Boiler making noise? What each sound means

This guide explains what banging, gurgling, kettling, humming, whistling and clicking mean. It covers the safe checks you can do first. If the noise doesn't stop, you'll need a boiler repair from a Gas Safe engineer. This guide is part of our common boiler problems series.

Why is my boiler making a noise?

A boiler gets noisy when water can't move through it freely, or when air gets in. Air, low pressure, sludge and limescale cause most boiler noises. A worn pump or a loose part can also hum or rattle.

Sludge is dirt and rust that builds up in the heating water over the years. Limescale is the hard, chalky layer that hard water leaves behind when it's heated. You'll know it from the inside of a kettle. Both can coat the heat exchanger. That's the part inside the boiler where heat from the flame passes into the water.

What should I check first?

Start with the pressure, the radiators and, in a frost, the condensate pipe. These checks are all done from the outside, and they sort out many gurgles and bangs. Never take the cover off the boiler.

1. Listen for where it comes from. Is it the boiler, the pipes or a radiator? Note when it happens: when the heating starts, when you run a hot tap, or all the time.
2. Check the pressure. Look at the gauge on the front of the boiler. With the heating off, it should usually read about 1 to 1.5 bar. If it's low, see how to top it up in our guide to a boiler losing pressure.
3. Feel the radiators. If any are cold at the top, air is trapped. Bleed them, then check the pressure again. Our guide to cold radiators shows how.
4. Open the radiator valves. If most of your TRVs are turned right down, little water can flow. A TRV is a thermostatic radiator valve, the numbered dial on a radiator. Open a few and listen again.
5. Check the condensate pipe in a frost. This is the small plastic pipe that takes waste water from the boiler to a drain. On a freezing day, look at the part outside for ice.
6. Write down any fault code, plus the boiler make and model. The engineer will ask for them.

What does each boiler noise mean?

Each kind of noise points to a different cause. Gurgling is often something you can sort yourself. Kettling, loud banging and a grinding hum usually need an engineer. This table sums up the common noises.

What is boiler kettling?

Kettling is when a boiler rumbles, pops or whistles like a kettle coming to the boil. It usually means limescale or sludge on the heat exchanger is slowing the water down. The water gets too hot in spots, turns to tiny bubbles of steam and makes the noise.

A weak pump can cause it too. The pump is the part that pushes hot water round your radiators. If it moves the water too slowly, the water sits in the boiler and overheats. Kettling often gets worse over time. It makes the boiler work harder and use more gas. It can also strain the heat exchanger, and some boilers cut out when they get too hot. Our guide to a boiler that keeps turning off explains that.

Kettling needs a Gas Safe engineer, as the cause is inside the boiler or the system. Depending on what the engineer finds, the fix may be a power flush, a new pump or descaling the heat exchanger. Descaling means clearing the limescale. In hard water areas, scale builds up faster, mainly if the system is topped up often.

Is a banging boiler dangerous?

Usually it isn't an emergency. Boilers have safety cut-outs that turn them off before they get dangerously hot. But banging is a sign something is wrong. Left alone, it wears the boiler out faster.

If the banging is loud and keeps going, turn the boiler off and call a Gas Safe engineer. Don't keep running it to see if it stops. If you smell gas at any point, leave the boiler alone and call 0800 111 999.

Why does my boiler bang when the heating comes on?

When the heating starts, the boiler fires up hard to warm cold water quickly. If the water can't flow freely, it boils in spots and bangs. Sludge, limescale, air or closed radiator valves can all slow it down.

• Kettling from scale or sludge on the heat exchanger.
• Air trapped in the radiators or pipes. Bleeding them often helps.
• Most TRVs turned down, so the water has few places to go.
• A weak or failing pump that can't move the water fast enough.
• Pipes knocking as they warm up. Metal pipes grow slightly as they heat. If they rub on a floorboard or a loose clip, they tick or knock.

Banging from the water pipes when you turn a tap off is different. That's usually on the plumbing side. See our plumber page.

Why is my boiler gurgling?

Gurgling usually means air in the system or low pressure. Bleed your radiators, then check the gauge reads about 1 to 1.5 bar. A short gurgle just after you've bled a radiator or topped up the pressure is normal, as the last air moves through.

In freezing weather, gurgling often means the condensate pipe has frozen. The waste water can't drain, so it backs up and gurgles. The boiler may then stop and show a fault. You can often thaw the pipe with warm water, never boiling. See our guide to a frozen condensate pipe for the steps.

If you keep needing to bleed radiators or top up the pressure, air or water is getting in or out somewhere. That needs an engineer to find.

Why is my boiler humming or vibrating?

A loud hum or vibration usually comes from the pump. The pump may be set to run too fast, or it may be wearing out. A loose fixing on the boiler or the pipes can also make it buzz or rattle.

A soft, steady hum while the boiler runs is normal. A hum that gets louder, grinds or rattles is not. Changing the pump speed or replacing the pump is a job for an engineer. On a combi boiler, the pump is inside the case. Our guide to hot water but no heating explains more about pumps.

Why is my boiler whistling?

Whistling usually means water is being squeezed through a narrow gap. Limescale, sludge or a partly closed valve can all cause it. A high whistle from the boiler is often a form of kettling.

Check the pressure, and open any TRVs that are turned right down. If a single radiator valve whistles, try opening it a little more. If the boiler itself still whistles, it needs an engineer.

Why is my boiler clicking?

A few clicks when the boiler starts or stops are normal. Repeated clicking without the boiler lighting is not. That usually means the spark is trying to light the gas, but the gas isn't lighting.

Normal clicks come from switches inside the boiler, the thermostat and any zone valves. A zone valve is a motorised valve that sends hot water to the heating or the hot water. If the boiler keeps clicking and won't fire up, check your gas hob lights. Then reset the boiler once. Our guide to a boiler that won't turn on has the full checks.

Which boiler noises are normal?

Every boiler makes some noise. A soft whoosh as it lights, a low hum while it runs and a few clicks are all normal. Pipes ticking as they warm up and cool down are normal too.

• A whoosh or soft thud as the burner lights.
• A low hum from the fan and the pump.
• Clicks from the thermostat, the timer and valves switching.
• The fan running on for a few minutes after the boiler stops, to cool it down.
• A short gurgle after you've bled the radiators.

A noise that is new, loud or getting worse is worth checking. So is any noise with a fault code or a smell.

Do you need a new boiler if it's making noises?

Usually not. Most noisy boilers can be fixed by bleeding, a new pump, a power flush or descaling. The noise is often caused by the water in the system, not the boiler itself.

A new boiler can make more sense when:

• the heat exchanger is badly scaled or failing on an older boiler, as it's one of the most costly parts to replace
• the pump and the heat exchanger are both worn on an older boiler
• the noise keeps coming back after a flush or a new pump, along with other faults

The engineer checks the boiler first, then talks you through the options. If a repair makes more sense, we'll say so. If not, our boiler installation page explains how it works. It starts with a free survey, and you get a fixed price.

When to call a Gas Safe engineer

Call a Gas Safe engineer if the noise carries on after you've checked the pressure and bled the radiators.Kettling, loud banging, grinding and clicking without lighting all point to a fault that needs a proper check. By law, paid gas work must be done by someone on the Gas Safe Register.

• the boiler rumbles, pops or bangs every time the heating comes on
• it hums loudly, grinds or vibrates
• it keeps clicking but won't light
• you keep needing to bleed radiators or top up the pressure
• the noise comes with a fault code, a lockout or a burning smell

The noise is only a starting point. The engineer first runs a thorough check of the boiler and the system. That finds the fault and the parts needed. If sludge is the cause, a power flush can clear it. That's a deep clean that pumps water and a cleaner through the pipes and radiators at speed. See our power flush page for how it works. For other faults, see boiler repair. We repair all makes.

A yearly boiler service helps catch noisy faults early. The engineer checks the boiler is safe, cleans key parts and spots wear before it causes a breakdown.

Other boiler guides

• Cold radiators: what cold at the top or bottom means, and how to bleed a radiator.
• Boiler losing pressure: what the gauge should read and how to top it up.
• Frozen condensate pipe: how to thaw it safely.
• Boiler keeps turning off: lockouts, short cycling and when to reset.
• Boiler won't turn on: the checks to try first, and what a pilot light is.
• Hot water but no heating: thermostat, timer and valve checks.
• Heating works but no hot water: checks to try, and what a diverter valve does.
• Leaking boiler: what to do first, and where the water is coming from.
• Boiler not working after a power cut: how to reset it, and set the timer clock.
• Boiler pressure too high: what is too high, and how to lower it safely.
• Boiler tripping the electrics: why it trips the fuse box, and who to call.
• All common boiler problems.

Boiler still making noise? Book a boiler repair and we'll find the cause.