Where this starts
Almost every boiler noise falls into three families, and they sound quite different once you know there are three. Standing next to it while it fires up and listening for thirty seconds separates them better than any description over the phone.
KETTLING is the important one. It is a rumbling, knocking or banging from the boiler itself shortly after the burner lights, and it rises and falls with the burner. The mechanism is exactly what its name suggests: limescale or corrosion sludge on the heat exchanger creates hot spots where water flashes to steam and immediately collapses back, and the noise is that collapse repeating. It matters because it is a symptom of the water and the system, not of the boiler as a machine, and because it means the exchanger is running hotter than designed.
A CONSTANT WHINE OR HUM that runs whenever the boiler runs and does not follow the burner is a rotating part: the circulating pump, the fan, or a bearing going. It tends to be steady rather than rhythmic, and it is often audible with the burner off but the pump still running after a cycle.
TICKING AND CREAKING that begins a few minutes after the heating starts and comes from the pipework rather than the appliance is thermal expansion. Copper grows as it heats, and a pipe passing through a tight joist notch or resting on masonry moves against it in small steps. It is a nuisance rather than a fault, and it belongs to the pipe route rather than the boiler.
WHAT MAKES KETTLING WORSE OVER TIME, and why it is worth acting on: the sludge and scale that cause it also reduce flow and heat transfer, so the boiler works harder for less output, cycles more, and the exchanger runs hotter. Systems that kettle noisily for years tend to arrive at a failed exchanger, which is normally the single most expensive part.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
Kettling from scale or sludge on the heat exchanger
Most commonRumbling, knocking or banging from the boiler starting shortly after the burner lights and following it, often worse in a hard-water area or an older system.
How to check: Whether the noise follows the burner. A noise that rises and falls with firing is water boiling locally, not a mechanical part.
Restricted flow through the boiler
Most commonKettling together with poor circulation — radiators slow to warm, a large temperature difference across the boiler, or the boiler cycling on and off rapidly.
How to check: How quickly the radiators warm and whether the boiler short-cycles. Kettling and poor flow usually have the same cause.
A failing pump or fan
CommonA constant whine, hum or grinding that runs with the appliance rather than with the burner, sometimes audible after the burner has stopped while the pump overruns.
How to check: Whether the noise is steady or rhythmic. Steady and independent of firing is a rotating part.
Pipework expanding against structure
CommonTicking, creaking or a single knock beginning minutes after the heating starts, from the pipe route rather than the appliance, on a predictable timetable.
How to check: Where the noise comes from. If it is under a floor or in a wall rather than at the boiler, it is the pipe and not the appliance.
Air in the system
CommonGurgling or bubbling rather than banging, often with cold spots at the tops of radiators and a pressure gauge that has been topped up more than once.
How to check: Whether radiators are cold at the top. Air collects at high points and gurgles as it moves.
The system running at too high a flow temperature
Less commonKettling that improves noticeably when the boiler's flow temperature is reduced, on a system set near its maximum, frequently after an installer left it at the default.
How to check: What the flow temperature is set to. A lower setting reduces local boiling and usually improves efficiency as well.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
Rising and falling with firing is kettling; steady whatever the burner does is a rotating part. That one observation splits the two largest families.
- 2
Ticking and creaking from under a floor is thermal expansion in the pipework, which is a clipping question and has nothing to do with the boiler.
- 3
Poor circulation usually accompanies kettling and has the same cause; air at the top of radiators points at a different and much simpler one.
- 4
A system losing water is taking in fresh water each time, and fresh water brings the oxygen and the minerals that produce both sludge and scale.
- 5
A boiler left at its maximum default kettles more readily and condenses less, so lowering it often reduces the noise and the bill together.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Always, for any work on the boiler itself — it is a gas appliance and only a registered engineer may open or service it.
- Where the boiler is kettling, since the remedy is usually a system clean and an inhibitor rather than anything done to the noise, and the diagnosis of what fouled it matters.
- Where system pressure keeps falling and needs topping up, because the make-up water is the cause of the fouling and the leak has to be found.
- Where there is any smell of gas, staining or sooting around the appliance, or anyone in the house has headaches or nausea — stop using it and treat it as an emergency.
- Where the appliance is an unvented or a sealed system showing signs of the pressure relief discharging, which is a safety component reporting something.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.