Combi Boiler Making Noise When Running Hot Water?

combi boiler

Last updated: May 27, 2022 at 10:53 am 

A combi boiler, or a combination boiler, is a boiler that manages both the central heating of a property as well as the hot water. It is called a ‘combi’ since it is a combination of a water heater and a boiler.

They are the most common type of boilers in the UK, largely since you do not need to have a hot water storage cylinder which means that homes can save space, this makes combi boilers particularly appealing to landlords and people who live in smaller homes or flats.

As with any boiler, combi boilers often make noises whilst being used, and generally just throughout the day. This can sometimes alarm homeowners, but much of the time the noises coming from boilers (both combi and otherwise) are completely normal.

How Combi Boilers Work

Combi boilers work by heating the water instantly as soon as the hot tap is turned on. When the hot tap is used, a valve within the boiler diverts the heat away from the central heating and towards the domestic water supply, providing you with hot water almost instantly.

This movement of the valve and heating of water, inevitably, makes a noise. Therefore, it might seem that combi boilers are noisy when hot water is being used – it is just the water heating up!

Likewise, just after running hot water, you might notice that your combi boiler makes noises. This will be the valve moving, and then the boiler working to heat up the central heating. The boiler will work to bring the room back up to the set desired temperature and will then modulate to cool down once the room is within the desired temperature range.

Also read: Why Your Worcester Boiler Is Making a Loud Vibrating Noise

What Noises Are Normal For a Combi Boiler

All boilers make noise – there are lots of moving parts in a boiler as well as water and air which make noises as they move around the system. Some of the normal noises for your combi boiler to make are:

Whooshing, Rumbling, or Humming

If your combi boiler suddenly starts to sound alive when you turn the hot tap on, do not fret. This is completely normal and will likely always happen when you use your hot water.

It is the noise of the valves within the boiler moving, and the water is heated.

Faint Humming or Buzzing

All boilers tend to make some level of humming or buzzing, as they’re always doing something, even if it is just keeping your home slightly warm or preparing to come on at a set time.
Whooshing, rumbling, or humming when the heating turns on

Just like with the hot tap, any boiler is likely to make some noise when the central heating comes on. This is just the noise of the water being carried around the home, as well as it being warmed up by the boiler. Usually, this noise subsides or quietens after a while, depending on the temperature and the boiler.

Other Noises a Boiler Might Make And Why

If your boiler makes noise generally, there might be a cause for concern. Sometimes, when boilers suddenly become noisy, it means that there is an issue somewhere in the system.

Signs that your boiler needs attention include…


Kettling is the name given to the tapping and knocking sounds that sometimes come from a boiler. These sounds can be relatively quiet or occasionally quite loud and are caused by a build-up of limescale within the boiler. This is particularly common in regions with harder water, as the minerals make boilers, kettles, and sinks more prone to limescale than in regions with soft water.

Often, kettling can only be fixed by removing the elements with limescale and replacing them. There is not particularly a short-term fix for kettling, as limescale isn’t a reversible thing. However, once fixed, a filter and specialist chemicals can be used to reduce the growth of limescale in the future.


If your boiler is making gurgling or popping sounds, it is likely an easy fix. Usually, gurgling or popping sounds coming from your boiler are a sign of trapped air somewhere in your pipes or radiators. To solve this, you will need to bleed your radiators.

You can bleed a radiator by using a bleeding key and twisting the radiator bleed valve. You will hear some hissing – this is the air escaping. When the hissing stops, tighten the valve. This should be done carefully, with a cloth beneath the valve and only when the heating is off, and the radiators are cool.


Banging or rattling from your boiler tends to mean that there is a mechanical issue within the boiler. This could be any number of things, but the most common mechanical issue with boilers involves the pump.

It is important that you call a professional over to check on your boiler if you begin to hear rattling or knocking sounds. They can then determine which element of the boiler needs looking at, and if need be, replace it.

Often, this is just caused by the pump coming slightly loose. A quick tighten is usually enough to sort out a rattling boiler.

What to Do if Your Boiler Is Too Noisy

The easiest thing to do if your boiler is making the noises it is supposed to make, but they’re too noisy for you, is to soundproof your boiler cupboard. You can do this in a few different ways.

Soundproof Panels

Soundproof panels are specifically designed to help soundproof rooms. They are often used in garages for band practice, and can really deaden the noise of your boiler, allowing you to enjoy your home in peace.

The panels are made from fibreglass and foam, and are lightweight, making them easy to attach to walls and ceilings. They work by preventing any reverberation and making it harder for sound waves to travel through the walls.

Carpet Tiles

Just like acoustic panelling, carpet tiles deaden the noise of your boiler by stopping it in its tracks. These can be easily attached to walls or placed on the floor, and for best results can even be piled on top of one another.

The tiles reduce any echo or reverberation, massively reducing the amount of noise that can exit your boiler cupboard.

Soundproof Curtains

Soundproof curtains are made from specific materials to enhance their soundproofing abilities. They have fibreglass in them and are usually dense and thick to provide more of a barrier to the sound that you’re trying to hide.

You can hang soundproof curtains on the inside and outside of your boiler cupboard for extra soundproofing.


Fibreglass appears in most sound cancelling materials, but it can also be used on its own. It works best when you can put it in between the studs of walls, but it will also work as a soundproofing material if just attached to walls for those who cannot damage the cupboard their boiler resides in.

Fibreglass is relatively inexpensive and is extremely lightweight, so will be easy to install for your soundproofing.

Soundproof Blankets

Similarly, to soundproof curtains, these are specifically designed to prevent any noise from escaping. They are great for stapling to walls, hanging on hooks, or rolling up on the floor.

Soundproof blankets can even be used as curtains if you’re unable to find the soundproof curtains you’d like!

You May Also Like

About the Author: AJ

AJ is a self-confessed soundproofing nut. He has written full-time on Quiet Living for the past 4 years, and has a wealth of knowledge about living a quieter life, soundproofing and fixing loud noises.

Leave a Reply

Your email address will not be published. Required fields are marked *