Boiler Kettling

What is boiler kettling?

As mentioned, kettling is basically the sound of water boiling inside your boiler. It often sounds like a boiling kettle, hence the name kettling. But what is causing the water to boil in the first place?

In a combi-boiler, water is prevented from boiling by the speed which it flows over the burner. If something were to reduce the water flow, the water would spend more time over the burner and therefore get heated to a higher temperature. This usually goes unnoticed until the flow has decreased to such an extent that the water begins to boil.

What causes boiler kettling?

So what causes flow reduction?

Limescale build-up

Limescale build-up in the system causes pipes in the heat exchanger to become narrower which reduces water flow. Homes located in hard water areas are particularly at risk from limescale build-up.

Sludge build-up

Over time sludge can build up in the system and begin blocking pipes. This sludge is made from Magnetite and is formed by iron deposits which wear off the inside of the radiators.

A leak in your system

A leaking pipe causes pressure in the system to drop which reduces water flow. A leaking pipe is easy to fix but make sure the leak has not been caused by a more serious problem such as high-pressure in the system.

Damaged pump

A damaged pump will not be able to pump water around the system fast enough. Replacing the pump should solve the problem. But make sure you get the rest of the system checked out. The pump may have failed due to a build-up of sludge.

Faulty thermostat

A faulty thermostat may send false readings to the boiler causing it to overheat. Check the settings on your thermostat and if you have a digital thermostat reset it. This may solve the problem without the need to call a heating engineer.

Is boiler kettling dangerous?

The short answer is no. Kettling certainly has the potential to cause serious damage to your home because boiling water creates steam which increases pressure inside the system. In theory, this could cause pipes to burst and spray scalding hot water into your home and electrical circuits.

However, all modern boilers have failsafe mechanisms built-in which shut the boiler down once certain pressure thresholds are breached. Your boiler will show signs of distress long before this happens though. So if your boiler is kettling you should get it checked out sooner rather than later.

How to prevent boiler kettling

If your boiler is kettling you need to get it checked out by a qualified Gas Safe engineer. There are some steps you can take to reduce the risk of kettling occurring in the first place, however.

Get your boiler serviced regularly

Regular boiler servicing can identify issues early before they become a major problem. You should aim to get your boiler serviced every 12 months even if it is no longer under warranty.

Read our Boiler Warranty Guide for more information.

Power-flush the system

A Power-flush will remove any limescale and sludge build-up in your system. A power-flush should be carried out every 5-6 years. In hard water areas, it may also be beneficial to add chemicals to the system to prevent limescale build-up.

Install a filter

Sludge build-up can be a major issue in older systems. Installing a magnetic filter on the return pipe can help to remove iron-based magnetite and other impurities before it reaches the boiler.

Get your system checked by a Gas Safe registered engineer

While kettling does not mean your boiler is about to fail, it is a sign that there is an issue with your central heating system. It also means your boiler is not working efficiently and therefore costing you money. So if your boiler is making funny noises you should definitely get it checked by a registered Gas Safe engineer to prevent further damage.