Hot water usually smells like rotten eggs because it contains hydrogen sulphide gas. When only the hot water smells, the likely source is a storage hot water system, where sulphate-reducing bacteria, anode-rod reactions or both can produce the odour. A smell alone cannot confirm that the water is safe, so avoid drinking or cooking with affected water until the source has been identified.
What causes the smell?
Sulphate-reducing bacteria can convert naturally occurring sulphates into sulphide in low-oxygen conditions. Warm, stagnant water may make this activity more noticeable.
Many storage tanks also contain a magnesium sacrificial anode. It corrodes instead of the steel tank, helping protect the tank from corrosion. Under some water conditions, its electrochemical reactions may contribute to hydrogen sulphide production. The smell does not necessarily mean the anode has failed.
How do I find the source?
Compare hot and cold water at several taps:
- Only the hot water smells: the storage hot water system is the likely source.
- Hot and cold water smell at several taps: the incoming water supply may be responsible.
- Only one outlet smells: the issue may be local to that fixture or branch.
To distinguish the water from a nearby drain odour, collect hot and cold samples separately in clean glasses and move away from the sink before checking them. Avoid scalding water and do not inhale closely.
These checks are for observation only. Do not open, drain, flush, disinfect or modify the system yourself.
What should I do next?
A persistent smell limited to hot water should be assessed by a licensed plumber. Diagnosis may involve the tank, water chemistry, bacterial activity and anode condition; any treatment must suit the system manufacturer’s requirements.
Hot water systems involve scalding water, pressure and, depending on the unit, electricity or gas. Leave servicing, temperature changes, disinfection and anode work to appropriately licensed tradespeople.
If both hot and cold water smell at several outlets, check your water supplier’s current service information and report the water-quality concern. For a persistent hot-water-only odour, our Sydney plumbing team can inspect the system safely.
If the smell is present in the air around a gas appliance rather than in the water, leave the area and contact the gas emergency service. Do not operate switches or flames.