What influence does hard or soft water quality have on the construction materials of hot water cylinders?

Water quality has a direct influence on the recommended material construction of a hot water cylinder, especially in relation to limescale, corrosion, pH, chlorides, water hardness, cylinder lifespan and heat transfer performance.

UK water classification

In the UK, soft water is generally classified as water with a total hardness of up to 100 mg/l as calcium carbonate, expressed as mg/l CaCO₃. The Drinking Water Inspectorate (the Drinking Water Inspectorate, often shortened to DWI, is the official body that regulates and checks drinking water quality in England and Wales) defines hardness as being caused by dissolved calcium and magnesium, and its consumer guidance uses the following bands: soft up to 100 mg/l CaCO₃, slightly hard 100–150, moderately hard 150–200, hard 200–300, and very hard above 300.

UK water classification

Classification	Hardness as CaCO₃
Soft	up to 100 mg/l
Slightly hard	100–150 mg/l
Moderately hard	150–200 mg/l
Hard	200–300 mg/l
Very hard	over 300 mg/l

UK map of hard and soft water areas

Here is a map of the UK and Ireland showing the general water quality in certain areas

Water quality map showing hard water and soft water areas for the UK and Ireland

The best method is to check with your local water supplier, because hardness can vary by supply zone rather than just county.

Hard water areas

In hard water areas, the main issue is scale formation. Hard water contains higher levels of dissolved calcium and magnesium salts, which can precipitate as limescale when water is heated. The Drinking Water Inspectorate explains that softening water just before a hot water tank or boiler can help prevent hardness salts from forming scale.

For hot water cylinders, limescale can build up around immersion heaters, heat exchanger coils, cylinder bases, thermostatic controls, valves and pipe connections. This can reduce heat transfer, slow recovery times, increase energy use and reduce the usable performance of the cylinder. CIBSE notes that minerals in hard alkaline water can accumulate as scale when heated, reducing heat exchanger performance and affecting valves and water flow.

In hard water areas, the cylinder material itself is only part of the answer. A good specification may include a stainless steel cylinder, suitable immersion heater design, scale-resistant components, access for maintenance, and possibly a scale reducer, water conditioner or softener, depending on the installation.

Soft water areas

In soft water areas, the concern is usually less about limescale and more about corrosion. Soft or acidic water can be more aggressive to metals. The Drinking Water Inspectorate states that soft acidic waters can cause corrosion of pipework and dissolution of metals such as copper, zinc and lead.

Some technical guidance uses a stricter band where soft water is up to 50 mg/l CaCO₃. Flexiheat UK would describe this as very soft or low-hardness water, particularly where the pH is also low. The DWI notes that soft acidic waters can cause corrosion of pipework and dissolve metals such as copper, zinc and lead, so hardness should be considered alongside pH, alkalinity, chlorides, conductivity and local water chemistry

This is why the choice between copper, stainless steel, glass-lined steel or enamelled steel cylinders should take local water chemistry into account. In softer or more aggressive water areas, stainless steel is recommended; most manufacturers won’t warranty any other cylinder than a stainless steel cylinder in soft water areas.

Stainless steel cylinders

Stainless steel is widely used for modern unvented hot water cylinders because it offers excellent corrosion resistance and does not rely on a sacrificial anode in the same way as many glass-lined steel cylinders. However, stainless steel is not completely immune to water quality issues. Chlorides, high temperatures and poor installation conditions can increase corrosion risk. The British Stainless Steel Association notes that 316 or duplex stainless steels are generally suitable for potable hot water heaters.

Recommendations

In simple terms, hard water usually increases the need for scale management, while soft or aggressive water increases the need for corrosion-resistant cylinder construction. The best hot water cylinder material should be selected by considering water hardness, pH, chloride level, storage temperature, potable water quality, immersion heater exposure, heat exchanger coil design and manufacturer warranty limits.

You can’t go wrong with stainless steel cylinders in soft or hard water areas