Heating systems are not silent, but new noises are information. A rumbling boil-and-whistle from the boiler is not the same fault as a bang under the floor when the heating shuts off, and neither is the same as a constant electrical hum from the pump. Each has a distinct cause and a distinct fix.
Below is how we separate them on site.
Kettling: rumbling or whistling from the boiler
Kettling is scale or sludge on the heat exchanger causing water to boil locally instead of flowing away. It sounds exactly like a kettle coming to the boil, and it means the exchanger is running far hotter than intended in patches. Left alone it shortens exchanger life and eventually causes overheat lockouts.
The fix is a proper system clean and, in hard-water or heavily sludged cases, exchanger treatment or replacement — plus a filter and inhibitor so it does not simply return.
Banging and creaking pipework
- A single sharp bang when a tap or valve closes is water hammer — usually solved with correct pipe support or an arrestor.
- Creaking as heating comes on and goes off is thermal expansion against joists or clips, and is cosmetic rather than harmful.
- Repeated knocking under floors often means pipe clipping is missing where it should not be.
Humming, whining and vibration
A constant hum usually comes from the circulation pump. Set too high for the system, a modern modulating pump will transmit noise through pipework; failing bearings produce a rougher whine. Airlocks make a distinctive gurgle that comes and goes with the heating cycle.
Pump speed and system balancing are worth checking before replacing anything — noise frequently disappears once flow rates are set correctly.
Noises that mean stop and call
- Any hissing accompanied by a gas smell — turn off the supply, ventilate, and call the gas emergency line.
- Loud, sustained boiling with the boiler cycling on overheat.
- New noise combined with visible water around the appliance.
Frequently asked questions
- Is kettling dangerous?
- It is not an immediate safety risk in the way a combustion fault is, but it damages the boiler over time and signals a system that needs cleaning. It should not be ignored for a season.
- Will a chemical additive quieten a kettling boiler?
- Sometimes, temporarily, on a lightly affected system. On a genuinely dirty system it is a stopgap; the debris needs removing rather than suspending.
- My pipes bang only when the heating turns off. Why?
- That is usually contraction against a fixing point, or a valve slamming shut. Both are fixable, and neither indicates a boiler fault.
- Can annual servicing catch this early?
- Yes. Inhibitor testing and filter inspection at each service are exactly how kettling gets caught before it becomes an exchanger replacement.
Get the noise identified before it becomes a repair
Describe when the noise happens — during firing, during hot water, or when the heating shuts down — and we can usually name the cause.