How does limescale actually damage your plumbing and heating systems?
Key points
1. Limescale does not cause damage overnight. It builds up in layers, remaining invisible for months, before leading to a loss of efficiency and eventually a breakdown. In a boiler, just 1 mm of limescale deposits can already reduce efficiency by 10 to 15 per cent.
2. In the areas of Belgium most affected (Flemish Brabant, Brussels, Walloon Brabant, where the fH level exceeds 25), fittings age two to three times faster without treatment: tap cartridges, water heater heating elements and boiler heat exchangers.
3. Repairing a fault after it has occurred is almost always more expensive than preventing it. Replacing a mixer tap, changing a hot-water tank ahead of schedule or having a boiler descaled as a matter of urgency can quickly end up costing more than the annual maintenance of a water softener (€80 to €180 per year).
Limescale forms when water is heated. Dissolved calcium and magnesium remain stable in cold water, but precipitate as calcium carbonate as soon as the temperature exceeds around 60°C – precisely the operating range of a boiler or a hot-water tank. The deposit clings to the metal walls and insulates them thermally, whereas the metal itself is designed to conduct heat.
It is this mechanism that explains the loss of efficiency observed in boilers affected by limescale. Scale conducts heat twenty to forty times less efficiently than the steel or aluminium in the heat exchangers: with just 1 mm of limescale, the heat exchanger already loses 10 to 15 per cent of its efficiency, as some of the heat remains trapped in the insulating layer instead of being transferred to the water. At 3 mm, the loss exceeds 30 per cent, and the appliance compensates by running for longer to achieve the same result, thereby consuming more gas or electricity.
The most telling sign is often heard before it is seen. A boiler or water heater that starts to rattle or crackle as the temperature rises indicates a build-up of limescale that is trapping steam against the hot wall in certain areas. This is one of the first signs to take seriously, well before a breakdown occurs.
One Belgian detail is worth noting here: houses fitted with a rainwater tank connected to the toilets or washing machine are not affected in the same way. Rainwater is naturally soft, containing virtually no calcium. Limescale therefore builds up in the systems supplied by the mains water network (kitchen, bathroom, boiler), which explains why certain appliances in the same house age very differently depending on their water source.
Boiler, hot water tank and heat pump: the most expensive breakdowns
A storage hot-water tank is subject to the same phenomenon, but in a more visible way. Limescale builds up primarily on the heating element, which is the hottest part of the appliance. A scale-encrusted heating element takes longer to heat the same volume of water, reaches higher local temperatures than expected, and eventually fails before the end of its expected lifespan. A tank designed to last 12 to 15 years breaks down after between 5 and 8 years when used with untreated hard water, which corresponds well with the halved lifespan observed in practice.
Air-to-water heat pumps, which have become increasingly common in Belgian renovation projects since the introduction of regional grants, are no exception. Their domestic hot water system operates on the same principle as a conventional boiler: a heat exchanger that transfers heat to the water in the system. Limescale builds up in the same way, with the same effect on efficiency. This is a point that installers often forget to mention when selling the unit.
Washing machines and dishwashers: wear and tear that’s less obvious, but just as real
Pipes: the loss of cross-sectional area that goes unnoticed
Limescale does not stop at visible appliances. It also builds up on the inner walls of pipes, in gradually thickening layers, without any leaks or breakdowns directly indicating its presence. A pipe with an internal diameter of 15 mm that accumulates 2 mm of limescale on its walls loses more than 40 per cent of its effective cross-sectional area. The cross-sectional area of a pipe is proportional to the square of its radius: a narrowing of just a few millimetres therefore has a disproportionate effect on the flow rate that can pass through it.
This explains why a shower with the correct flow rate at the time of installation loses pressure year after year, without any visible faults elsewhere in the system. The phenomenon primarily affects small-diameter pipes (sink, shower and washing machine supply pipes), where the relative loss of cross-sectional area is greatest for the same amount of limescale build-up in millimetres.
A timeline that always follows the same pattern
Without treatment, deterioration progresses at a fairly steady rate in areas with hard water:
Period | What’s happening | Visible sign |
| 0–6 months | The deposit is forming at the hotspots | No outward signs |
| 6–18 months | Initial deposits on taps and shower screens | White marks, joints that harden |
| 1–3 years | Wear and tear on thermostatic cartridges | The mixer tap is more difficult to adjust; the water pressure has dropped slightly |
| 3–5 years | Loss of boiler efficiency (10–15 per cent) | Rising energy bills, a knocking noise |
| 5–8 years | Water heater or boiler breakdowns | Breakdown, emergency plumbing call-out |
| Ages 8–15 | Early replacement of equipment | Boiler or water heater replaced before the normal lifespan |
Warning signs to look out for before a breakdown
In areas with hard water, certain signals recur from one property to the next, and they generally appear in the following order:
- White marks that keep reappearing on the taps and shower walls, despite cleaning
- A gradual drop in pressure in the shower or at the tap, with no identifiable leak
- A knocking or crackling noise coming from the boiler or water heater whilst it is heating up
- A thermostatic mixer tap that is becoming increasingly difficult to operate, or a temperature that drifts
- A gas or electricity bill that rises without any change in usage
- Household detergents and fabric conditioners are being used up more quickly than before
A single one of these signs does not necessarily mean much. If two or three of them appear in the same year, in a house that is more than five years old and has never had a water treatment system installed, it is worth checking the actual water hardness at your address.
In a new-build or recently renovated property, the first sign almost always appears in the bathroom, before the boiler: this is where hot water most often comes into contact with cold surfaces (glass, chrome, seals), where limescale precipitates and is immediately visible. A bathroom renovation project is therefore often the right time to decide on a water softener, before fitting taps or a shower screen that will then need to be protected.
Why does the speed of destruction depend on your postcode?
The rate at which limescale damages a system depends primarily on the water hardness at the specific location, not on a national average. A water heater installed in Namur, in an area with soft water of around 5 to 12°fH, will last its full fifteen years without any problems. The same appliance installed in the Hageland or in Brussels, where the hardness regularly exceeds 25°fH, will start to show signs of wear and tear as early as the fifth year.
You can find out the exact water hardness at your address by contacting your water supplier directly: SWDE or IECBW in Wallonia, Vivaqua in Brussels, De Watergroep or Farys in Flanders. This information is free of charge, available on request, and is the only figure that really matters – far more so than a provincial average, which can mask significant variations from one municipality to another. Our guide ‘Water softeners: are they really useful in Belgium?’ sets out the water hardness by province and the threshold at which a water softener actually becomes cost-effective. This is the starting point before investing in a device that you might not necessarily need elsewhere.
Frequently asked questions
How long does it take for limescale to cause real damage to a boiler?
Limescale build-up begins as soon as the boiler is put into service in hard water, but the first measurable effects on efficiency generally appear between 2 and 4 years, when the layer reaches a thickness of 1 mm. Complete breakdowns tend to occur more frequently between 5 and 8 years, depending on the water hardness and how the boiler is used.Can limescale completely block a pipe?
This is rare in a well-maintained domestic system, but not impossible in an old, small-diameter pipe that has not been serviced for several decades. The most common scenario is a gradual reduction in flow, not a total blockage.Do you need to descale a boiler every year, even if you have a water softener?
Mandatory annual boiler servicing is still necessary, whether you have a water softener or not, but a properly adjusted water softener significantly reduces the need for specific descaling to remove limescale during this service.Does limescale damage washing machines and dishwashers in the same way as it does boilers?
The mechanism is the same (deposits on the heating element), but the effect is less dramatic: reduced performance and longer cycles rather than a sudden breakdown. Regular descaling limits the damage if there is no water softener.Can a water softener installed at a later date repair the damage that has already been done?
No, a water softener protects equipment installed after it has been commissioned; it does not dissolve limescale that has already built up. Professional descaling is still required for existing equipment before the softened water takes over.