Limescale and your boiler: why winter exposes the problem

In short: Has your boiler started rumbling, does the shower take ages to run hot, or does the hot water keep switching between hot and lukewarm? Then limescale is a likely cause. It builds up all year in the part of your boiler that heats your tap water, but you only start to notice it in winter, when the boiler works hardest. A regular service keeps your boiler safe, but it does not stop new scale from forming. Only treating the water before it reaches the boiler does that.

Why you notice limescale in winter

From October, your boiler goes from occasional use to running almost all the time. Three things change at once:

• The heating runs for hours every day, often after months of rest.
• The whole household uses more hot water: longer showers, baths, more washing.
• The water coming into your home is colder, so the boiler has to heat each litre for longer.

Every time hard water is heated, some of the dissolved calcium turns back into solid limescale. A layer that formed unnoticed over the summer becomes a problem as soon as the boiler has to deliver more, and usually on the coldest mornings, when you need it most. That is also why heating engineers are so hard to book in autumn: it is servicing season, and it is when many households find out their boiler is scaled up.

1. What limescale does inside your boiler

A gas or oil boiler has two separate water circuits, and limescale affects them very differently.

The heating circuit is a closed loop. The same water keeps circulating between the boiler and your radiators or underfloor heating. Its minerals settle once, then stop. Scale only becomes a problem here if the circuit is refilled often, for example because of a small leak or a recurring drop in pressure. Every top-up brings fresh hard water into the system.

The hot water circuit works the other way round. In a combi boiler, fresh water flows through a heat exchanger every time you turn on a hot tap. With a separate hot water tank, the tank refills with fresh water throughout the day. If your water is hard, this is where limescale builds up day after day.

Condensing boilers, now the standard in Belgian homes, have compact heat exchangers with very fine passages. They save energy, but they cope less well with scale.

2. Four signs to watch for this winter

1. Your boiler rumbles like a kettle. When scale covers the heat exchanger, water trapped underneath overheats and forms small steam bubbles. You hear it as a rumbling, crackling or whistling sound when the boiler starts.

2. You wait longer for hot water, or it keeps changing temperature. In a combi boiler, a scaled exchanger needs more time to reach the temperature you set. At the tap, the water may switch between hot and lukewarm.

3. The hot tap runs weaker than the cold one. If the cold tap runs normally but the hot one is clearly weaker, the blockage is often in the boiler's hot water circuit.

4. More breakdowns or more frequent top-ups. Error codes, the boiler cutting out, a boiler that will not restart, or pressure you keep having to top up. If you are using more gas or oil without the weather being any colder, that can point the same way.

These signs can have other causes, such as air in the system, a faulty pump or a worn expansion vessel. Please do not open the boiler yourself: ask your heating engineer to find the cause.

3. Boiler servicing: what your heating engineer checks

In Belgium, servicing your central heating boiler at regular intervals is a legal requirement. The rules depend on your region:

The service must be carried out by a certified technician, who gives you a certificate. If you rent, you as the tenant usually arrange it.

What a standard service covers:

• the general condition of the boiler and burner, with cleaning where needed;
• combustion values and emissions;
• the flue and the air supply for combustion;
• ventilation of the boiler room and the safety devices.

What is often not included: descaling the hot water heat exchanger, or checking how hard your water is. A standard service is about safety and combustion, not limescale. Many people only find this out after the visit.

Four questions to ask your heating engineer on the day:

• Can you see scale on the hot water side of the boiler or in the tank?
• Has the pressure dropped since your last visit, and how often has it been topped up?
• Would you recommend descaling the heat exchanger, and what would that involve?
• Given how hard my water is, what would you suggest to limit scale in future?

4. Prevent rather than descale

Descaling treats the symptom, not the cause. The technician dismantles or flushes the heat exchanger with an acid solution. That takes time, it costs money, and it has to be done again as long as hard water keeps flowing into the boiler. On newer condensing boilers, some manufacturers recommend replacing the heat exchanger rather than descaling it again and again.

Prevention means treating the water before it reaches the boiler, with a system fitted on the main water supply. There are three approaches:

• Salt-based water softener (ion exchange). Removes calcium and magnesium and replaces them with sodium. It works, but it needs regular salt top-ups, uses water to regenerate and sends salty water down the drain. It is advisable to leave some hardness in the water so it does not become corrosive for your pipes.
• CO₂ treatment (SoluCalc). Adds a small, controlled amount of food-grade CO₂ to the water. The calcium stays dissolved instead of settling as limescale. No salt, no regeneration, no salty wastewater. The hardness of your water stays the same, because the minerals remain in the water in dissolved form. After installation, existing deposits in pipes and appliances dissolve gradually over time. Maintenance is minimal: the CO₂ bottle is replaced from time to time.
• Magnetic and electronic devices. Cheap and easy to fit, but independent test results are mixed.

Whichever treatment you consider, how well it protects the hot water side of your boiler depends on the type of boiler, how your hot water is produced (combi, tank, heat pump) and how it is set up. That is why we look at your installation with you before we recommend anything, so you know exactly what to expect.

Treating the water where it enters your home does not replace the legal service. The two go hand in hand: the service keeps your boiler safe, and prevention limits new scale between visits.

5. How hard is your water?

Your water supplier publishes the hardness for each municipality, usually in French degrees (°f): SWDE, inBW or CILE in Wallonia, De Watergroep, Farys, Water-link or Pidpa in Flanders. But you can also simply check your water's hardness in our water hardness calculator below.

Is there limescale in my area?

Depending on your geographical region, water hardness can vary significantly and affect your household appliances and sanitary installations. Use our calculator to check the water hardness in your area.

Calculate

Frequently asked questions

How often does my boiler need servicing in Belgium? In Wallonia, every 3 years for a gas boiler and every year for an oil boiler. In Flanders, every 2 years for gas and every year for oil, for boilers of 20 kW and above. The service must be carried out by a certified technician.

I rent. Do I pay for the boiler service, or does my landlord? In most cases, you as the occupant arrange and pay for the regular service and keep the certificate. Check your lease, though, as it may say otherwise.

Why do I keep having to top up my boiler pressure? A pressure drop that keeps coming back usually means a small leak, a worn expansion vessel or a faulty safety valve. It is worth having it checked: besides the hassle, every top-up brings fresh hard water, and with it new scale, into the heating circuit.

Can limescale damage my combi boiler? Yes. Scale on the hot water exchanger reduces heat transfer, restricts the flow and can cause noises, the boiler cutting out and parts such as sensors and valves wearing out early.

Is descaling included in a standard boiler service? Usually not. A standard service focuses on safety and combustion. Ask your technician whether descaling the hot water side would make sense for you.

Will a water treatment system protect my boiler? Treating the water before it reaches the boiler limits new scale on the hot water side. How well it works depends on the technology and on your installation, so have it assessed before you decide.

Wondering what your water hardness means for your boiler? Ask for personalised advice: an advisor or an installer from our network near you will look at your installation with you.