A hot water cylinder bangs when a tap is opened if the sudden change in flow sends a pressure surge through the pipework. That surge is called water hammer. It vibrates or hums instead if a pipe is moving against a joist, a floorboard or a wall while water is drawn. On an unvented cylinder, a flat expansion vessel removes the air cushion that damps those surges, so the knock gets louder over time.
Banging or vibrating from the cylinder: what separates the two noises
A bang is a single knock that lands the moment a tap, shower or WC valve moves. A vibration is a hum or buzz that lasts as long as the water keeps flowing.
| Criterion | Banging (knock as the tap moves) | Vibrating (hum while water flows) |
|---|---|---|
| When you hear it | A single knock at the instant a tap or valve opens or closes | A steady hum or buzz for as long as water is being drawn |
| Usual mechanism | Water hammer: flow stops or reverses suddenly and a pressure surge travels through the pipes | Pipework moving and striking a joist, floorboard or wall, often where a clip has broken |
| Where it seems to start | Heard around the house at once, often loudest near the cylinder and its connections | One spot: a floor void, an airing cupboard wall or a run behind plasterboard |
| Part played by the cylinder | On an unvented cylinder the air in the expansion vessel damps the surge, so a flat vessel makes the knock harsher | The cylinder and its pipes act as a sounding board; the movement itself is in the pipe run |
| If it carries on | In extreme cases water hammer can damage the system | Repeated movement wears clips and rubs joints where the pipe makes contact |
| Who deals with it | An engineer looks at incoming pressure, valve types and the expansion vessel charge | An engineer finds the moving run and refixes or cushions it |
Both noises are about pipework and pressure far more often than a fault inside the cylinder shell. Telling them apart changes who looks at what. A knock sends attention to pressure and valves. A hum sends attention to the route the pipe takes under the floor or through a wall.
When the noise is a single bang: water hammer at a tap or valve
When a hot water cylinder bangs the moment a tap opens, water hammer is the usual explanation. Water hammer is a pressure surge caused by a sudden stop or reversal of flow. Manufacturer guidance names it as the most common reason for knocking pipes. It can happen only when a tap is turned, or simply be worse at that moment.
Any outlet can trigger it. A quick-closing kitchen tap, a shower mixer or the fill valve on a WC all change the flow abruptly. That is why the bang often follows someone else using water in another room. The sound carries along the pipes, so the cylinder cupboard is rarely where the problem sits.
In extreme cases water hammer can damage the system. Repeated knocking whenever taps, showers or the WC are used is worth having checked by a heating engineer rather than lived with.
When the cylinder hums or vibrates while the water runs: pipework on the move
When the cylinder hums or vibrates for as long as water is flowing, something in the pipe run is moving. Copper expands as it warms. That movement shows up as ticking, vibration or knocking, and it is clearest the first time cold pipework heats through. A broken pipe clip lets the pipe swing and strike a floorboard or a wall.
Where a pipe merely sits against a wall, a joist or a second pipe, cloth, foam or felt between the two ends the contact. Vaillant describes exactly that situation in its guidance on noisy heating pipes. On an accessible run in a cupboard or loft it is a small job. Under a finished floor it means lifting boards, which is engineer's work.
The cylinder gets blamed because it amplifies the sound. A large metal vessel with rigid connections carries vibration well, so a hum from three metres away can seem to come from inside the tank.
Why a flat expansion vessel on an unvented cylinder makes banging worse
Banging that has grown worse over months on an unvented cylinder is worth checking against the expansion vessel. The air in that vessel damps water hammer surges as well as taking up thermal expansion, and the air volume shrinks with time.
An unvented cylinder is fed straight from the mains, so it carries built-in protection against pressure build-up and thermal expansion. Water expands by up to 4% of its volume when heated. The expansion vessel provides that space. Heatrae Sadia also lists damping the shock of water hammer as a function of the air in the vessel.
Water leaving the expansion valve now and then is the published sign that the air volume has shrunk too far. Manufacturer guidance says the air charge has to be renewed periodically. Checking the charge, recharging the vessel and cleaning the strainer are tasks for a competent engineer, normally alongside an annual service. There is more in our notes on why the expansion vessel loses its charge and on servicing an unvented cylinder.
Vented or unvented cylinder: which noises belong to which system
A vented cylinder is fed by a cold water cistern, usually in the loft, and vents to the atmosphere through an open pipe. It therefore runs at lower pressure. Banging heard above the ceiling after someone has run a bath normally belongs to that cistern refilling, not to the cylinder.
An unvented cylinder has no loft tank at all, so the refill noise disappears. What remains is mains pressure in every hot pipe, plus the pressure reducing and expansion controls at the cylinder. Noise on an unvented system is therefore concentrated around the cylinder valves and the pipe runs they feed. Knowing which type you have narrows the search before anyone lifts a floorboard.
Cylinder noise: what you can check yourself and what needs a registered engineer
- Read the boiler pressure gauge and note what it shows when the system is cold
- Check the settings on the room thermostat, the cylinder thermostat and the programmer
- Use the reset button on the boiler if the controls call for it
- Bleed radiators that feel cold at the top
Those four checks are the safe limit for a householder with a noisy hot water system. Gurgling is worth noting as a symptom: it points to trapped air travelling with the water. Clearing an airlock on the hot water side is not a householder task.
The safety side of an unvented cylinder belongs to a qualified engineer. That covers the expansion vessel, the pressure reducing valve, the expansion and temperature relief valves and the strainer. In England, installing or replacing an unvented cylinder over 15 litres is notifiable under the Building Regulations, and Approved Document G requires at least two independent safety devices besides the thermostat. Who may carry out that work is set out in our guide on installing an unvented hot water cylinder. Work on gas appliances, flues or inside a boiler case falls under the Gas Safety (Installation and Use) Regulations 1998 as amended and is for Gas Safe registered engineers.
Banging or vibrating: which explanation to follow for your cylinder
Follow the water hammer trail if the noise is one sharp bang that lands exactly as a tap, shower or WC valve moves. It usually sounds through the whole house and occurs on cold draw-offs as well. Follow the moving pipework trail if the sound is a hum or buzz that lasts while water flows, comes from one place and is loudest on a run that has just heated up.
Both can exist on one system, which is why the two are worth separating before anything is dismantled. Where an unvented cylinder has been in service for several years and water occasionally leaves the expansion valve, the vessel charge is the sensible first item on an engineer's list. Where the house is quiet until a particular shower is used, the pipe run to that shower is the sensible first item.
If you would like the cylinder, its valves and the pipework around it looked at, a free home survey and a written fixed price are the starting point for Megaflo and unvented cylinder work.
Sources
- Vaillant – How to fix noise in central heating pipes
- Viessmann – How to stop noisy central heating pipes
- Heatrae Sadia – A–Z glossary of heating terminology
- Heatrae Sadia – Back to basics: how does an unvented cylinder work
- Heatrae Sadia – Love Your Cylinder FAQs (PDF)
- Baxi – Back to basics: cylinders
- Approved Document G: sanitation, hot water safety and water efficiency (PDF)
- CNC Building Control – Permission to install or replace a hot water cylinder
- Ideal Heating – What is kettling in a boiler?
- Gas Safety (Installation and Use) Regulations 1998