Scale deposits on the hottest surfaces, which are always heat transfer surfaces. It insulates them, so appliances burn more fuel to deliver less heat; it restricts flow in narrow passages; and it causes elements to overheat inside their own deposit until they fail.
The mechanism
Calcium bicarbonate is soluble in cold water. Heat the water and it decomposes, precipitating calcium carbonate – solid, crystalline, and adherent – along with carbon dioxide. The hotter the surface, the faster the reaction, which is why scale forms preferentially on the hottest thing in contact with the water.
In a heating appliance, the hottest thing in contact with the water is always the heat exchanger, the coil or the element. That is not a coincidence – it is the defining problem. Scale forms exactly where you least want it, and nowhere else much.
Calcium carbonate conducts heat far less well than the copper or steel it forms on. A layer measured in fractions of a millimetre has a measurable effect on heat transfer. That is why a scaled element runs hotter, a scaled coil heats slowly, and a scaled heat exchanger costs fuel.
Where it costs the most
Combi plate heat exchangers
The defining hard water fault in a combi. A plate heat exchanger transfers heat from the boiler’s primary water to your incoming cold water as it passes through, and it does so through a stack of very thin plates with very narrow passages. Narrow passages transfer heat well, and they block readily.
Scale in a plate exchanger does two things at once: it insulates, so the water leaves cooler, and it restricts, so less water passes. The result is the pattern everybody in this county recognises – hot water that is not as hot as it was, at a flow that is not as good as it was, declining gradually over years until somebody notices.
Hot water cylinder coils
The indirect coil is the heat exchanger between boiler water and stored water. Scale on the outside of that coil means the boiler runs longer to heat the cylinder and never quite reaches temperature. The symptom is a cylinder that takes an age to heat, and a boiler running far more than it should for hot water.
Immersion heater elements
The most visible example. Scale forms on the element, insulates it, so the element must run hotter to push the same heat into the water. Hotter running deposits more scale. The feedback loop ends with the element overheating inside its own insulating shell and burning out. See immersion heaters.
Boiler heat exchangers on open-vented systems
Where the same water is not recirculated – which is the case for domestic hot water rather than heating – scale accumulates continuously. A sealed heating circuit is less affected, because the same water circulates and its dissolved mineral is deposited once and then exhausted.
The heating circuit is a special case
This is worth understanding because it changes what you should worry about.
A sealed heating system circulates the same water indefinitely. Whatever hardness that water contained is deposited once, early in the system’s life, and then there is no more to deposit. So limescale is not really the problem in a heating circuit – the problem there is magnetite, the black iron oxide produced by corrosion, which is a different substance with a different cause and a different solution. See power flush, chemical flush, or neither.
Domestic hot water is the opposite: fresh hard water passes through continuously, so the deposition never stops. That is why hot water components scale and heating components sludge.
Two exceptions matter. A heating system that is repeatedly topped up because of a leak is receiving fresh hard water continuously, so it gets both problems. And a combi’s plate exchanger sits on the domestic hot water side, so it scales.
What it costs you, concretely
- Fuel. An insulated heat exchanger means the appliance burns more to deliver the same heat.
- Flow. A restricted plate exchanger means poorer hot water performance.
- Components. Immersion elements, plate exchangers and tap cartridges all fail sooner.
- Appliances. Washing machines, dishwashers and coffee machines all heat water and all suffer.
- Detergent. Measurably more soap and detergent is needed in hard water to achieve the same result.
- Time. Descaling shower heads, kettles, taps and screens is a recurring domestic chore that simply disappears with softened water.
- Warranty risk. Several manufacturers reference water treatment in hard water areas in their installation requirements.
We deliberately do not publish figures for any of these. We have no licensed source for the cost of scale in a specific installation, and a plausible-looking number would be fiction – see what things cost for our position on that.
Removing it, and preventing it
Removing existing scale
Descaling chemicals will dissolve calcium carbonate, and they are used routinely on plate heat exchangers, cylinders and elements. Two cautions: aggressive descalers can attack other materials in the system, so the chemical has to suit the components; and descaling treats the symptom, so unless something changes, it returns.
Softened water is mildly aggressive towards existing scale and will gradually dissolve deposits over months. It is a slow reversal rather than an instant one, and it is a genuine benefit of softening that people do not expect.
Preventing it
- A water softener removes the hardness, so no scale forms anywhere downstream. The comprehensive answer – see water softeners.
- A scale inhibitor prevents scale adhering to heat exchange surfaces without removing hardness. Protects the equipment, does not change the water – see scale inhibitors.
- Lower stored water temperature, within the limits that legionella control requires. Scale formation accelerates sharply with temperature, so storing at the correct temperature rather than higher genuinely reduces it.
- Regular descaling of shower heads, aerators and kettles, which is maintenance rather than prevention.
Hot water declining gradually on a combi? That is almost always the plate heat exchanger, and it is a hard water problem rather than a boiler fault.
Questions we get asked about this
Does limescale damage pipes?
It reduces the bore very slowly over decades, which is rarely the practical problem. The damage that matters is on heat transfer surfaces – heat exchangers, coils and elements – which is where it forms fastest and costs most.
Why has my combi’s hot water got worse over the years?
Almost certainly scale in the plate heat exchanger, which both insulates and restricts. It is the single most common hard water complaint in this county and it develops so gradually that nobody notices until it is substantial.
Can a scaled heat exchanger be cleaned?
Frequently, yes – a chemical descale on a plate exchanger is a routine job. Whether it is worth doing depends on the appliance’s age and how quickly it will re-scale without treatment.
Does limescale affect my central heating radiators?
Not much. A sealed heating circuit deposits its hardness once and then has no more to deposit. The problem in radiators is magnetite from corrosion, which is a different substance entirely.
Will a softener remove scale that is already there?
Gradually. Softened water is mildly aggressive towards existing deposits and will dissolve them over months. It is not instant, and it is a real benefit people rarely expect.
Should I turn my cylinder temperature down to reduce scale?
Not below what legionella control requires for stored water. The correct approach is to store at the right temperature and blend down at the outlet with a thermostatic mixing valve, rather than storing cool.
Need this doing?
Call and describe the problem — you will speak to an engineer, not a call centre. Written quotes, no estimates over the phone.