How To Descale A Boiler And Why

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Last updated: October 31, 2023

Left untreated, limescale can be a boiler destroyer, shortening its lifespan and reducing its efficiency, potentially leading to expensive heating bills and repairs. So, getting on top of this tricky mineral deposit is not only good for your boiler, but also for your bank balance.

We look at how limescale can build up in central heating system, the damage it can potentially do and what you can do to descale your boiler and keep everything in a happy balance.

What Causes Scale In Boilers?

Limescale can be a scourge in many homes, particularly if you live in a hard water area and is caused by minerals in the water, namely calcium carbonate and magnesium ions. Every time your boiler heats up water, these minerals start to form deposits which, if left unchecked, gradually build up to create clumps of limescale in your central heating system. As well as where the water flows – such as taps, pipes and radiators – limescale is also prone to build up on the heat exchanger of your combi boiler.

Over time, these limescale deposits get thicker and harder, eventually causing damage to mechanical parts and inner parts of your heating system as well as putting your boiler under increased pressure as it finds it harder to pump water around an increasingly restricted pipe and radiator system. And this pressure means your heating bills could well build up too.

While limescale is not harmful to humans, it can be harmful for your boiler. Signs of limescale build-up in your boiler and central heating system include:

Why Is It Important to Descale a Boiler?

If you do suspect that limescale is present in your boiler, it is important not to ignore it. Letting limescale to build up can lead to a whole host of problems further down the line for your central heating and ultimately your wallet as it can cause an eventual damage and even a total breakdown of your boiler.

Plan for boiler descaling

Descaling your boiler every so often should be an essential part of your central heating maintenance plan as a boiler system, particularly the sealed systems you find in a combi boiler, is vulnerable to the damage and corrosion that limescale build-up can wreak. And this is because hot water has a much higher evaporation rate than cold water, leaving more limescale behind.

Descaling is also a preventative measure and will keep any limescale levels in the water flow to a more manageable level, improving performance and ensuring your boiler continues to work at its efficient optimum for longer, helping to save on your energy bills.

How to Descale a Boiler

Removing limescale is best left to the professionals if you are in anyway unsure. But if you have some DIY and heating systems know-how, it is possible to do your own home boiler descale. And it will involve using a limescale remover, specifically formulated for central heating systems.

Here’s our step-by-step guide to descaling a boiler:

Turn off your boiler and let your central heating system cool down, ensuring the water has also been switched off

Drain off the system by opening all your taps until the water flow stops

If you have a water tank: 

Add the recommended amount of limescale remover to your cold water tank, then fill the rest of the tank with cold water so that the water covers the feed connection

Turn your system and water back on and leave to heat up for 4 hours as per instructions and so the descaling process can be completed

Meanwhile, every 15-20 minutes, turn on the hot taps to get hot water running through the system

Once complete, flush with cold water and drain the system several times to complete the clean and get your system re-set and working.

Combi boilers:

Combi boilers don’t have a separate water tank so the boiler descaling process will focus on the heat exchanger. This could mean removing the heat exchanger to descale it separately – do seek expert help from a qualified heating engineer if you are in anyway unsure how to descale a heat exchanger.

How to Prevent Scale Build Up

Prevention is better than the cure so, as well as descaling your boiler when needed, you can also take steps to help stop the limescale building up in the first place. Limiting limescale in the boiler system is more than worth the time and effort as it should also help both your boiler and central heating last longer, improve performance and remain energy efficient. So, as well as descaling your boiler system, here’s what else you can do:

  • Use a water softener

A water softener is a device which you can install in your central heating system to tackle the issue of limescale at its source. Rather than removing the limescale, a water softener will change its composition by using ions to positively charge the main components of limescale, including calcium, magnesium and iron. This effectively ‘softens’ the water and prevents the limescale from developing or building up.

  • Install a scale reducer

As well a water softener to soften hard water as it goes from the mains and into your boiler, you can also install a limescale reducer. This is a magnetic device which works to collect the minerals in hard water and that can cause limescale to form. You can also buy scale reducers which alter the structure of limescale so it cannot stick inside your radiator pipes and form an obstructive lining.  Central heating inhibitors can also be used to prevent the formation of sludge, which is a gooey mix of rust, dirt and limescale.

  • Get a powerflush

While a powerflush is recommended when fitting a new boiler to ensure it gets off to a limescale and sludge free start, it is also worth booking in an occasional powerflush. As the name suggests, this process uses a mix of sludge busting chemicals and water which is pushed through your central heating system at high pressure to flush all that limescale, corrosion, dirt and grime out. A powerflush should only ever be conducted by a heating engineer and can be done as part of an annual service, if needed.

And finally…

If you are in any doubt about how to prevent limescale or how to maintain your boiler, always call out a Gas Safe registered engineer to advise and do the work for you.

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