Sulphur smell from the hot tap is almost always bacteria in the hot water cylinder — easy to fix. From the cold tap, contact your water company.
If the rotten egg smell comes from the cold tap as well as the hot, this could indicate contamination of the mains supply. Contact your water company immediately.
Hot water cylinder bacteria (most common). Sulphate-reducing bacteria can grow in domestic hot water cylinders that are not maintained at the correct temperature. These bacteria convert sulphates (naturally present in water) into hydrogen sulphide gas — which has the characteristic rotten egg or sulphur smell. This is almost exclusively a hot water issue and is entirely preventable.
Bacterial growth in tap aerators. The fine mesh aerator fitted at the end of most taps is a warm, moist environment where bacteria can accumulate. A biofilm here can produce localised smells. This affects only the immediate flow rather than the whole supply.
Stagnant water in unused pipes. Holiday properties or taps that are rarely used can develop bacterial growth in stagnant water. Running taps for 2–3 minutes flushes this out.
Cold tap smell (rare, serious). If the cold mains supply smells of sulphur or drains, this is unusual and should be reported to your water company immediately.
Hydrogen sulphide from a hot water system is generally harmless at domestic concentrations. However, the bacteria responsible for producing it — including Legionella — can pose a health risk, particularly for elderly people, those with respiratory conditions, or the immunocompromised. Legionella is killed when water is maintained at 60°C.
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Frequently asked questions
This is the classic sign of bacteria growing in a hot water cylinder set below 60°C. Sulphate-reducing bacteria produce hydrogen sulphide gas, which smells of rotten eggs or sulphur. Raising your hot water temperature to 60°C kills these bacteria and resolves the problem within 24 hours.
If the smell comes only from your hot tap and your hot water is below 60°C, it is usually harmless bacteria producing hydrogen sulphide. If the smell comes from the cold tap, contact your water company immediately — this could indicate contamination of the mains supply.
Raise your hot water cylinder thermostat to 60°C and run the hot taps for 5 minutes to flush the system with the hot treated water. This kills sulphate-reducing bacteria. Also clean tap aerators (the mesh filter at the end of taps) as bacteria accumulate there.
Bacteria in domestic hot water systems set below 60°C can include Legionella, which causes Legionnaires' disease — a serious respiratory illness. Legionella risk is highest when water is between 20°C and 45°C, water is stored or recirculated, and in vulnerable people. Maintaining hot water at 60°C eliminates this risk.
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