Why is my boiler making noises? Banging, kettling, gurgling

Most boiler noises fall into a handful of categories, and each one points to a different cause. A faint whoosh and a low hum are normal. Banging, rumbling, gurgling, loud humming and whistling are not. This guide runs through each, what it means, and whether it is a job for you or for a Gas Safe registered engineer.

The one to treat as urgent is a loud bang or boom when the boiler fires up. That can be delayed ignition, a gas-side fault, and it should be looked at quickly. The rest are usually safe to book in for a normal appointment.

Points to take away

• A healthy boiler makes a soft whoosh on ignition and a quiet, steady hum.
• Rumbling like a kettle is kettling: scale or sludge in the heat exchanger.
• Gurgling is usually trapped air or low pressure, both DIY fixes.
• A loud bang on start-up is the noise to get checked promptly.
• Growing hum or vibration means a worn pump or fan bearing.

Reading time: about 6 minutes.

What a healthy boiler sounds like

When the heating calls for heat, you should hear the fan spin up, a soft whoosh as the burner lights, then a low, even hum while it runs. From the radiators, a faint tick as the metal warms is normal. Anything louder, sharper or newer than that is worth identifying.

Banging or booming

A single loud bang when the boiler ignites. Most likely delayed ignition. Gas enters the combustion chamber, does not light straight away, builds up, then ignites with a thump. Causes include a dirty or misaligned ignition electrode, low gas pressure, or a partly blocked burner. Get it checked promptly. This is a gas-side fault, not a “book it next month” noise.

Banging or knocking while running. Often kettling (see below), or trapped air, or pipes expanding against a joist or a tight clip (water hammer). Check the pressure is 1.0 to 1.5 bar cold first. If pipes knock as they heat and cool, an engineer can fit clips or a cushion where a pipe passes through timber.

Kettling: a rumbling, bubbling noise

Kettling sounds like a kettle heating up: rumbling, bubbling, sometimes rising to a whistle. It is loudest after the heating has been on a while.

The cause is a restricted heat exchanger. In hard water areas, that is limescale. Hard water is common across London, the South East, the East Midlands and much of the East of England. Elsewhere it is more often magnetite sludge, a black iron-oxide grit from corroding radiators. Either way, water flow slows, hot spots form, and water flashes briefly to steam.

Kettling is not an immediate danger, but it stresses the boiler and gets worse. It needs an engineer to flush the system or clean the heat exchanger.

Gurgling

Gurgling usually means one of two things, both fixable without an engineer.

• Trapped air. Bleed the radiators, starting downstairs and working up. Top the pressure back up afterwards.
• Low pressure. Not enough water is passing over the heat exchanger. Repressurise the boiler to 1.5 bar. Our step-by-step guide to repressurising a boiler covers how.

In cold weather, gurgling from the boiler followed by a lockout is often a frozen condensate pipe. Pour warm water over the external pipe and reset the boiler.

Humming, buzzing or vibrating

A quiet hum is normal. A hum that has got louder, or turned into a buzz, drone or vibration, points to a component under strain.

• Pump. Worn bearings make a pump drone or vibrate. A failing pump also causes poor circulation and cold radiators.
• Fan. A fan with worn bearings whines or rattles, usually worst on start-up.
• High mains pressure. Can make the filling loop or valves buzz. A plumber can fit a pressure-reducing valve.

None of these are emergencies, but a worn bearing only goes one way. Book it in before the part fails and takes the heating with it.

Whistling

Usually kettling at a higher pitch, or trapped air, or water forcing through a partly closed valve. Try bleeding the radiators and checking pressure. If it persists, it is the same heat exchanger problem as kettling and needs a flush.

How worried should you be?

Use this to decide whether to book a normal appointment or push for a quick one.

Fixing kettling: flush, filter, and whether it comes back

An engineer has three options depending on how bad it is. A chemical clean, adding a cleaner to the system and running it, then draining and refilling with inhibitor, costs £120 to £250. A power flush, forcing water and chemicals through each radiator, costs £300 to £600 and is more thorough. Fitting a magnetic filter afterwards, £90 to £150, catches new sludge before it reaches the boiler.

If the heat exchanger is already heavily scaled and the boiler is over 10 years old, weigh a flush against the cost of a new boiler. Our guide to the signs a boiler needs replacing covers when repair stops making sense.

Get free quotes from Gas Safe registered engineers covering your postcode, or browse the directory to find heating companies registered in your area.