Boiler banging noise? Causes and what to check

A boiler banging noise is most often kettling: limescale or sludge on the heat exchanger makes water overheat and flash into steam bubbles, which collapse with a bang or rumble. Trapped air, low pressure and a failing pump can cause similar noises, and you can check some of these yourself safely.

A boiler that has suddenly started banging is telling you something has changed inside the system. It is rarely an emergency, but it is worth sorting before the noise turns into a breakdown.

Is a banging boiler dangerous?

Usually not in itself, but it should not be ignored. Kettling puts extra strain on the heat exchanger, and a boiler working this hard wears faster and uses more gas. If the banging comes with a smell of gas, scorch marks, black sooty staining or the boiler flame looking wrong, turn the boiler off, leave the property if you smell gas, and call the National Gas Emergency Service on 0800 111 999. Our guide on what to do if you smell gas covers the steps.

Common causes of a boiler banging noise

Kettling

Kettling sounds like a kettle coming to the boil: rumbling, popping or banging from inside the boiler, often while the heating or hot water is running. It happens when the water in the heat exchanger cannot move fast enough to carry the heat away. Parts of it overheat, steam forms, and the bubbles collapse noisily.

The usual causes are a restriction in the water flow from limescale or sludge, or a pump that is not moving water quickly enough.

Limescale from London’s hard water

London has hard water, and when hard water is heated it leaves limescale behind. It builds up inside the boiler’s heat exchanger, especially on combis, which heat mains water every time you run a hot tap. Scale narrows the waterways and creates hot spots, which leads to kettling. Our guide to hard water and limescale in your boiler explains the options, including scale reducers.

Sludge in the heating system

Over time, the inside of radiators and pipes corrodes and forms black sludge. It collects in the boiler as well as in the bottom of radiators. Clues include radiators that are cold at the bottom and black water when you bleed them. A power flush clears it out, and a magnetic filter helps stop it coming back.

Air in the system

Air pockets can make gurgling, knocking and trickling noises in the boiler and pipes, particularly when the heating first fires up. Bleeding the radiators usually helps. See our guide on how to bleed a radiator.

Low water pressure

If the system pressure is too low, water does not circulate properly through the boiler and it can overheat and bang. Low pressure also lets air in more easily. Check the gauge: with the heating cold it should be about 1 to 1.5 bar.

A failing or blocked pump

The pump pushes water round the heating system. If it is worn, partly seized or blocked with sludge, water moves too slowly through the boiler and kettling follows. A pump on its way out may also hum, whine or grind.

Banging in the pipes rather than the boiler

If the bang comes from the pipes when you turn a tap off quickly, that is more likely water hammer than a boiler fault. Loose pipes rattling against joists can also knock as they expand and cool. Our post on noisy pipes and water hammer covers this.

What is safe to check yourself?

You can work through these without tools and without opening the boiler.

1. Check the pressure gauge with the heating cold. If it is below 1 bar, top it up following the boiler manual or our repressurising guide.
2. Bleed the radiators if any are cold at the top or gurgling, then check the pressure again.
3. Note when the noise happens. Heating only, hot water only, or both? Straight after the boiler fires up, or after it has been running a while? This helps an engineer narrow it down.
4. Feel the radiators. Cold bottoms point to sludge. Hot near the boiler and cold further away can point to the pump.
5. Check the condensate pipe in freezing weather. A frozen condensate pipe makes a gurgling sound and can lock the boiler out. Warm water (not boiling) will thaw it.
6. Turn the boiler off if the noise is violent, and call us.

Do not remove the boiler casing, tap the pump, or try to descale the boiler yourself. Anything inside the case is work for a Gas Safe registered engineer only.

How an engineer fixes it

Depending on the cause, the fix might be a chemical clean or power flush of the system, descaling or replacing the heat exchanger, replacing the pump, recharging the expansion vessel, fitting a magnetic filter or scale reducer, or rebalancing the radiators. An annual boiler service is a good time to catch early signs of scale and sludge before they become noisy.

If the boiler is old and the heat exchanger is badly scaled, replacing parts may not be worth it. We will tell you honestly if a new boiler makes more sense.

When to call an engineer

• The banging is loud, frequent or getting worse
• It continues after bleeding the radiators and topping up the pressure
• The boiler locks out or shows a fault code
• You can hear the pump grinding or whining
• There is any sign of leaking, scorching or a gas smell

Book a boiler repair online, or call Lee on 020 8050 5913. If he can talk you through a safe fix on the phone, there is no charge.