Why is my boiler making noise?

A noisy boiler can be down to a number of things. Sometimes it is perfectly normal for a boiler to make a noise, such as when the pilot light is ignited or when the pump circulates water around the boiler’s burner and heat exchanger. However, there are other times when a noisy boiler is a cause for concern.

Banging or whistling noises, for example, are a sign of boiler kettling; the term used to describe water reaching boiling point inside the boiler, which creates steam and causes pressure to potentially rise to higher levels. Gurgling noises, on the other hand, are caused by trapped air passing through the system and moving water in the boiler and sometimes the radiators.

If you hear unusual sounds coming from your boiler system, you should first identify the potential causes.

Boiler kettling is a common cause of a noisy boiler. Water within the system is typically heated to around 70 °C inside the boiler circuit before being pumped throughout a home. However, the flow of water can be slowed down, causing the water to heat too quickly and reach boiling point (100 °C).

When this happens, steam is generated and air becomes trapped inside the system, giving off a whistling sound; hence ‘kettling’. Boiler kettling can also create banging noises.

There are several reasons why water might be pumped around at a slower rate:

Limescale build up

It is common for homes in hard water areas to experience limescale build up. Over time the mains water causes limescale to build up in pipes and block the flow of water. By slowing the flow of water as it passes over the heat exchanger, the system heats the water to boiling point and the steam that is given off increases pressure inside the system.

If you live in a hard water area, your system will use the same water, and it’s possible that this can cause limescale inside the pipework and heat exchanger. That will reduce the effective cross-section of the pipes and heat exchanger channels and slow the flow down, as well as making it more turbulent.

Faulty thermostat

A broken thermostat can give an incorrect reading to the boiler and cause it to heat water to dangerous levels. This is sometimes the case in older boilers, where faults can crop up as the boiler reaches the end of its lifespan. New appliances have many more safety devices that prevent this from occurring in the event of a faulty thermostat.

Damaged pump

The boiler pump is essential for pushing water round the system at the right speed to heat water to a suitable temperature. If the pump becomes faulty, it is likely that water will be pushed at a slower rate and overheat.

If you suspect your boiler is kettling, you should call a Gas Safe installer to inspect your system. Boiler kettling can be dangerous and should only be inspected by a qualified professional. If the source of the issue is limescale build up, they will most likely need to perform a chemical clean or power-flush.

If the issue stems from faulty parts, these parts will need to be replaced.

Gurgling noises are a sign that air has become trapped inside the system and is mixing with the water. While this is usually not a significant problem, it should be resolved as soon as possible. The simplest way to do this is by bleeding your radiators.

How to stop gurgling noises

Turn on your heating system to allow your radiators to warm up. Once they are warm, carefully feel the surface of each radiator. If certain parts of the radiator feel cooler it is likely that a pocket of air is trapped there. Turn off the heating and allow the radiators to cool.

Next, place the radiator key in the radiator valve of every radiator with potential trapped air and place a bucket. Turn the key anti-clockwise until you hear a hissing noise - this means that air is leaving the system.

Once the hissing has stopped and only water is coming out, close the radiator valve by turning the key clockwise. Repeat this process for the other affected radiators. You can now check the water pressure on your boiler to see if the level has decreased, and if the gurgling noises have stopped.

Humming boiler noises can be created if water arrives from the mains at too high a pressure. This can cause valves in the boiler or around a home’s pipes to hum. A humming noise can also be caused by a pump sending heated water around the pipework too quickly, or by the bearings in the boiler fan humming as it moves.

How to stop humming noises

Humming noises should be inspected by a boiler engineer. If the issue is related to the boiler pump or fan, the boiler casing would need to be taken off and this can only be done by a qualified professional.

There are two common reasons for whooshing noises originating from a boiler.

Air intake pipe blockages

The air intake pipe is attached to the back of the boiler and leads outside of the property. Objects and debris can block the entry to the air intake pipe and create a whooshing sound. If you place your hand over the pipe and cannot feel air moving through the pipe, there is likely a block. If you can feel air movement, the likely cause is the air filter.

Air filter blockages

Air filters are usually located inside the boiler and can be blocked by the buildup of dust and other materials. The debris on the filter blocks the passage of air and causes the whooshing sound. It is very important that only a Gas Safe Registered Engineer works on the appliance.

How to stop whooshing noises

Whether the air intake pipe or air filter is at fault, the solution is to call a Gas Safe engineer to resolve the issue. Obvious blockages to the air intake pipe can be resolved with caution, but anything that is not immediately visible should be investigated by a qualified professional.

A noisy boiler, while alarming, is usually not an immediate cause for concern. Boilers are built with safety mechanisms to shut the boiler down before it becomes dangerous. However, this is not 100% guaranteed, and it is always advisable to arrange for a boiler engineer to inspect your boiler as soon as possible.

• If a boiler is making a strange and unexpected noise, there are several potential reasons for it.
• Boiler kettling occurs when water is heated to boiling point inside the boiler and steam is released, trapping air in the system and increasing pressure levels.
• Gurgling noises are caused by trapped air, and can be resolved by bleeding radiators.
• Drone noises can also be caused by a faulty pump, and must be replaced by a qualified heating expert.
• Whooshing noises are caused by blockages either to the air intake pipe or to the air filter.
• Strange boiler noises are usually not immediately dangerous but must nonetheless be investigated by a boiler engineer as soon as possible.