Why is my boiler making noises? What each sound means

A boiler is never completely silent, so the first job is to work out whether the sound is normal. Then you can do the few safe checks below and decide whether you need an engineer. None of these checks involve opening the boiler casing or touching any gas parts.

Safety: If you smell gas, or the boiler has soot or black marks around it, a yellow or orange flame instead of a crisp blue one, or anyone at home has headaches, dizziness or nausea, stop. Turn the appliance off if it's safe to, open doors and windows, get everyone out into fresh air and call the National Gas Emergency Service on 0800 111 999.

Which boiler noises are normal?

Ideal Heating lists three sounds you don't need to worry about:

• A quiet whirring while the heating is on. That's the pump moving water round the system.
• A deep, short rumble when the boiler lights, as the burner starts heating the water.
• Clicking as switches and valves turn on and off.

If the noise is new, louder than usual or keeps going, it's worth looking into. British Gas notes that a noisy boiler is not usually dangerous in itself, because boilers have safety devices that switch them off. Even so, a noise is often the first sign of a fault, and it's cheaper to deal with early.

Why is my boiler banging or making a kettle noise?

Banging, popping or a sound like a kettle coming to the boil is often called kettling. Vaillant explains that it happens when water inside the boiler gets so hot it boils, usually because the flow of water has slowed. Common causes are limescale on the heat exchanger, sludge in the system, a faulty boiler thermostat or a pump that isn't working properly.

There's no safe home fix for kettling. Vaillant says it should only be inspected by a qualified professional. An engineer may clean the system with chemicals or a power flush, fit a magnetic filter, add inhibitor to the water or replace a faulty part.

Why are my pipes banging when I turn a tap off?

If the bang comes from the pipes, not the boiler, and happens just as you turn a tap or shower off, it's probably water hammer. Ideal Heating says the sudden stop in flowing water makes the pipes knock. An occasional knock is not urgent, but hard, frequent hammering can damage pipes and joints, so ask a plumber or heating engineer to find the cause.

Why is my boiler gurgling?

Gurgling usually means air is trapped in the heating system. Both Ideal Heating and Vaillant say the first step is to bleed your radiators, which is a safe job for most people. Vaillant also says to check the boiler's pressure gauge afterwards, because bleeding can lower it. Your manual tells you the right reading and how to top it up.

In freezing weather, gurgling can also mean the condensate pipe has frozen. This is the plastic pipe, usually white, that runs from a condensing boiler to a drain outside. If you can reach it safely, pour warm water, not boiling, slowly over the outside section, then reset the boiler as your manual describes. Only ever thaw the outside pipe.

If the gurgling comes back soon after bleeding, air may be getting into the system somewhere. Ideal Heating says the system may then need draining, which is a job for an engineer.

What does a whistling or high-pitched boiler mean?

Ideal Heating says whistling is usually caused by trapped air, though kettling or a pump that needs attention can also cause it. British Gas links whistling to limescale building up in the heat exchanger, and says a high-pitched noise can be a sign of overheating. Try bleeding the radiators first. If the whistle stays, book an engineer.

Is a grinding, droning or humming boiler serious?

These sounds usually point to moving parts. Ideal Heating says grinding is caused by a faulty pump and advises switching the system off and calling an engineer straight away. Vaillant says a loud drone, like an aeroplane taking off, is usually a failing pump too.

Vaillant lists several causes of humming: mains water arriving at too high a pressure, a pump running too fast, or worn bearings in the boiler's fan. Ideal Heating adds that loud vibrating can come from loose parts, a faulty thermostat, debris in the heat exchanger or an unbalanced system. All of these need an engineer to diagnose, because fixing them means opening the boiler.

Can hard water make a boiler noisy?

Yes. Hard water leaves limescale, and South East Water says this can build up in kettles, boilers and hot water pipes. Limescale on a boiler's heat exchanger is one of the main causes of kettling. If you live in Berkshire, it's worth knowing your water hardness. South East Water says most water in southern England comes from chalk aquifers and is hard, and Thames Water says all the water in its region is hard. You can check your own supply by entering your postcode on your water company's website.

If you're in a hard water area, ask your engineer whether a scale reducer or inhibitor would help, and keep up a yearly service so scale is spotted early.

When should you call a Gas Safe registered engineer?

Only a Gas Safe registered engineer should work inside a gas boiler, on its gas pipework or on the flue. Book one if:

• The boiler bangs, pops or sounds like a kettle.
• You hear grinding, droning or loud humming.
• Gurgling or whistling carries on after you've bled the radiators.
• The noise comes with a fault code, a leak or pressure that keeps dropping.
• The boiler keeps switching itself off or locks out again after one reset.

When the engineer arrives, ask to see their Gas Safe ID card. The Gas Safe Register says to check both the front and the back, so you know they're qualified for the work you need. If you rent, report the noise to your landlord rather than booking an engineer yourself.

Turn the boiler off and keep away from any electrics if the boiler is leaking water. Don't open the boiler casing, and don't keep pressing reset if it locks out more than once.