Boiler noises: what banging, kettling and gurgling mean

When to stop and call straight away

Most boiler noises are not emergencies. A few are. Turn the boiler off and call an engineer if:

• you hear a bang from the boiler on ignition, on gas or oil;
• the noise comes with smoke, a smell of fumes or oil, or soot around the boiler or flue;
• your carbon monoxide alarm sounds;
• the pressure gauge is near 3 bar or water is discharging outside.

If you smell gas, do not touch switches. Turn off the gas at the meter if safe, open doors and windows, leave the property and call the NI Gas Emergency Service on 0800 002 001.

For oil boilers, see also oil boiler smoke or smell.

What you can safely check yourself

• Boiler pressure on sealed systems: about 1 to 1.5 bar cold is typical. See boiler pressure low.
• Bleed the radiators if you hear gurgling or trickling. See bleeding radiators.
• The condensate pipe outside, if it is cold and the boiler is a condensing model.
• Thermostat and controls: some noises come from the boiler cycling on and off rapidly.

Do not take the boiler case off, adjust the pump, or work on the burner.

Banging

A bang or “whump” on ignition

On a gas boiler, this is often delayed ignition: gas builds up for a moment before it lights. On an oil boiler, it can mean oil is igniting late, the electrodes are worn, or the chamber has unburnt oil in it after repeated resets.

This needs an engineer. Do not keep resetting a boiler that bangs on start-up.

Banging in the pipes

Banging or knocking along the pipes, sometimes called water hammer, is more often caused by:

• air in the system;
• sludge or scale restricting flow;
• pipes expanding as they heat and rubbing against joists or clips;
• valves closing quickly.

It is not usually dangerous, but persistent banging puts strain on joints and the pump. Bleed the radiators first; if it continues, get it looked at.

Kettling (rumbling, like a kettle)

Kettling is a rumbling or boiling sound from the boiler. It happens when water overheats in the heat exchanger and briefly turns to steam. Common causes:

• limescale on the heat exchanger, especially in hard water areas;
• sludge reducing flow through the system;
• a failing pump or wrong pump speed;
• low pressure or blocked filters.

Kettling makes the boiler work harder and can shorten the life of the heat exchanger. It needs an engineer to diagnose. The fix might be a system clean, a scale treatment, a magnetic filter or a pump repair.

Gurgling

Air in the system

Gurgling or trickling, especially when the heating first comes on, is usually air. Bleed the radiators and check the pressure.

Frozen or blocked condensate pipe

On condensing boilers (most gas boilers fitted in recent years, and modern oil boilers), gurgling before the boiler stops in freezing weather points to the condensate pipe. If the pipe runs outside at ground level and you can reach it safely, pour warm (not boiling) water along it or hold a hot water bottle against it, then reset the boiler once.

Do not climb ladders to reach a pipe. If the boiler does not restart, call an engineer.

Whistling or squealing

A high-pitched whistle can come from:

• scale in the heat exchanger restricting flow, similar to kettling;
• a pump running too fast or starting to fail;
• air trapped in the boiler.

Persistent whistling should be checked on your next service, or sooner if it is getting worse.

Humming or vibrating

A low hum can come from the pump, the fan or pipework vibrating. A pump set to too high a speed can hum through the whole system. A fan bearing that is wearing can get louder over time. Neither is urgent on its own, but both are worth mentioning to an engineer.

Clicking

A few clicks as the boiler starts are normal: valves opening and the ignition sparking. Repeated clicking with no flame, especially followed by a lockout, points to an ignition fault. Reset once; if it continues, call an engineer.

Oil boiler noises

Oil burners are louder than gas boilers by design. You will hear the motor and fan start and the flame roar. What is not normal:

• rumbling or pulsing once lit, which usually means combustion is poorly set;
• a bang at ignition;
• screeching from the burner motor.

These are all checked on an annual oil service, where the burner is set up with a combustion analyser. See oil boiler not firing for lockouts.

How a service helps

Many boiler noises start small and get worse. On a service, our engineer checks combustion, the heat exchanger, the condensate trap, the pressure and the expansion vessel, which covers most of the causes above. Tell us about any noise when you book so the engineer can listen for it.

Book a repair or service

Repair visit £85 for the first hour, then a fixed quote before any further work. Gas service £79, oil service £89. Book online or email [email protected].