Hard water in Norfolk: what limescale does to your boiler, cylinder and shower, and what actually helps

Water hardness is a measure of dissolved calcium and magnesium. Rain falling on Norfolk soaks down through chalk, which is almost pure calcium carbonate, and by the time it is pumped back up from the aquifer it carries a good deal of that chalk in solution. Most of the county's supply comes from groundwater rather than upland reservoirs, so it arrives at the tap hard, and in places very hard.

Your water company publishes the hardness for each supply zone; a Norwich postcode typically comes back somewhere in the "hard" to "very hard" bands, which in practical terms means well over 250 milligrams of calcium carbonate per litre. Soft-water areas such as the west of Scotland or Cornwall sit below 100.

None of this makes the water unsafe. Hard water is fine to drink and some people prefer the taste. The trouble starts when it is heated.

Calcium carbonate is less soluble in hot water than cold. Heat the water and some of the dissolved chalk comes out of solution and settles on whatever surface is hottest. In a kettle that is the element. In a combi boiler it is the plate heat exchanger that makes the hot water, and in a system boiler it is the main heat exchanger and the coil inside the cylinder.

The scale that forms is a poor conductor of heat. A layer only a couple of millimetres thick means the boiler has to burn longer to push the same heat through into the water, which shows up first as higher gas use and then as the boiler running hotter than it was designed to. Metal that is repeatedly overheated on one side fatigues, and that is how a heat exchanger eventually cracks or a cylinder coil pinholes.

The sound a scaled boiler makes is distinctive: a rumbling or "kettling" as water trapped behind the scale flashes to steam. If your boiler has started doing that, it is telling you something.

You do not need a test kit to know whether scale is building up. Walk round the house and look for these.

• Shower heads and tap aerators that spray unevenly or lose pressure. Unscrew one and the holes will be half blocked with white crust.
• Hot water that runs slower than cold at the same tap. The hot pipework is narrowing from the inside; the cold pipework is not.
• A combi that runs hot then cold in the shower. The plate heat exchanger is partly scaled and cannot pass enough water to keep the temperature steady.
• An immersion heater that trips or takes far longer to heat the cylinder than it used to. Scale on the element is insulating it from the water.
• White marks on the glass shower screen, the draining board and inside the dishwasher door.
• Rumbling from the boiler a few minutes after it fires.

Any one of these on its own is a nuisance. Three or four together mean the scale has reached the parts you cannot see, and it is worth having the boiler or cylinder looked at before the heating season rather than after the first breakdown.

The immediate cost is fuel. A scaled heat exchanger transfers heat less efficiently, so the boiler burns for longer to reach the same temperature. Over a winter that adds up, although exactly how much depends on the boiler and how heavily it is scaled.

The larger cost is parts. A plate heat exchanger on a combi is a routine replacement job in a hard-water area, and a main heat exchanger is one of the most expensive components on any boiler. A cylinder coil cannot be replaced on its own; the cylinder goes. Scale is the single most common reason a boiler that should last fifteen years is replaced at ten.

There is also a safety angle that rarely gets mentioned. A combi that overheats because of scale relies on its thermostat and overheat cut-out to shut it down. Those devices are tested at a boiler service, which is one more reason the annual visit matters more here than it does in a soft-water town.

There is a whole shelf of products sold as the answer to hard water. They fall into three groups, and only two of them are worth your money.

Scale inhibitors on the boiler feed

A small in-line device, usually a cartridge of polyphosphate or a magnetic or electrolytic unit, fitted on the cold feed to a combi. These do not remove hardness; they change how the calcium crystallises so it is less likely to stick. They are cheap, they are a reasonable first line of defence, and most boiler manufacturers now ask for one in hard-water areas. Cartridge types need refilling every year or so, which is easy to do at the same time as the service.

A whole-house water softener

The only thing that actually removes hardness. A softener swaps calcium and magnesium for sodium through an ion-exchange resin and regenerates itself with salt you top up every few weeks. Fitted on the incoming main with a hard-water tap left for drinking, it protects every appliance in the house. It costs more up front and needs a bit of space under a sink or in a garage, but it is the fix that ends the problem rather than slowing it down. Households with a combi and a family showering every day tend to get the most from one.

Things that do not do much

Stick-on magnets, plug-in "electronic descalers" and clip-on gadgets sold online have very little independent evidence behind them. If a product claims to soften water without salt and without any consumable, treat the claim with caution.

Prevention is one job; the scale that has built up over the last ten years is another. Tap aerators and shower heads come off and soak in white vinegar overnight. A combi's plate heat exchanger can be chemically descaled in place by circulating a citric acid solution through the hot water side, which is quicker and far cheaper than replacing it, provided the scale has not already cracked the plates.

The heating circuit is a different matter. Radiators and the primary side of the boiler do not scale in the same way because the same water goes round and round, but they do fill with black magnetite sludge, which is a separate problem with a separate cure. If radiators are cold at the bottom or the boiler is noisy on the heating rather than the hot water, the answer is usually a power flush rather than a descale.

A cylinder that has scaled heavily is harder to save. Immersion elements can be swapped, but a scaled coil stays scaled. When a vented cylinder is on its way out, it is worth pricing an unvented replacement with a softener at the same time, because a new hot water cylinder fed with softened water should last a great deal longer than the one it replaces.