Hot water cylinder making noise: what each sound means

How to work out which hot water cylinder noise you have

Sound on its own is ambiguous. Sound plus timing is close to a diagnosis, so before anything else, note when the noise happens relative to what the system is doing. Does it start when the boiler fires or when the immersion switches on, and fade once the cylinder is up to temperature? Does it only happen at the instant a tap or an appliance stops? Is it there right now with nothing running and nothing heating? That single observation splits the whole list into groups.

Then work out where in the room the sound is coming from, because people describe almost any noise in an airing cupboard as coming from the cylinder. Put a hand flat on the cylinder shell while it is heating. A genuine cylinder noise is felt as vibration through the shell. Water hammer is felt through the pipework and sometimes through the floor joists, and it will be far stronger at a pipe clip than on the vessel itself. Pump noise is loudest at the pump body and travels along whatever the pump is bolted to.

The last thing worth noting is whether anything changed just before the noise started. A new washing machine or dishwasher, a lever tap fitted in a bathroom, a radiator job, a period with the water off for street works, or a cylinder that has been drained and refilled all introduce specific noises. A noise that arrived the same week as a new appliance is almost certainly hammer, and a noise that started after the system was drained is usually air.

• Builds during heating and stops when the cylinder is hot: scale on the heat source
• One bang at the moment a tap or appliance shuts off: water hammer
• Continuous, with nothing running and nothing heating: something is passing water
• Steady hum or whine unrelated to heating: a pump
• Ticking, tapping or creaking that comes and goes as the system warms and cools: expansion
• Gurgling when a hot tap is opened: air in the pipework or the primary circuit

Kettling: a rumbling or boiling sound while the water heats

This is the noise most people mean when they say the cylinder is making a noise. It starts quietly a few minutes into a heating cycle, builds to a rumble or a sound like a kettle coming to the boil, and dies away once the water is up to temperature. It is caused by limescale, and in London that is a question of when rather than whether.

The mechanism is worth understanding because it explains why the noise matters. The hottest surface inside a cylinder is either the immersion element or the outer face of the internal coil that carries water from the boiler. Hard water deposits calcium carbonate preferentially on the hottest surface it touches, so scale forms as a hard crust exactly there. Once that crust exists, a thin film of water gets trapped underneath it against metal that is much hotter than the surrounding water. That film flashes to steam, the bubble forces its way out and collapses in cooler water, and the rumbling you hear is thousands of those collapses per minute.

Left alone it gets worse in a way that has a cost attached. Scale is a good insulator, so the element or coil can no longer shed heat into the water at the rate it is producing it. The metal runs hotter and hotter, the element distorts, and it either fails outright or starts tripping its thermal cut-out. On the boiler side, a scaled coil transfers less heat, so the cylinder takes noticeably longer to reheat and the boiler runs for longer to achieve less.

The other clue that points at scale is reheat time. If the cylinder is both noisier and slower than it was a year ago, and the hot water runs out sooner than it used to, you are not imagining a connection. Those are three symptoms of the same crust. What happens next depends on where the scale is. An immersion element that has furred up is replaced, which also lets an engineer see the state of the inside of the cylinder through the boss. A scaled coil is a different conversation, since the coil is built into the vessel and cannot be swapped on its own. If you want the background on why scale arrives this quickly here, why hot water systems fail earlier in London puts the chemistry against the working life of a whole installation, our page on hard water and hot water cylinders in London sets out what the deposit actually does inside a vessel, and the immersion heater repair page covers what changing a furred element involves in practice.

Kettling is a warning rather than an emergency. If it is accompanied by violent knocking, water that is too hot to put a hand under, or a thermal cut-out that keeps tripping, treat it as overheating instead: switch off the immersion at its fused spur, turn the boiler off, and leave the cold water supply on so the cylinder can cool.

Banging and knocking when a tap or appliance shuts off

If the bang happens at the moment water stops flowing rather than while the cylinder is heating, the cylinder is innocent. This is water hammer, and it is a pressure shock in the pipework.

Water in a pipe is a moving column with real mass. When a valve closes quickly, that column has to stop in a fraction of a second, and the energy has to go somewhere. It becomes a pressure spike that travels back down the pipe, reflects off the first solid thing it meets, and travels again. The bang you hear is the pipe moving against a joist, a clip or a bit of floorboard as that shock passes.

The valves that cause it are the fast closing ones. Solenoid valves in washing machines and dishwashers shut almost instantly, which is why hammer so often appears the week a new appliance is installed. Quarter turn lever taps do the same thing, and a ballvalve in a loft tank that is bouncing on its seat produces a rapid knocking rather than a single bang. On an unvented system there is more energy in the pipe to begin with, because the hot side is at mains pressure, so the same appliance that was quiet in an old vented house is loud in a converted flat.

Hammer is worth fixing, and not only because it is irritating at six in the morning. It is a mechanical shock delivered to the same joints hundreds of times a week. Compression joints work loose, push fit connections creep, and the thing that eventually weeps under a floor was usually being hammered for a year first. The fixes are ordinary plumbing: refit and add missing pipe clips so the pipe cannot move, fit a shock arrestor near the offending appliance, and check that any pressure reducing valve on the inlet is still holding the pressure it is supposed to.

• One sharp bang as an appliance fills or stops: a solenoid valve, fit an arrestor at that appliance
• Repeated rapid knocking: a float valve bouncing in a cold water tank
• Rattling along a run rather than a single bang: loose or missing pipe clips
• Bang that started after a new tap or machine went in: the new valve is closing faster than the old one
• Hammer plus an unusually forceful cold tap: worth checking the inlet pressure on the system

Why a hot water cylinder ticks and creaks as it warms up

This is the noise that worries people most and needs doing least. A cylinder full of water gains a great deal of heat in a short time, and everything it touches changes size as a result. The copper or stainless shell grows, the pipework connected to it grows more, and all of it is restrained by clips, by holes drilled through joists, and by the cupboard it sits in. The movement happens in tiny jumps as friction gives way, and each jump is a tick or a creak.

You can usually recognise it by its pattern. It follows the heating cycle rather than the water flow, it appears both while the cylinder is warming and again half an hour after everything has switched off as it cools, and it is irregular. It does not build into a rumble and it has nothing to do with taps being opened.

It only becomes worth acting on when it is loud enough to be a nuisance, and the fix is mechanical rather than plumbing: find the point where a pipe is passing through a tight hole or resting directly on a joist and give it room, or pack the contact point so it slides quietly instead of gripping and releasing. Nothing inside the cylinder is involved.

Hissing, trickling or the sound of running water with everything off

A noise that is present when nothing is running and nothing is heating is the one to take seriously, because it means water is moving somewhere it should not be. On a sealed hot water system, water moving with every outlet closed is either a valve passing or a leak.

Start at the tundish, the small open funnel in the pipework near the cylinder with a visible air gap in it. It exists so that any discharge from the safety valves is seen rather than piped away quietly. Look into it and listen. If water is trickling through it continuously, hot or cold, idle or heating, a relief valve is not seating. Typically a fragment of scale or grit has lodged on the seat the last time the valve lifted, and it now holds the valve fractionally open. On a metered supply this runs day and night and shows up on a bill long before anybody notices the sound. A relief valve that is passing is replaced rather than cleaned, because it is a safety device and its seat has already demonstrated that it does not seal. The distinction worth drawing before anyone is called out is between a valve that is passing continuously and one that lifted once after a pressure event and then reseated properly, because the first is a part that has failed and the second is a symptom of something upstream of it that has not been dealt with.

If the tundish is dry, the next candidates are ordinary plumbing rather than the cylinder. A toilet with a leaking flush valve produces a soft continuous hiss that is easily blamed on the airing cupboard next door. A fill loop or a filling valve left fractionally open passes water into the heating side. Beyond that, a hiss with no visible source and no explanation is one of the classic signatures of a concealed leak under a floor or in a wall, particularly in a flat where the pipework runs in screed.

There is a quick test for that. Turn every tap off, make sure no appliance is filling, and look at the water meter if you have one accessible. If the dial is still creeping, water is leaving the system somewhere with nothing open, and that needs tracing properly rather than guessing.

A hiss or trickle that is continuous is the one noise on this page that costs money every hour it is left. It is also the only one that can be a hidden leak. If the tundish is dry and you cannot find the source, do not leave it running for weeks in the hope it stops.

Humming, whining or vibration that has nothing to do with heating

A constant hum or whine that runs regardless of whether the cylinder is heating is a motor, and in a domestic hot water system there are only a few places a motor can be.

The most common is a shower pump or a whole house pump, usually sitting in the airing cupboard next to the cylinder, which is why it gets blamed on the cylinder. A healthy pump makes a steady noise only when water is being drawn. A pump that runs on after the shower has stopped, cycles on and off by itself, or has developed a harsh whine and a strong vibration is failing. Worn bearings produce the whine, and a pump that hunts on and off with no outlet open usually means either a non return valve is passing or there is a small leak on the run it serves.

A secondary return pump is the other one. In larger flats and houses with a long run to the bathroom, a small pump circulates hot water continuously so it arrives at the tap quickly. It runs on a timer or a stat, so its noise comes and goes on a schedule that has no relationship to the boiler. Those pumps are usually clamped to pipework, so a hum that started out of nowhere is often just a mounting bracket that has loosened and is now transmitting vibration into the structure.

Electric thermal store units of the kind fitted in many purpose built flats, Pulsacoil and similar, have a pump built into the unit and a plate heat exchanger. When the pump bearings in one of those wear, the noise is distinctive: a rising whine whenever hot water is drawn, sometimes with a rattle from the casing. That is a component level repair on the unit rather than anything to do with a conventional cylinder.

• Hum only while water is being drawn: a pump doing its job, check the mounting if it is transmitting into the floor
• Pump running on after the tap is closed: a passing non return valve or a leak on that run
• Harsh whine with strong vibration: worn bearings, the pump is on its way out
• Hum on a timer with no tap open: a secondary return pump circulating hot water
• Whine and rattle from an all in one electric unit: the internal pump rather than the cylinder

Why a hot water cylinder gurgles, and where the air comes from

Gurgling is air moving through water, and in a hot water system that air came from somewhere identifiable. If it started after the system was drained for a repair, the refill trapped air at a high point and it will usually clear itself over a few days of normal use, helped along by running each hot tap hard for a minute.

If it has appeared on its own, the question is where air is getting in. On a vented cylinder fed by a tank in the loft, a cold feed that is partly blocked or a float valve that cannot refill the tank quickly enough lets the level drop while a bath is running, and air is drawn into the feed. That produces glugging at the taps and sometimes a hammering in the pipework as slugs of water and air arrive in turn. On the boiler side, air in the primary circuit gurgles as it passes through the coil, and it generally points at the heating side rather than the hot water side.

Air is also the reason a hot tap can splutter and spit for a few seconds before running normally. That is worth noticing rather than ignoring, because a system that keeps making air after it has been vented several times is usually making it for a reason, and finding the reason is the actual repair.

Which noises are urgent and which can wait

Very little of what a cylinder does is an emergency, but a small number of sounds mean the stored water is being overheated, and those are in a different category from everything else on this page.

The sounds that mean stop are violent knocking or a distinct boiling from inside the cylinder rather than a soft rumble, kettling combined with water at the taps that is scalding with no cold mixed in, and a tundish discharging water that is too hot to touch. Together they describe water being driven past its safe temperature with the final mechanical protection now doing the work. In that situation switch off the immersion at its fused spur and turn the boiler off, leave the cold water supply on so the cylinder can refill and cool, and do not use the hot water until it has been checked. Turning the cold supply off is the instinct to resist, because the incoming cold is what allows an overheating cylinder to come back down. A cut-out that has tripped afterwards leaves a household with no hot water but the heating works perfectly well, which is a confusing combination until you know that the two circuits are separated at the cylinder rather than at the boiler. Overheating has a wider set of symptoms than sound alone, and our page on a hot water cylinder overheating goes through the rest of them. Where the noise follows the boiler firing rather than the immersion switching on, boiler diagnosis is the work that separates a fault in the appliance from a scaled coil inside the cylinder, and the two produce a rumble that sounds almost identical from the hallway.

Everything else is a repair that can be booked. Kettling should be dealt with in the next few weeks rather than the next few hours, because it is quietly destroying an element. A continuous hiss or trickle should be looked at promptly since it is either wasting water or hiding a leak. Water hammer is a maintenance job. Ticking and creaking needs nothing unless it annoys you.

• Stop and switch off the heat sources: boiling, violent knocking, scalding water, very hot tundish discharge
• Book promptly: continuous hiss or trickle with everything off, a meter that turns with no tap open
• Book soon: kettling and rumbling, a pump whining or running on
• Fix when convenient: water hammer, rattling pipes, a loose pump bracket
• Leave alone: ticking and creaking that follows the heating cycle

If a thermal cut-out on an immersion heater has tripped, resetting it once to restore hot water is reasonable. Resetting it a second time is not, because each reset restores the fault that overheated the water and leaves the safety chain as the only thing standing between the system and an unsafe condition.

What to check yourself before calling an engineer

Some of this is genuinely a ten minute job with a torch, and it is worth saying so. Working out whether you have hammer, expansion noise or a passing valve costs nothing and it changes what needs to happen next.

Go through it in order. Look into the tundish with everything off and confirm it is dry. Run each hot tap briefly and listen for air. Have somebody shut a fast closing tap or start a washing machine while you stand by the pipework and feel for the shock. Then start a heating cycle, put a hand on the cylinder shell, and see whether the noise is in the vessel or in the pipes around it. Note what you find, because an engineer arriving to that is starting the job well ahead.

What not to do is short. Do not remove an immersion cover with the circuit live. Do not open or adjust anything on the inlet control group to see what happens, because it is set to a specific pressure and the relief valves are safety devices. Do not try to free a relief valve that is discharging by working it back and forth. Do not pour a descaling product into a hot water system in the hope of quieting a rumble, because what it loosens ends up on the seats of the valves that are protecting you.

When we attend a noise complaint the order is always the same: identify the noise against the heating cycle, separate the cylinder from the pipework and the pump, check the tundish and the relief valves before touching a heat source, and only then decide whether a part has failed or a component has scaled. The rate is fixed before we attend and the total depends on how long the job takes, so a noise that is identified properly on the first visit costs less than one that gets parts fitted at it speculatively. We have twenty engineers across London and the ones who work on unvented cylinders hold the qualification required for that work. If the cylinder has not had an unvented cylinder service within the last year, the safety valves are worth going over at the same visit, and if you also find water on the floor around the base, the hot water cylinder leaking page is the one that covers what that points at.

• Confirm the tundish is dry with every tap closed and nothing heating
• Note whether the noise follows the heating cycle or the taps
• Put a hand on the cylinder shell and on the nearest pipe clip while it is happening
• Check whether a new appliance or tap went in shortly before the noise started
• Watch the water meter with everything off if you can hear running water
• Photograph the cylinder label and the inlet group so the right parts come with the engineer