Why does my hot water smell like rotten eggs but not my cold?

Because the smell is being made inside the hot water cylinder. Bacteria in water held below about 60 degrees produce hydrogen sulphide, and on cylinders with a magnesium anode the reaction with the anode makes it worse. The cold water has not been stored warm, so it does not smell.

Is hot water that smells of rotten eggs dangerous?

The smell itself is not usually harmful at these levels. What matters is that it shows the cylinder is running too cool, which is the same condition that lets Legionella multiply. Get the stored temperature back to 60 degrees or above and find out why it dropped.

Will turning up the cylinder thermostat stop the smell?

Often, if low temperature was the cause. Set the cylinder thermostat to 60 degrees, let the tank heat fully and flush each hot tap. If the smell comes back within days, suspect an anode reaction or sediment in the tank that needs draining.

Can I remove the anode rod to stop the smell?

No. The anode is what stops the tank corroding, and removing it usually voids the cylinder warranty and shortens its life. On an unvented cylinder it also means draining a pressure vessel. The right fix is the replacement anode the manufacturer approves, fitted by a qualified engineer.

Why does only one bathroom have smelly hot water?

Hot water sitting still in a long pipe to a little used bathroom cools and goes stale. Run that tap for a few minutes after the cylinder has fully heated. If it still smells when the rest of the house is fine, the smell is probably the basin or bath trap rather than the water.

Hot Water Smells Like Rotten Eggs? Why, and How to Fix It

If the smell of rotten eggs comes only from the hot tap, it is almost always coming from inside the hot water cylinder. Bacteria that live happily in lukewarm water produce hydrogen sulphide, the gas that smells of eggs, and they thrive where the stored water is kept too cool, where the cylinder has sat unused, or where a metal anode inside the tank gives them something to react with. If the cold smells too, the problem is somewhere else: the drain under the basin, the loft tank, or the mains supply. The first job is to work out which, and a clean glass tells you in two minutes. A smelly cylinder is rarely dangerous in itself, but the conditions that cause it are the same ones that let Legionella grow, so it is worth fixing rather than living with.

Is it the water or the plughole?

A lot of egg smells blamed on the water are coming from the drain. Running water disturbs whatever is sitting in the trap under the basin, and the smell rises exactly where you are standing. Fill a clean glass from the hot tap, walk into another room, and smell it there. Then do the same with the cold.

If neither glass smells in the other room, the water is fine and the smell is the trap or the waste pipe. If the hot glass smells and the cold does not, it is the hot water system. If both smell, it is either the cold supply feeding everything, or a loft tank feeding both the hot and the bathroom cold taps.

Neither glass smells away from the sink: a drain or trap smell, not the water.
Only the hot glass smells: bacteria in the cylinder or a reaction with its anode.
Hot and cold both smell, at every tap: the mains supply, so ring your water company.
Hot and bathroom cold smell, kitchen cold is fine: the cold water tank in the loft.

Why a cylinder starts to smell

The bacteria behind the smell are harmless in themselves and are found in ordinary water. They feed on sulphates in the supply and give off hydrogen sulphide. In a cold pipe they are flushed away before they amount to anything. In a large tank of water held at 30 or 40 degrees for days, they settle into the scale and sediment at the bottom and multiply.

So the smell tells you about temperature and turnover. The usual stories are a cylinder thermostat turned down to save money, an immersion heater that has failed so the boiler coil is only just warming the tank, a holiday let or a second bathroom where the hot water sits for weeks, or a cylinder heated by a heat pump that never takes the water up to 60 degrees. A cylinder that has been drained and refilled, or a new one that has stood for a while before being commissioned, can also smell for a short time.

The anode rod, on cylinders that have one

Some cylinders, typically enamelled or glass lined steel vessels and some unvented models, are protected by a sacrificial anode, a magnesium rod that corrodes so the tank does not. Magnesium reacts with sulphates and the bacteria in the water, and that reaction is one of the classic sources of an egg smell that keeps coming back however often the cylinder is flushed.

Most copper vented cylinders and most stainless steel unvented cylinders do not have a magnesium anode, so on those the answer is almost always temperature. Where there is one, the options are a replacement anode, an anode of a different material, or a powered anode where the manufacturer offers one. Any of these on an unvented cylinder means draining it and disturbing the pressure vessel, so it is a job for an engineer with the unvented qualification, and the manufacturer should confirm the replacement part.

What you can safely do tonight

Check the cylinder thermostat, the small dial strapped to the side of the cylinder. Stored hot water should sit at 60 degrees or above, which is also the temperature that keeps Legionella under control. If it has been turned down, turn it back up and let the cylinder heat fully. Our guide to what temperature a hot water cylinder should be set at explains why that figure matters.

Once the cylinder is fully up to temperature, run each hot tap in turn for a couple of minutes to flush the lukewarm water that has been sitting in the pipes. In many cases that is enough for a smell caused by low temperature to clear over a day or two. Take care: 60 degrees water scalds quickly, so run the taps rather than putting hands under them, and keep children out of the bathroom while you do it.

Do not try to heat the cylinder beyond its normal setting by bridging a thermostat or leaving an immersion on with its stat removed. An unvented cylinder that overheats will discharge scalding water through its relief valves, and a vented one can boil.

When the smell is from the loft tank

On a tank fed system the cold water tank in the loft supplies the cylinder and usually the bathroom cold taps too. Old tanks without a proper fitted lid collect dust, insects and sometimes a dead bird or mouse. The water goes stale and the smell comes out of both hot and cold in the bathroom, while the kitchen cold, which comes straight off the mains, is fine.

If you can reach it safely, look in the tank with a torch. Scum, debris or anything dead means the tank needs draining, cleaning and fitting with a sealed lid and insect screens, or replacing, and the cylinder it feeds needs flushing afterwards. Many owners take the chance to convert to an unvented cylinder at that point and lose the loft tanks altogether. If you are not sure which system you have, our guide to unvented vs vented cylinders shows how to tell.

Other smells from the hot tap

A metallic smell or taste from the hot tap only, particularly with a brown or orange tinge to the water, is corrosion rather than bacteria. That is usually either a failing cylinder or heating water getting into the hot supply through a leaking coil, and it is covered in our guide to brown water from hot tap problems. A musty or earthy smell in hot and bathroom cold together points back at the loft tank. A chlorine smell in every tap is the water company treatment and is normal. A smell of gas anywhere in the house is not a plumbing question at all: leave the property and call the gas emergency line.

What we do on the visit

We start by confirming where the smell is coming from, because a trap, a loft tank and a cylinder need completely different work. On the cylinder we check the stored temperature against what the thermostat is asking for, test the immersion element and its stat, and look at whether the boiler coil is actually heating the whole tank. If the cylinder has an anode, we check the manufacturer data and inspect or replace it. Where the tank is full of sediment and scale, which is common in London hard water, we drain and flush it, then take it fully up to temperature before we leave.

If the smell has come from a cylinder that is also showing its age, leaking, corroding or failing to heat, we will tell you plainly whether it is worth repairing. Hot water cylinders are our core work and we have 20 engineers across London. Our rate is on the pricing page and agreed before we attend.

Questions we get asked

Why does my hot water smell like rotten eggs but not my cold?: Because the smell is being made inside the hot water cylinder. Bacteria in water held below about 60 degrees produce hydrogen sulphide, and on cylinders with a magnesium anode the reaction with the anode makes it worse. The cold water has not been stored warm, so it does not smell.
Is hot water that smells of rotten eggs dangerous?: The smell itself is not usually harmful at these levels. What matters is that it shows the cylinder is running too cool, which is the same condition that lets Legionella multiply. Get the stored temperature back to 60 degrees or above and find out why it dropped.
Will turning up the cylinder thermostat stop the smell?: Often, if low temperature was the cause. Set the cylinder thermostat to 60 degrees, let the tank heat fully and flush each hot tap. If the smell comes back within days, suspect an anode reaction or sediment in the tank that needs draining.
Can I remove the anode rod to stop the smell?: No. The anode is what stops the tank corroding, and removing it usually voids the cylinder warranty and shortens its life. On an unvented cylinder it also means draining a pressure vessel. The right fix is the replacement anode the manufacturer approves, fitted by a qualified engineer.
Why does only one bathroom have smelly hot water?: Hot water sitting still in a long pipe to a little used bathroom cools and goes stale. Run that tap for a few minutes after the cylinder has fully heated. If it still smells when the rest of the house is fine, the smell is probably the basin or bath trap rather than the water.