Noisy Hot Water Cylinder: What Each Sound Means

What is that noise from my hot water cylinder?

Start with timing, because the cause follows the moment the sound happens. Listen at the cylinder while the heating or immersion is running, then again an hour after it has switched off, then again when someone shuts a tap quickly. Three different sounds in three different moments point to three different faults.

A rumbling or crackling noise while the cylinder is heating is almost always limescale on the element. A ticking or creaking noise as it warms up or cools down is metal moving. A single bang when a tap, shower valve or washing machine valve closes is water hammer. A continuous hiss, or a sputtering from a pipe above or beside the cylinder, is a valve or vent pipe releasing something, and that is the one to take seriously.

• Rumbling, popping or a kettle-like rolling sound while heating: scale on the immersion element or coil
• Ticking or creaking while warming up or cooling down: pipes and the shell expanding against clips and supports
• One bang when a tap closes: water hammer in the supply pipework
• Rhythmic knocking from the loft: a worn float valve bouncing in the feed tank on a vented system
• Hissing, spitting or steam near a valve or vent pipe: stop using the heat source and read the safety section below

Why does my cylinder sound like a kettle?

Kettling is the commonest cylinder complaint in hard water areas, and London is one. Scale forms a crust on the immersion element, which is the heated metal rod that sits in the cylinder. The crust insulates the element from the water, so the metal runs hotter than it should, and the thin film of water trapped against the scale boils in tiny pockets. Each pocket collapses with a pop, and together they sound like a kettle coming to the boil. Scale on a boiler coil can sometimes make similar noises, though an immersion element is the more common source.

The sound is a warning, not a fault in itself. A scaled element still heats, but it takes longer, uses more electricity for the same result and fails sooner because the sheath runs at a higher temperature. You often see the noise and a longer recovery time together. If a cylinder that used to heat in an evening now takes most of the night, read hot water runs out fast alongside this page.

There is a trap here. People turn the thermostat down to quieten the cylinder. That does reduce the noise, but stored hot water is meant to be held at 60°C or above to control legionella, according to HSE guidance on hot and cold water systems. A cylinder that is quiet only because it is set well below that has swapped a nuisance for a hygiene problem. If the sound only goes away at a low setting, the element needs descaling or replacing, not a lower dial.

Why does my cylinder creak when it heats up or cools down?

A copper or stainless steel cylinder grows as it heats and shrinks as it cools. So do the pipes joined to it. If a pipe is clipped tightly against a joist, or the cylinder is resting on a stand or cradle that grips it, the metal slides in short stutters and you hear ticks and creaks. The noise usually appears some minutes after heating starts and again as the cylinder cools overnight.

This is rarely a fault in the cylinder. It is a fixing problem, and it tends to show up after a pipe has been moved, a cylinder has been refitted, or insulation has been added or removed. The checks are cheap and safe. Listen at the pipes rather than the cylinder to find where the sound starts. Look for pipes pressed against timber or notched through a joist without a sleeve. Look at the cylinder cradle for anything wedged between it and the shell.

Creaking that comes with a drip, a damp patch or staining under a connection is different. Movement can work a compression joint loose, and a slow weep at a joint is a leak that has not yet become visible. If you see moisture, follow the leak-source guide instead of treating it as noise.

Why does something bang when I turn off a tap?

Water hammer is the pressure shock that happens when flowing water is stopped suddenly. The moving column of water has momentum, and when a valve slams shut it has nowhere to go, so the pipe jumps against its clips. The bang is loudest where the valve is, which is not necessarily at the cylinder, so walk the house while someone operates each tap.

The usual causes are loose pipework, a quick-closing valve on a washing machine, dishwasher or shower, and mains pressure that is higher than the fittings were chosen for. An unvented cylinder has a pressure reducing valve at its inlet that sets the pressure the cylinder works at, and the manufacturer specifies the setting, so a hammer that started after work on the cold supply is worth mentioning to the engineer.

A vented cylinder adds one more cause. The cold water tank in the loft has a float valve, sometimes called a ball valve, that opens as the level drops and closes as it rises. When the washer inside it wears, the valve can flutter or vibrate as it closes, and the shock travels down the cold feed. A rhythmic knocking that follows the sound of the tank filling points there. Lift the tank lid and watch the arm. If it bounces as the tank fills, the valve is the culprit, and the water level in the tank should sit below the overflow with the valve fully closed.

Anything involving the expansion vessel, the combination valve or the tundish on an unvented cylinder is a different matter. That equipment protects the cylinder from overpressure, and it is not a householder repair. See TPRV dripping on an unvented cylinder for what those parts do.

When is a noisy cylinder dangerous?

Most cylinder noises are annoying rather than dangerous. A few are not. Treat these as stop signs:

• Steam from a vent pipe or from the tundish, or a loud hissing that does not stop when the heating switches off
• Water that is far hotter than the thermostat setting, or a rumbling that gets louder each time it heats
• A burning smell, scorch marks or discolouration at the immersion cover or the wiring
• A cut-out that keeps tripping, where each reset is followed by the same noise

What does an engineer check, and what follows?

The order matters because each step rules out the cheaper explanation. The engineer listens first, with the heat source on and off, to separate heating noise from flow noise. Where the sound is thermal, the next steps are the immersion element and thermostat condition, how much scale has formed, and whether the thermostat and the high-limit cut-out behave as they should. Where it is flow noise, the engineer follows the pipework, the float valve if the system is vented, and the inlet pressure on an unvented one.

Repair is the usual outcome for noise. A scaled element can be replaced, a loose pipe re-clipped, a float valve renewed. Replacing the cylinder is worth discussing only if the shell itself is the problem, for example heavy scaling throughout the cylinder, a coil that has failed, or leaks that follow the noise. The decision rule is simple: if the noise comes from a replaceable part, replace the part. If the cylinder has several parts failing together, read repair or replace before committing.

The hot water cylinder diagnosis page explains how the engineer works through these checks, and cylinder water hardness and scale covers why scale keeps coming back in hard water areas.