What Is Limescale and How Does It Form?
Limescale – also referred to as calcium carbonate deposit or simply scale – is the white or grey chalky residue that forms when hard water is heated. Tap water in much of the United Kingdom contains dissolved minerals, primarily calcium and magnesium bicarbonates, which are picked up as water passes through chalk and limestone rock. The concentration of these minerals defines what is called water hardness.
When this water is heated above approximately 60 degrees Celsius, the bicarbonates undergo a chemical change and precipitate out of solution as calcium carbonate. This solid compound adheres to the surfaces of whatever heated it – in a coffee machine, that means the boiler walls, heating element, thermoblock, internal pipework, and any valve or fitting that water passes through at high temperature.
The scale builds up gradually with each use. A thin initial layer does not cause obvious problems. As it accumulates over weeks and months, however, the effects become increasingly significant. Scale acts as thermal insulation, reducing the efficiency of the heating element and forcing it to work harder to reach the target temperature. It narrows internal passages, restricting water flow and increasing pressure load on the pump. It can eventually block components entirely or cause them to overheat and fail.
Water Hardness in the UK
The UK's water supply varies considerably in hardness by region. Water hardness is measured in parts per million (ppm) or degrees Clarke (degrees of general hardness). Broadly:
- London and south-east England are among the hardest water areas in the country, with many areas exceeding 300 ppm
- The Midlands and east of England tend to have moderately hard water, typically 200–300 ppm
- Scotland, Wales, the Lake District, and parts of the north-west of England generally have much softer water, often below 100 ppm
In practical terms, this means that a machine in London or Kent will accumulate limescale significantly faster than the same machine used in Glasgow or Snowdonia. Machines in hard water areas need to be descaled more frequently to achieve the same level of protection. Most machine manufacturers base their recommended descaling intervals on moderately hard water; if you live in a hard water area, you should descale more often than the manufacturer's default guidance suggests.
You can check your local water hardness through your water supplier's website or by purchasing an inexpensive water hardness test strip.
What Happens When Scale Is Not Addressed
The progression from scale accumulation to machine failure follows a reasonably predictable pattern, though the timeline varies with water hardness, usage frequency, and machine type.
Performance Degradation
The first effects are typically gradual and easy to miss: the machine takes slightly longer to heat up, extraction feels slightly slower, or coffee temperature is marginally lower than it used to be. These changes often happen so incrementally that they are not noticed until the machine is significantly scaled. The change in coffee quality is real, though subtle at first – water that cannot reach the correct temperature will extract coffee inconsistently.
Component Strain and Failure
As scale accumulates on the heating element, it creates a layer that the element must heat through rather than directly heating the water. This causes the element to run hotter than designed in order to compensate, accelerating wear and increasing the likelihood of the element burning out prematurely.
Similarly, scale in the pump pathways increases the resistance the pump works against. Pumps are rated for a certain working pressure, and a machine with partially scaled pathways may push the pump towards the top of its operational range during routine extraction, shortening its service life.
In thermoblock machines – which heat water on demand rather than storing a reservoir of hot water – scale accumulation in the narrow thermoblock channels is particularly damaging. The channels are small by design and can become significantly restricted even by moderate scale buildup, affecting both temperature accuracy and flow rate.
Total Blockage and Irreversible Damage
Heavily scaled machines may reach a point where internal passages are so restricted that the pump cannot force water through at the required pressure, or where the heating element burns out. In some cases, scale buildup is so severe that descaling solution alone cannot remove it, and physical removal by a technician is required. In extreme cases, components are damaged beyond economical repair.
At Wolentra, a significant proportion of the domestic machine failures we see could have been avoided or significantly delayed by timely descaling. The component failures that result from advanced scale accumulation – burned-out elements, seized pump valves, cracked thermoblocks – are more expensive to repair than a replacement pump or element alone, because the damage is often more widespread.
How Often Should You Descale?
Manufacturer guidance on descaling intervals is a starting point rather than a fixed rule. Most manufacturers recommend descaling every two to three months for daily home use in average water conditions. However, this guidance does not account for variations in water hardness or usage patterns.
A more practical approach is to adjust frequency based on your specific circumstances:
- Very hard water (above 300 ppm): Descale every four to six weeks if the machine is used daily
- Hard water (200–300 ppm): Descale every six to eight weeks
- Moderately hard water (100–200 ppm): Descale every two to three months
- Soft water (below 100 ppm): Descale every three to four months
These are approximate guidelines. If your machine has a descaling alert function, this is worth paying attention to, though these systems typically use fixed timers rather than measuring actual scale accumulation. In hard water areas, the alert may not trigger frequently enough.
For commercial machines, descaling should be approached more conservatively. A machine processing hundreds of shots per day in a hard water area may accumulate scale quickly enough to warrant monthly descaling.
How to Descale: The Process
The exact steps for descaling vary depending on machine type. Below is a general approach that applies to most domestic espresso machines and capsule machines. Always consult your specific machine's manual before beginning, particularly for machines with automated descaling programmes.
Choosing the Right Product
Use a descaling solution specifically formulated for coffee machines or espresso machines. Look for products that are citric acid based or proprietary descalers from brands like Puly, Cafetto, or those produced by your machine's manufacturer. These are formulated to dissolve calcium carbonate effectively without damaging rubber seals, aluminium boilers, or other components.
Avoid using white vinegar as a descaler. While vinegar is acidic enough to dissolve some scale, it can corrode aluminium components, degrade rubber gaskets and O-rings over time, and is notoriously difficult to fully rinse from the machine's internal surfaces. Machines that have been descaled with vinegar sometimes retain a perceptible vinegar taste in subsequent drinks, even after extensive rinsing.
General Descaling Steps
Before beginning, ensure the machine is cool and empty of coffee. Remove and rinse any removable components. If the machine has a manual descaling mode (rather than an automated programme), proceed as follows:
Fill the water tank with the descaling solution mixed according to the product instructions. Most solutions require dilution with water; using a concentrated solution does not necessarily descale more effectively and may leave residues. Run approximately half of the solution through the machine via the group head and steam wand, pausing briefly between each burst to allow the solution to dwell and work on scale deposits. Allow the machine to rest for ten to fifteen minutes with the remaining solution in the tank, then run the remainder through.
Empty and thoroughly rinse the water tank. Refill with fresh, clean water. Run at least two full tanks of clean water through the machine – via the group head and steam wand – to flush all traces of the descaling solution. This step is important. Residual descaling solution affects taste and should not remain in the system.
For machines with an automated descaling programme, follow the programme's instructions, which will vary by manufacturer but typically walk you through the process with prompts.
After Descaling: What to Expect
A recently descaled machine should heat up slightly faster and produce more consistent extraction than a heavily scaled one. In machines that were significantly scaled, the improvement in coffee temperature and extraction quality can be noticeable.
If you descale your machine and there is no improvement in performance, or if symptoms such as slow extraction or weak steam persist, the problem may not be scale alone. There may be a fault – a failing pump, a worn valve, or a damaged element – that descaling cannot address. In this case, a professional assessment is the appropriate next step.
It is also worth noting that a machine that has been heavily scaled for a long period may not return fully to its original performance after a single descale. Long-standing scale that has hardened significantly may require multiple descaling cycles or manual cleaning by a technician. If you have not descaled a machine in several years, do not expect one treatment to fully resolve all effects of scale accumulation.
Reducing Scale Accumulation
While descaling is necessary regardless of water quality, there are steps that can slow the rate of scale buildup and reduce the frequency with which descaling is needed.
Use filtered water. A water filter – either a jug filter or an in-line filter attached to a plumbed machine – removes a significant proportion of the calcium and magnesium that cause scale. The reduction in mineral content varies by filter type. Machines that use proprietary filter systems in their water tanks (as many commercial and prosumer machines do) can benefit substantially in hard water areas.
Use a water softener. For plumbed commercial machines, an in-line water softener is a worthwhile investment in hard water areas. These replace calcium and magnesium ions with sodium ions, which do not form scale. Water that has been softened does not produce limescale in the boiler.
Do not leave water standing in the tank for extended periods. Fresh water has not had the opportunity to absorb further minerals from tanks or pipes. Refilling with fresh water each day does not reduce the mineral content of what you use, but it is good general hygiene practice.
When Professional Descaling Is Needed
Routine descaling can be carried out by the machine owner and does not require specialist equipment. However, there are situations where professional descaling is advisable.
If a machine has not been descaled in a very long time and has accumulated heavy scale deposits, a single consumer descaling cycle may not be sufficient. A technician can assess the degree of scale buildup, use stronger solutions where appropriate, and physically clean components that cannot be reached through the water circuit. For boilers that have significant scale adhesion, this may be necessary before the machine can function normally again.
Commercial machines with complex water circuits, multiple boilers, or heat exchangers benefit from periodic professional descaling as part of a wider service visit, ensuring that all circuits are addressed and that the machine is checked for any wear or damage at the same time.
If you have questions about descaling your specific machine, or if you are seeing signs of scale-related performance issues that routine descaling has not resolved, contact Wolentra to discuss your situation.
Summary
Limescale is an unavoidable consequence of using tap water in any heated appliance. In a coffee machine, it is one of the most significant factors affecting both performance and lifespan. Regular descaling – adjusted in frequency to reflect your local water hardness and usage pattern – is one of the most cost-effective things you can do to maintain your machine.
The key points are: use a product designed for coffee machines, not vinegar; rinse thoroughly after descaling; adjust the interval based on your water hardness; and address scale buildup before it becomes severe enough to cause component damage. When in doubt, descaling more frequently than the manufacturer's baseline recommendation is unlikely to harm a machine in good condition and may significantly extend the interval before problems develop.
