Black Bits in Your Hot Water? What They Are and What to Do

Three quick tests before you do anything else

Collect a few of the bits on a white kitchen towel or in a clear glass. These three checks take five minutes and narrow it down to one or two causes.

• Which taps? Run every hot tap and every cold tap for a minute. Hot only, every tap: the hot water system. One tap only: that tap or its hose. Hot and cold: the mains or the loft tank.
• The rub test. Rub a speck between finger and thumb. Soft and smeary like a rubber eraser: perished rubber. Gritty and staining your fingers dark grey: sludge or corrosion.
• The magnet test. Dry the bits on the towel and hold a fridge magnet just above them. If they jump to it, it is magnetite, the black iron oxide sludge that forms inside radiators and heating pipes.

Soft rubbery bits from one or two taps

This is the most common cause and the least worrying. Flexible tap connectors, the braided hoses under basins and sinks, have a rubber lining. So do tap washers, the O-rings in mixer taps and the hoses on some showers. Over time, and faster in hot water, the rubber perishes and sheds small black flakes into the water.

The giveaway is that the bits appear at one tap, or a couple of taps on the same run, and not at the rest. Taking off the aerator on the end of the spout often shows a little collection of them caught in the mesh. The fix is to replace the flexible hoses or the washers at that tap. If the braided hose under the sink looks swollen, kinked or rusty at the ends, replace it soon regardless, because a failed hose under a sink is one of the most common causes of a sudden flood.

Fine black grit that sticks to a magnet

If the bits are magnetic, they are almost certainly magnetite from the heating system. Heating water and the water you wash in are supposed to be kept completely apart. When magnetite turns up in the hot tap, something has let the two mix.

On a tank fed hot water cylinder, the usual culprit is the coil inside the cylinder. Boiler water runs through that coil to heat the stored water, and when the coil corrodes through, heating water can leak into the cylinder. You will often see it with brown or grey hot water, a metallic smell, and the small header tank in the loft topping itself up far more than it should. Our guide to brown water from the hot tap covers the same fault from the colour side, and header tank overflowing covers what the loft tanks tell you.

On a combi boiler, the same thing can happen inside the boiler if the plate heat exchanger that heats your tap water cracks. Heating water and mains water then mix inside the boiler. A combi whose pressure keeps rising for no obvious reason, together with dirty hot water, points that way. Our page on boiler pressure too high explains the pressure side.

Heating water often contains chemical inhibitor as well as sludge. If you suspect it is getting into your hot water, do not use the hot tap for drinking, cooking or filling the kettle until it has been checked.

Black or dark flakes from an old copper cylinder

Old copper cylinders and pipes can shed flakes of dark corrosion on the inside, especially after the water has been off and turned back on, or after a drain down for other work. The flakes are usually larger and more papery than rubber and are not magnetic. A few after a repair that clear within a day are normal. Flakes that keep coming week after week are a sign the cylinder is deteriorating, and our guide to signs your cylinder needs replacing covers what else to look for.

Black specks in the cold water as well

If black specks come out of the cold taps too, particularly the kitchen cold tap, which comes straight off the mains, the cause is outside your hot water system. Deposits of manganese and iron build up in water mains and get stirred loose when the flow changes, for example after a burst or maintenance work in the street. Run the kitchen cold tap for a few minutes. If it clears, it was disturbed sediment. If it keeps happening, contact your water supplier, because the mains is their responsibility.

On a tank fed system, check the cold water storage tank in the loft as well. An old tank without a fitted lid collects dust, insects and debris, and that ends up in the cylinder and the bathroom taps. A tank that dirty needs cleaning or replacing and a proper lid fitting.

Is it safe to use the water?

Rubber flakes are unpleasant rather than dangerous, but the hot tap is not meant for drinking in any case, for reasons our guide on whether you can drink hot tap water explains. Washing and bathing are generally fine while you arrange a repair. The exception is heating water getting into the hot supply. Until that has been ruled out, use the kitchen cold tap for anything you drink or cook with.

What an engineer will do

We repeat the tap by tap check, test the bits, and look at where the hot water is coming from. For rubber, we replace the hoses and washers at the affected taps. If heating water is getting into the hot supply, we prove where, by checking the header tank behaviour on a tank fed system or the boiler pressure and heat exchanger on a combi. A leaking cylinder coil means a new cylinder, and our hot water cylinder replacement page explains the options. On an unvented cylinder, any work on the cylinder itself is done under our unvented cylinder repair service by an engineer with the G3 qualification.

When to call us

Call us on 020 4652 5111 or send an enquiry if the bits are magnetic, the hot water is also discoloured, they come from every hot tap, or the problem keeps coming back after you have changed a hose. A photo of the bits on a white towel and of your cylinder or boiler label saves time on the day. We have 20 engineers across London, and our rate is on the pricing page and agreed before anyone attends.