hot tub — different from a chlorine smell.
- Cause: bacterial growth in under-sanitised water and/or biofilm in the jet plumbing.
- Solution: sanitising shock, pipe cleaning (biofilm flush), often a water change.
- Prevention: sustained sanitisation, regular pipe cleaning, clean filter.
It wasn’t a chlorine smell. On lifting the cover, a clear waft of rotten eggs — sulphur, that smell that makes you step back. The first reflex was the wrong one: “too much product, I’ll air it out”. Complete misdiagnosis. The sulphur smell in a hot tub doesn’t tell a story of excess chemistry: it tells a story of bacteria, and water that no longer defends itself. Here’s what was hiding in the pipes, and how the hot tub got its healthy water back.
Sulphur ≠ chlorine: two smells, two opposite causes
This is the confusion to clear up straight away. The chlorine smell (sharp, “swimming pool”) signals chloramines — spent sanitiser. The rotten-egg smell (sulphur, hydrogen sulphide) signals something else entirely: bacterial activity in under-sanitised water. Both share one thing, and it’s the most counter-intuitive: in both cases, active sanitisation is lacking, not the reverse. Airing it out or “letting it rest” only makes it worse.
The prime suspect in a hot tub: the biofilm of the jet plumbing. That warm water circulating through dark pipes is a paradise for bacteria. Surface treatment of the water doesn’t reach it — the colony thrives in shelter, and releases its smells every time the jets fire up.
The deep clean
Treating the visible water wasn’t enough — you had to go into the pipes. 1. Sanitising shock on the water in place, dosed on the hot tub’s volume, jets open to circulate the sanitiser through the whole circuit. 2. Pipe flush: a circuit-cleaning product (line flush) added to the water, jets running for a good while to lift the biofilm — and there, the show: greyish residues come back up, the proof of what was hiding. 3. Full drain: on a small colonised volume, restarting from fresh water is simpler and safer than recovering. 4. Rinse, filter cleaned, refill and rebalance — pH, sanitisation, and a sanitiser suited to warm water (bromine, naturally stable at 37 °C).
Result: no more sulphur smell, and water that finally smelled of… nothing. Which, for a hot tub, is the best of perfumes.
Prevention — so it doesn’t come back
Biofilm comes back if you let it. The routine adopted: genuinely sustained sanitisation (an under-sanitised hot tub is an invitation), photo test twice a week (small volume, it moves fast), pipe flush at every water change, and filter cleaned regularly. The app schedules this maintenance as reminders — the drain at the right time depending on the age of the water and the usage, the circuit flush, the eye on the sanitiser.
What this case teaches
A hot tub’s rotten-egg smell is a signal of under-sanitisation and biofilm, not excess product — the reflex “I’ll air it out and wait” makes it worse. The real solution runs through the pipes, where surface treatment reaches nothing. It’s exactly the kind of invisible maintenance you forget: the app plans it for you, calculates the shock suited to the small volume, and reminds you of the circuit flush. A well-maintained hot tub smells of nothing — and that’s precisely the goal.
Frequently asked questions
Why does my hot tub smell of rotten eggs?
The sulphur smell (hydrogen sulphide) generally signals bacterial activity in insufficiently sanitised water, often nesting in the biofilm of the jet plumbing. It isn't a chlorine smell: it's the sign that sanitisation is lacking, not that there's too much of it.
How do I remove biofilm from a hot tub's pipes?
A pipe-flush product (line flush) circulates a cleaner through the whole jet circuit to lift the biofilm, followed by a drain and a rinse. It's the step that simple water treatment doesn't do — hence the smells that come back if you skip it.
Should I drain a hot tub that smells bad?
Often yes, after flushing the pipes: colonised water is more easily renewed than recovered. On a hot tub's small volume, draining is quick and restarts from a healthy base.