Boiler making a loud noise: what each sound usually means

Stop and switch off if you notice any of these

Most boiler noises are a fault developing rather than an emergency. A few are not. If you smell gas, do not touch any switches, turn the gas off at the meter if it is safe to do so, open windows and call the National Gas Emergency Service on 0800 111 999.

• A loud bang when the boiler lights, especially with a smell of gas or burning.
• Violent banging or boiling sounds with pipes near the boiler too hot to touch.
• Water or steam coming from the small copper pressure relief pipe on an outside wall.
• Scorch marks or soot on or around the boiler casing, or a carbon monoxide alarm sounding.

In any of these cases, switch the boiler off at its controls and its fused spur, and leave it off until a Gas Safe registered engineer has checked it.

Match the sound to the likely cause

Try to describe the noise as precisely as you can and note when it happens: when the boiler lights, while the heating runs, only when hot water is on, or after the boiler switches off. That timing narrows down the cause far more than the loudness does.

Banging and whistling: kettling

Vaillant describes banging and whistling as water reaching boiling point inside the boiler, producing steam and pushing the pressure up. It is called kettling because it sounds like a kettle coming to the boil. In hard water areas, which includes most of London, limescale building up on the heat exchanger is a common cause. Sludge from corroded radiators, a slow or failing pump, or valves left closed can do the same thing by stopping the water carrying the heat away quickly enough.

Kettling does not usually fix itself, and it puts strain on the heat exchanger. A boiler that kettles may also lock out on an overheat fault. An engineer will check the flow, the pump and the condition of the system water, and may recommend descaling or cleaning the system. Our guide to boiler kettling noises covers what that investigation should establish, and power flushing explains when a full system clean is the right answer.

Gurgling: air, low pressure or a frozen pipe

Gurgling is most often air moving round the heating system. It tends to follow work on the heating, a drop in pressure, or radiators that have not been bled for a long time. Radiators that are cold at the top are a strong sign.

Bleed the radiators with the heating off and cool, starting downstairs, then check the pressure gauge. Bleeding releases water as well as air, so on a sealed system the pressure often needs topping up afterwards; most boilers want roughly 1 to 1.5 bar when cold, but check your user guide. If the pressure keeps falling, there is a leak somewhere. In freezing weather, gurgling from the boiler itself often means the condensate pipe outside has frozen and the water is backing up.

Humming, droning and vibration

A low drone is usually the pump. It may be set to run faster than the system needs, or it may be wearing out. A pump that hums but does not move water will often lead to cold radiators and overheat faults. Vaillant also lists high mains water pressure on the valves and worn fan bearings as causes of humming.

None of these are things to adjust yourself. Our guide to heating pump repair or replacement explains how an engineer decides whether to reset, adjust or replace a pump.

Whooshing, roaring and repeated clicking

A boiler needs a clear supply of air and a clear flue to burn gas cleanly. A whooshing or roaring sound can mean the intake or flue is partly blocked, perhaps by a bird's nest, leaves or debris, or that the fan is struggling. Look at the flue terminal from the ground, but do not climb to it and do not poke anything into it.

Rapid clicking without the burner lighting is the ignition trying to fire. It is often a gas supply issue, so check the hob works and a prepayment meter has credit. If the boiler then shows a fault code, note it down and look at our guide to a boiler that is not working for the next checks.

Noises from the pipes rather than the boiler

Not every noise comes from the boiler. Pipes creaking or ticking as the heating warms up and cools down is expansion, and it is usually harmless. A sharp bang when a tap is turned off is water hammer, which affects the water supply rather than the boiler. If the noise is mainly at night, our guide to heating pipe noises at night helps you tell the two apart. And if the sound only started straight after a service, our article on boiler noise after a service explains what to report back.

Getting a noisy boiler looked at

Call if the noise is new, getting louder, comes with a fault code or rising pressure, or if the heating or hot water has become unreliable. Noisy boilers rarely get better on their own, and catching kettling or a failing pump early usually means a smaller repair.

Loyal Plumbers has 10 vans covering London and everywhere inside the M25, 24 hours a day, and our boiler work is carried out by Gas Safe registered engineers. When you call, describe the noise, when it happens, the boiler make and model, what the pressure gauge reads and whether any radiators are cold in places. A short phone recording of the sound helps too. Our call-out charge is £60, and the engineer explains what they have found and what the repair involves before starting.