What is kettling in a boiler? Is it dangerous?

You might have heard strange rumbling, popping, whistling or banging noises coming from your boiler. We’ve already written about the sounds your boiler can make. A lot of them are perfectly normal, and are the sounds of pumps, switches, flowing gases and liquids, and the flames of the burner. Before you conclude it’s kettling, make sure you have ruled out the sounds of a healthy boiler – it could save you a call-out fee.

If your boiler is in the open, for example in your kitchen, you’ll probably have got used to the sounds it makes, and will immediately notice anything different. If it’s hidden away in a utility room, garage or outdoors, you might not. It’s a good idea to have a listen to yours every so often, just to get used to its normal operating sounds. Anything loud, irregular or non-mechanical in sound is probably cause for further investigation.

How does a boiler work?

Many people assume a gas combi boiler brings water in from outside, boils it over a flame and sends it to the hot taps. This isn’t quite the case. The water that’s heated by the burner is in an enclosed loop, and is pumped over the flame and through a heat exchanger. A heat exchanger is basically a loop with hot water piped through in one direction, with mains water being piped through in the opposite direction. The heat from the loop’s boiled water quickly gets transferred to the mains water passing the other way, which quickly becomes hot. Then the cooled water in the loop goes over the flame again, until the tap is turned off.

There are two important things to note. First, the hot water that comes out of your taps has not passed directly over the flame. It would simply be too hot if that were the case. It picks up its heat from the boiling water that has gone over the flame.

Second, although they are called “boilers”, they don’t boil water. As you might recall from your science classes at school, water boils at 100 °C. That means it changes from liquid to gas (steam). The water inside the boiler loop usually heats up to around 70 °C, and the hot water that comes out of the tap will be around 55 °C (or whatever you set it at). A nice hot bath is around 40 °C; a baby’s bath should be no hotter than 37 °C.

In the same way, water from the boiler also gets sent to radiators and (in a system or heat only boiler) your hot water cylinder, which are usually on the same system. The cylinder is also a heat exchanger, although it works much more slowly than a combi boiler.

What is kettling?

As we mentioned above, water inside the boiler doesn’t boil. Or at least it shouldn’t. It is prevented from reaching 100 °C by the speed and volume at which t is pumped over the flame.

However, if something were to slow down the flow, there’s a good chance that the water will linger over the burner long enough for it to actually boil. Suddenly, a system designed to accommodate hot but sub-boiling water will have bubbles of steam inside it, and the pressure will have increased dramatically through expansion. (Remember, expansion of steam is basically how a steam engine works – it’s a powerful force that can move a train.)

What can slow the flow? Several things.

Build-up of limescale

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.

Build-up of sludge

Magnetite is an iron-based chemical that wears off the insides of radiators, and can cause a build-up of sludge that will slow down flow.

Damaged pump

If the pump isn’t working to its full potential, it won’t push the water through the heat exchanger quickly enough.

Boiler thermostat

A damaged or wrongly set up thermostat could give the wrong reading to the boiler, which could cause it to overheat.

So that’s where the word “kettling” comes from. It’s where your water is being boiled just like inside a kettle. You might have noticed that a boiling kettle is often louder than a radio or a person talking, but in the case of a kettle it’s completely normal – you’re heating the water up until it turns into steam, which makes a lot of noise as it moves and as gas bubbles collapse.

Is kettling dangerous?

Kettling has the potential to be dangerous, as theoretically the high pressure could burst internal pipework and cause steam emission and electrical hazards.

However, modern boilers should have built-in safeguards to detect the symptoms of overheating and shut down the boiler before it reaches that stage.

Steps to prevent kettling

There are some ways you can prevent or cure kettling, but none of them are DIY jobs – always make sure you use a Gas Safe engineer to fix your boiler. They will probably do one of the following jobs:

Clean or replace the heat exchanger

The most common kettling remedy is to remove the heat exchanger and scrub it. If it’s a limescale build-up it will need some serious cleaning, and a replacement might be a better option. If the problem’s particularly severe, it’s probably time for a new boiler.

Add a chemical to the water in the system

Chemicals can be added to the central heating system to reduce the build-up of sludge. This will keep the pipes clean and free-flowing.

Install a filter

A filter on the return pipe to the boiler will strip the impurities as they are picked up in the central heating system. A magnetic filter is particularly good at trapping the iron-based magnetite.

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