What is Kettling in a Boiler?

What is Kettling in a Boiler

You may have heard a rumbling, popping, whistling or banging noises from your boiler. If it is in the open, for example in the kitchen, you’ll have got used to the sounds it makes and will notice anything different. Any loud, irregular or non-mechanical sound sometimes referred to as a kettling noise should be cause for investigation.

What is Kettling in a Boiler?

The water in your boiler doesn’t actually boil – or at least it shouldn’t. However, something slows down the flow, water could stay over the burner long enough for it to boil. This causes bubbles of steam, and the pressure increases dramatically as the steam expands.

Several things can slow the flow:

Build-up of limescale

If you are in a hard water area it can cause limescale inside the heat exchanger. That reduces the cross-section of the pipes and slows the flow down.

Build-up of sludge

Magnetite, an iron-based chemical, wears off the inside of radiators and causes a build-up of sludge that slows downflow.

Damaged pump

If the pump isn’t working well, it won’t move water through the heat exchanger fast enough.


A faulty thermostat could give the boiler the wrong reading, making it overheat.

Is kettling dangerous?

Kettling is potentially dangerous, as the high pressure could burst pipework and cause steam emission. Modern boilers have safeguards to detect overheating and shut the system down before it reaches that point.

Steps to prevent kettling

There are ways a Gas Safe engineer can prevent kettling:

Clean or replace the heat exchanger

The most common remedy is to remove the heat exchanger and clean it. If the problem’s severe, it – or the full boiler – may need replacing.

Add a chemical to the water in the system

Chemicals can be applied to the central heating system to reduce sludge. This also keeps the pipes clean.

Install a filter

A filter will strip impurities as they are picked up by the central heating system. A magnetic filter is good for trapping iron-based magnetite.

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