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Failing to treat limescale, or hard scale, in heating or water systems can lead to increased energy costs, more maintenance, earlier component replacement and higher cleaning costs. For example, 1.6 mm of scale results in a 12% drop in heat transfer, which means more energy is needed to heat water. Scale forms at rates of up to 1.5 mm a year.

All components in contact with hard water are at risk. Where hard water comes into contact with hot components, there is a higher risk of scale formation. Boiler heat exchangers are particularly at risk where hard scale forms in the presence of corrosion products. Areas of low flow or components such as valves are often prone to failure.

Part L

The 2006 edition of Part L requires that heat gains and losses from heating systems be limited. In the case of Part L1 for dwellings, further guidance is given in the Domestic Heating Compliance Guide 2006. It requires provisions to reduce limescale accumulation in heating systems where water hardness exceeds 200 mg/l (parts per million) calcium carbonate equivalence. About 50% of England and Wales is supplied with hard water in this category.

Specification options:

Ion exchange water softeners

These are plumbed-in units comprising a resin tank, a brine tank and controls.

• The resin tank is where the hard water is chemically altered; the calcium and magnesium ions are replaced by sodium. Twin tank systems enable water to be supplied from one tank while the other is being regenerated. Uniform small resin beads are most effective.
• The brine tank provides the sodium to regenerate the resin.
• Controls The simplest control is a timer mechanism. Control by volume of water used is more economical. Computer-controlled systems are the most efficient and regulate regeneration to respond to consumption patterns.

These are electronic or magnetic systems that cause physical changes to hard water crystals so that the propensity to form scale on the water or heating system is reduced. An environmental benefit is that the systems do not use chemicals.

Physical water conditioners may be plumbed in or clamped on to pipework. Mains-powered equipment is suited for larger water systems.

• Magnetic conditioners are generally used for individual appliances. The magnetic field alters the scale-forming minerals so these combine with each other rather than form scale. These are limited in capacity and duration of effectiveness. Mains-powered conditioners are more powerful.
• Electrolytic systems add dissolved metal ions (iron or zinc), which attract the scale-forming minerals. Basic systems rely on water as the transfer medium. More efficient systems include resistors. Mains-powered and controlled electrolytic action in effect treats the hard water.
• Electrostatic systems cause the ions to repel so scale-forming minerals do not combine.
• Electronic systems apply a variable electrical field that alters the shape of scale-forming mineral crystals so they do not form scale.

Good practice guidance

BS 7953 is the code of practice for treatment of water in domestic hot-water central heating.

CIBSE and BSRIA also publishes guides on water treatment.

Systems may be certified to German standard W 512, which relates principally to chemical treatment systems.