Brown Water From the Hot Tap? What It Means for Your Cylinder

Hot only, or hot and cold?

Run the kitchen cold tap into a clear glass, then the hot tap into another, and hold them against something white. The kitchen cold comes straight off the mains on almost every system, so it is your control.

If the cold is brown too, the water is arriving discoloured. Work on a main in the street, a burst nearby or a change in the flow through old iron mains stirs up sediment, and it normally clears within a few hours. Your water company, which is Thames Water for most of London, can tell you whether there is work in your area. If the cold is clear and the hot is brown, keep reading, because the cause is in your house.

• Hot and cold both brown: the mains supply. Ring the water company.
• Brown for a few seconds after the water has been off or after work: sediment, usually clears.
• Hot and bathroom cold brown, kitchen cold clear: a rusting cold tank in the loft.
• Hot only, brown or orange with a metallic smell: corrosion inside the cylinder.
• Hot only, dark grey or black, and the heating keeps needing topping up: a leaking coil.

Sediment after the water has been off

Scale and fine sediment settle at the bottom of every cylinder, and London water, being hard, produces plenty of it. When the supply has been off, the cylinder has been drained, or someone has been working on the pipes, the rush of water on refilling lifts that sediment and it comes out of the hot taps as a brown or cloudy tinge.

Run the hot tap for a few minutes. If the colour fades and does not return, that was all it was. If the cylinder is regularly throwing out sediment, a proper drain and flush helps, and our guide to how to drain a hot water cylinder explains what is involved on each type of system.

A rusting cold water tank in the loft

Older tank fed systems were often installed with a galvanised steel cold water tank in the loft. The zinc coating wears through after decades, the steel rusts, and the rust feeds straight down into the cylinder. Because the same tank usually supplies the bathroom cold taps, the tell is brown water from the hot taps and the bath and basin cold, while the kitchen cold is clear.

Look in the tank with a torch if you can get to it safely. Orange scale on the walls, rust around the outlet or a rusty sludge on the bottom confirms it. A galvanised tank in that state is not worth repairing. It gets replaced with a plastic tank with a sealed lid, and the cylinder it feeds is flushed through. Some owners use the moment to change to an unvented cylinder and remove the loft tanks entirely.

Corrosion inside the cylinder

A copper cylinder that is reaching the end of its life corrodes from the inside, usually low down where sediment has sat for years. Steel cylinders with an enamelled lining, and unvented cylinders that rely on a sacrificial anode, start to rust once the lining is damaged or the anode is used up. In each case the rust comes out in the hot water, often with a metallic smell or taste, and it gets steadily worse rather than clearing.

Discoloured water is one of the clearer signs your hot water cylinder needs replacing, particularly on a cylinder that is also slow to heat, leaking at the seams or more than a couple of decades old. Where an unvented cylinder still has a replaceable anode and the vessel itself is sound, replacing the anode may be enough, and that is something the annual service should be checking.

Heating water leaking in through the coil

A cylinder heated by a boiler has a coil inside it. Boiler water runs through the coil and never mixes with the water you use. If the coil corrodes through, the two start to mix. Heating water is dark grey to black with iron oxide sludge and carries corrosion inhibitor, so the hot taps run dirty, and the effect often comes and goes as pressures on each side change.

The side effects give it away. On a sealed heating system the boiler pressure keeps dropping, or occasionally rises, with no leak you can find. On a tank fed heating system the small feed and expansion tank in the loft overflows, or the cold water tank level behaves oddly. If you have discoloured hot water alongside a boiler that keeps needing its pressure topped up, a failed coil is the first thing to rule out. A coil cannot be repaired inside the cylinder, so this is almost always a cylinder replacement.

If the hot water is dark and you suspect heating water is getting in, do not use it for bathing or washing up until it has been checked, and do not keep topping up the boiler pressure. Turn the boiler feed to the cylinder off if you know how, and call an engineer.

Combi boilers and brown hot water

With a combi there is no cylinder and no loft tank, so hot only discolouration is rarer. The usual causes are old steel or iron pipework in the house, a corroded section of pipe, or occasionally a fault inside the boiler hot water heat exchanger. If a combi is producing dirty hot water and losing pressure at the same time, stop using the hot water and have the boiler checked.

What we do on the visit

We start with the same glass test, then check the loft tank on a tank fed system, the pressure history on a sealed heating system, and the condition of the cylinder and its connections. Where a coil leak is suspected, we isolate the heating side and watch whether the pressure on the heating system and the colour of the hot water change, which proves it one way or the other without guesswork.

If the fix is a flush or a new loft tank, we can usually do it on the day. If the cylinder is finished, we will explain why and what the options are, and whether it is worth repairing an old cylinder at all is a question we will answer straight. Hot water cylinders are our core work, with 20 engineers across London. Our rate is on the pricing page and agreed before we attend.