Smelly or Discoloured Hot Water: Is It the Cylinder?

Is it only the hot water that is smelly or discoloured?

This is the one test that decides the whole page, and it takes two minutes. Fill a clean white bowl or a clear glass from the cold tap in the kitchen, then another from the hot tap, and compare them side by side. Do the same at a second outlet, ideally one in a different room.

If both are discoloured or both smell, the cylinder is not the source, because cold water at the kitchen tap normally comes straight from the main and does not pass through it. The usual cause is then the incoming supply or the pipework before it. Disturbed deposits in water mains can turn water brown, orange or white, according to the Drinking Water Inspectorate, so the first call is to your water company. If the fault comes back repeatedly, the Inspectorate notes the supply pipe into the property may be worth checking.

If only the hot water is affected, the stored water or something that touches it is the cause. That is where a cylinder diagnosis begins. For anything about whether the water is safe to drink or wash in while you wait, follow your water company or Drinking Water Inspectorate guidance. Cylinder checks cannot answer that.

Why does my hot water smell of eggs?

A rotten-egg smell is hydrogen sulphide, and it is produced when certain bacteria feed on sulphur compounds that are naturally in the water. Warm water that has sat still is a good place for them. The smell is most often linked to stored water that stays lukewarm for long periods. Some tanks contain a sacrificial anode, which can feed the reaction, but many UK cylinders do not have one, so the engineer first checks what yours contains rather than assuming.

The pattern tells you more than the smell. If the smell only appears after the house has been empty, or the cylinder has been switched off for some time, stagnation is the likely cause. If it appears every day, a cylinder that never reaches a high enough temperature is a better suspect. Hot water should be stored at 60°C or above, and reach the outlets at 50°C or above within a minute of running, according to the HSE guidance on hot and cold water systems. A cylinder with a failed element, a thermostat set low or a coil that is not transferring heat will sit in the warm range, and that range suits bacterial growth.

This links the smell to faults covered elsewhere on this site. If the cylinder is not getting hot, start with hot water cylinder not heating up and how to reset a hot water cylinder.

Why is my hot water brown, orange or black?

Colour points to the cause more reliably than smell does. The common sources, in the order an engineer would consider them, are these.

• Sediment: scale flakes and sand-like deposits settle at the base of the cylinder and come out when water is drawn hard, giving a cloudy white or tan tint and grit in the bottom of a glass
• Rust from steel components: iron in older pipework or fittings can colour only the hot side if the stored water has dissolved it, and it often clears after a minute of running
• Primary water mixing in: on an indirect cylinder, a failed heating coil can let water from the heating circuit into the stored water, which is dark brown or black, may smell of inhibitor and often comes with a falling boiler pressure or a topping-up feed tank
• Corrosion of the cylinder itself: typically shows as rust-coloured water together with a leak or a damp patch at the base

What can I safely check before an engineer visits?

Keep the checks to things that do not involve opening the cylinder, touching valves or working on electrics. Run each hot tap for two minutes and note whether the colour or smell fades. Fading points to sediment or pipework that has been disturbed. Staying constant points to the cylinder or the coil.

Check the temperature setting if there is a dial on the outside and record it, but do not turn it up in the hope of curing the smell. Note whether the heating feed tank in the loft, if you have a vented system, keeps needing water, because that points at the coil. Look at the base of the cylinder and the pipework for damp, staining or drips.

Do not drain the cylinder yourself unless you have been shown how. Draining an unvented cylinder involves isolating the electrics and the supply first, and reconnection involves the expansion vessel and the safety valves, which are not householder work. Draining a hot water cylinder safely explains the sequence and where the limits are.

What does the engineer check, and does it mean replacing the cylinder?

The order starts with the cheapest explanations. The engineer first confirms the hot-only pattern, then checks the stored temperature at the cylinder and at the taps, because a low temperature explains smell without any part having failed. Next comes the heat source: element and thermostat on an electric or immersion cylinder, or flow and return temperatures across the coil on an indirect one. Where a coil leak is suspected, the engineer looks for pressure loss on the heating side and for primary water appearing in the cylinder.

The cylinder is then inspected for sediment and scale, and for signs of corrosion at the base and at the connections. A flush cures sediment. A failed element, a thermostat set too low or a stuck two-port valve are repairs, and the smell often goes once the cylinder reaches temperature and is run through properly.

Replacement is on the table only when the shell is corroding through or a coil has failed in a way that is not worth repairing. Cylinder repair or replace sets out the decision rule. The hot water cylinder diagnosis service covers this sequence, and hot water cylinder lifespan and signs of failure helps if the cylinder is already old.

This page does not give advice on health effects. If anyone is unwell and you think the water is the reason, speak to a doctor, and report the water quality to your water company.