Brown water from the hot tap only usually points to the hot water side of your plumbing, not the mains. Treat it as discoloured water if the water carries a brown, orange or black tint. Sediment from the cylinder, its pipework or a loft cistern is then the usual place to look. Treat it as smelly water from the hot tap if it runs clear but tastes metallic or bitter — that taste comes from long contact with copper, iron or galvanised pipes and fittings.
Discoloured hot water or a smell from the hot tap: which symptom do you have?
Both symptoms show up at the hot tap, but they lead to different checks. The colour of the water and the behaviour of the cold kitchen tap separate them.
| Criterion | Discoloured hot water | Smell or taste from the hot tap |
|---|---|---|
| What you notice | A brown, orange or black tint in the water itself, often strongest at the first draw-off | Water looks clear, but smells or tastes metallic, bitter or otherwise unusual |
| Likely source in the house | Sediment stirred up in pipework, in the cylinder or in a loft storage cistern | Water standing in copper, iron or galvanised pipes, tanks or fittings |
| What the cold kitchen tap tells you | Clear cold water alongside brown hot water moves attention to the hot side | A taste or smell on both taps moves attention away from the cylinder |
| When to ring your water supplier | If the colour lasts longer than 24 hours and does not clear | If the smell or taste appears suddenly and makes the water unpalatable |
| Work it usually leads to | Cylinder service, draining and flushing, cistern cleaning and disinfection | Checks on pipework, fittings, cylinder temperature and little-used outlets |
The Drinking Water Inspectorate lists the condition of plumbing inside a property as one cause of discoloured water, alongside disturbance in the mains. Where only your property is affected, its advice is to bring in a qualified plumber to find and put right faults in pipes and fittings. Brown water at the hot tap alone does not prove the cylinder is at fault. It does narrow the search to the hot side.
When discoloured hot water points to your cylinder or a loft cistern
Discoloured hot water points to the hot side of your plumbing when the cold kitchen tap runs clear. On an open-vented system the cylinder is fed from a cold water cistern in the loft, while the kitchen cold tap comes straight off the mains. That split explains how colour can appear at one tap and not the other. Our guide on telling a vented cylinder from an unvented one shows which arrangement you have.
The Drinking Water Inspectorate names three house-side causes: the condition of the supply pipe, the condition of the plumbing inside the building, and a storage cistern that needs cleaning. It advises that storage cisterns are inspected at least once a year, and preferably every six months. Settled sludge can be flushed out or vacuumed away and the system disinfected. The cistern also needs a rigid, close-fitting, lockable lid and fine mesh screens on its openings, so light, insects and dirt stay out.
Cleaning a cistern, draining a cylinder and testing its valves is work for a plumber. If the colour has been there for less than a day, the Inspectorate suggests running the tap to clear the line. Colour that lasts longer than 24 hours is a reason to contact your water supplier.
When a smell or taste from the hot tap is the symptom to act on
A smell or taste at the hot tap, with the water running clear, points to what the water has been sitting in. Water in long contact with copper, iron or galvanised pipes, tanks or fittings can take on a metallic or bitter taste, according to the Drinking Water Inspectorate. Little-used outlets and a house left empty make that contact longer.
Temperature is the second factor. HSE guidance for hot and cold water systems sets hot water storage at a minimum of 60°C, with 50°C reached at the outlet within one minute, and 55°C in healthcare premises. Legionella bacteria do not survive above 60°C and are dormant below 20°C, multiplying between 20°C and 45°C where nutrients are present. Baxi advises setting the cylinder thermostat to 60°C, and that setting is one of the few things you can check yourself — see what temperature a hot water cylinder should be set to.
Those HSE duties sit with people in control of premises, landlords among them, rather than with owner-occupiers. The same guidance asks that outlets are used at least weekly, that rarely used outlets are flushed through, and that shower heads are descaled and cleaned. Where a property has stood empty for a long period, HSE expects a flushing regime or another measure such as draining down.
Hot water from the tap is not recommended for drinking, because it can carry raised levels of metals such as copper. If water suddenly changes colour, do not assume it is fit to drink without asking your water supplier. The same call is worth making if a strong taste or smell appears suddenly, or if discolouration lasts more than 24 hours.
Milky or cloudy hot water is a different thing from brown hot water
Milky or cloudy hot water is almost always air, not sediment. Fill a glass and watch it: if it clears from the bottom upwards, tiny air bubbles are leaving the water. The Drinking Water Inspectorate uses exactly that as the test for white or cloudy water.
Worcester Bosch explains why combi boilers do this. Heating drinking water turns calcium bicarbonate into calcium carbonate and releases carbon dioxide as millions of tiny bubbles. The water clears as it cools in the glass. The effect is stronger in hard water areas and at lower flow rates.
Heatrae Sadia says the same about stored hot water: in some areas aeration makes hot water look cloudy, which is normal and clears quickly. Brown, orange or black water behaves differently. The colour stays in the glass, or it settles out as sediment at the bottom.
What a cylinder service checks when hot water runs discoloured
A cylinder service covers five points, and two of them bear directly on discoloured hot water. Heatrae Sadia lists them as follows.
- Safety of the electrical components and the wiring
- Operation of the temperature and pressure relief valve and the expansion valve
- A clear discharge through the tundish, the open gap in the discharge pipe that shows a valve has let go
- The sacrificial anode, checked and replaced if needed
- The condition of the seals
A sacrificial anode is a metal rod inside the cylinder that corrodes in place of the steel around it. Vaillant fits a magnesium anode in its cylinders, has it checked for the first time after two years and yearly after that, and replaced with an original part when needed. Heatrae Sadia recommends servicing a cylinder every 12 months, in step with the boiler service. Our guide on annual servicing of unvented cylinders and Megaflo systems sets out what the visit covers.
Who may work on the cylinder behind discoloured hot water in England?
Work on an unvented cylinder — one fed straight from the mains rather than from a loft tank — belongs to someone competent in unvented hot water systems. Installation falls under Building Regulation G3 in England and Wales, and calls for a competent plumbing and electrical installer. Approved Document G says competence can be shown by registration in a Competent Person Scheme for that work, or by a valid skills card for unvented hot water systems. If the installer is not registered in such a scheme, building control must be notified before the cylinder goes in.
Two further rules frame the work. The Water Supply (Water Fittings) Regulations 1999, which apply in England and Wales, exist to prevent contamination and waste of the water supplied. No fitting may be installed or used so that it causes contamination, and every fitting must resist corrosion or be protected against it. Any work on gas pipework, a flue or inside the boiler casing is separate again: in England that is for a Gas Safe registered engineer, under the Gas Safety (Installation and Use) Regulations 1998 as amended.
Discoloured or smelly hot water: which one you are dealing with, and what to arrange
Treat it as discoloured water if the water carries a brown, orange or black tint while the cold kitchen tap runs clear. The hot side is then where to look: the cylinder, its pipework and any loft cistern. Treat it as a smell or taste problem if the water runs clear but tastes metallic or bitter. The likely factors there are water standing in pipes and fittings, little-used outlets and the cylinder temperature.
Both cases share one step. If colour lasts more than 24 hours, or a strong taste or smell appears suddenly, contact your water supplier before you draw any conclusion about the plumbing. Where the cylinder turns out to be the common factor, a service, a repair or a replacement is the practical next move. You can arrange a free home survey and a written fixed price first — our page on Megaflo and unvented systems shows what we fit and service.
Sources
- Drinking Water Inspectorate — Brown, black or orange water
- Drinking Water Inspectorate — Discoloured water
- Drinking Water Inspectorate — White or cloudy water
- Drinking Water Inspectorate — Taste and odour in drinking water
- Drinking Water Inspectorate — Water storage
- Worcester Bosch — Why is the hot water from my combi boiler sometimes cloudy?
- Heatrae Sadia — Get servicing savvy about cylinders
- Heatrae Sadia — Importance of servicing your cylinder
- Vaillant — uniSTOR operating and installation manual (PDF)
- HSE — Hot and cold water systems
- HSE — Legionella and landlords' responsibilities
- HSE — Domestic gas: frequently asked questions
- Approved Document G with 2024 amendments (PDF)
- The Water Supply (Water Fittings) Regulations 1999
- Baxi — Top tips for maintaining your boiler