Descale the heating surfaces and the sludge at the base, not the tank walls. Withdraw the immersion element and soak it in citric or sulphamic acid, then flush the bottom of the tank through the drain valve. Every millimetre of scale on a heat transfer surface adds roughly 7% to the energy needed, so in water above 180 mg/L of calcium carbonate this is a two- to three-year job.
How do I know I have a scale problem?
Five signs, usually in this order: reheat takes noticeably longer than it used to; the tank rumbles or kettles as it heats; the immersion heater fails prematurely; sediment appears when the drain valve is opened; and the usable volume falls even though the thermostat still reaches setpoint.
Scale is calcium carbonate, and its solubility falls as water gets hotter — the reverse of most salts. It therefore precipitates preferentially on the hottest surfaces in the system: the immersion element sheath, the outer surface of the coil, and the base of the tank where the sludge settles.
| Hardness (as CaCO₃) | German °dH | Classification | Practical action |
|---|---|---|---|
| 0–60 mg/L | 0–3.4 | Soft | None; watch the anode instead, soft water consumes it faster |
| 61–120 mg/L | 3.4–6.7 | Moderately hard | Inspect the element every 5 years |
| 121–180 mg/L | 6.8–10.1 | Hard | Descale every 3–5 years, store at 60 °C not 70 °C |
| Above 180 mg/L | Above 10.1 | Very hard | Descale every 2–3 years, consider softening or dosing |
What does scale actually cost?
More than most owners assume. Calcium carbonate is a poor conductor, so a scale layer behaves as insulation between the heat source and the water. The widely cited figure from the Carbon Trust is that a 1 mm layer increases the energy input needed for the same heat output by about 7%; at 3 mm the element or coil is working roughly a quarter harder.
There is a second cost. Scale on an immersion element traps heat against the sheath, which is why heavily scaled elements fail early and why the thermal cut-out starts tripping. On a plate heat exchanger the effect arrives faster still, because the plates run at high heat flux with a very thin water film.
How do I descale a cylinder safely?
Work on the components, not the vessel. Flooding an enamelled tank with strong acid risks the coating and will strip the magnesium anode; using hydrochloric acid anywhere near stainless steel risks chloride pitting.
- Isolate the electricity supply to the immersion heater and the water supply to the tank, then open a hot tap to release pressure.
- Drain enough water to fall below the element boss — or fully, if you also intend to flush the base.
- Withdraw the element and soak it in a citric acid or sulphamic acid descaler at the strength on the packaging. Brush the sheath, do not scrape it.
- Flush the base through the drain valve with the cold feed briefly opened, to lift the loose sludge out rather than dissolving it in place.
- Inspect and replace the anode while the tank is open. It is the same access, and a spent anode is far more dangerous to the tank than the scale.
- Refit with fresh gaskets and PTFE, refill, vent, then restore power and check for leaks.
A plate exchanger or fresh water station is cleaned differently: circulate a descaling solution through the DHW side with a small pump for 30–60 minutes, then flush thoroughly with clean water.
What temperature should I store at in hard water?
Sixty degrees, and no more. Precipitation accelerates sharply with surface temperature, so raising a store from 60 °C to 70 °C markedly increases scaling while adding nothing to safety — legionella is already controlled at 60 °C. Storing lower than 55 °C reduces scaling further but requires a periodic pasteurisation cycle, and it also cuts the usable hot water the tank delivers.
Fit a thermostatic mixing valve so 60 °C storage arrives at the tap at 43–48 °C. That combination — store hot, deliver blended — is the best compromise between hygiene, scaling and scald risk.
How do I stop it coming back?
- Softening the hot supply is the only measure that removes hardness rather than managing it. Softened water is more aggressive to steel, so keep the anode inspection on schedule.
- Polyphosphate or silicate dosing does not remove hardness; it keeps the crystals in suspension so they leave with the water instead of adhering. Cartridges need replacing on the manufacturer's cycle to do anything at all.
- Magnetic and electronic conditioners are widely sold, but independent evidence for them remains mixed — treat any claimed saving as unproven until it is measured on your own system.
- Annual service. Pulling the element once a year in very hard water takes twenty minutes and prevents both the scale and the anode problem in one visit.
Frequently asked questions
Can I put descaler straight into the hot water cylinder?
It is not advisable. Strong acid can attack the enamel lining, will consume the magnesium anode, and chloride-based products risk pitting stainless steel. Descale the immersion element out of the tank and flush the base sludge through the drain valve instead.
How often should a cylinder be descaled in a hard water area?
Every two to three years above 180 mg/L of calcium carbonate, and every three to five years between 120 and 180 mg/L. Below about 120 mg/L an inspection every five years is usually enough.
Does limescale really reduce efficiency that much?
Yes. A 1 mm scale layer on a heat transfer surface increases the energy needed for the same output by roughly 7%, according to the Carbon Trust figure used across the industry. Scale also traps heat against immersion elements and shortens their life.
Should I install a water softener for my hot water cylinder?
It is worth it above roughly 200 mg/L of calcium carbonate, especially where a plate heat exchanger or a heat pump cylinder is involved. Bear in mind softened water is slightly more aggressive to steel, so keep inspecting the sacrificial anode every two years.