Whole life costing: Hard water

Peter Mayer

You can treat hard water with chemicals or condition it with
electricity but one thing’s for certain – if you want to meet
Part L1, you’re going to have to do something about it.
Peter Mayer of Building LifePlans counts the costs of the various options.

Hard water problems

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 1.6mm of scale results in a 12% reduction in heat transfer which means that more energy is needed to heat water. Scale forms at rates of up to 1.5mm a year.

All components in contact with hard water are at risk. Where hard water comes in 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 should 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 typical consumption patterns.


Physical water conditioners

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 these systems do not use chemicals.

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

• Magnetic conditioners are generally used for individual appliances. The magnetic field alters the scale–forming minerals to 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.


Specification options


Domestic hard water treatment and systems Capital cost
Net present value for 60 years £/unit Service life
Ion exchange water softeners      
Water softener up to 5 persons; 1,500 litre capacity; timer controlled 590 2,440 10-20
Water softener up to 5 persons; 1,500 litre capacity; water volume controlled 667 2,610 10-20
Water softener up to 8 persons, up to 2,000 litre capacity. Computer controlled to predict usage pattern with diagnostics 964 3,230 10-20
Physical water conditioners      
In-line magnetic physical water conditioner; flow rate to 25 l/min for individual appliance 135 1,700 3-10
Electrolytic physical water conditioner; based on zinc, variable flow rate; mains powered 494 2,090 5-10
Electrolytic physical water conditioner; based on zinc; basic system 171 1,420 5-10
Electronic (modulating frequency) physical water conditioner for combi systems; mains powered 216 1,610 5-10


Table notes

• A discount rate of 3.5% is used to calculate net present values.

• Capacity based on water hardness of 300 mg/l (parts per million) calcium carbonate (CaCO3) equivalence.

• Costs are based on averages from sample of systems available on the market with generic allowances for installation and maintenance. Service lives are based on the average of the range. Regular inspection and servicing account for the bulk of the through life cost.

• Direct comparison of options may not be possible as they relate to different functional solutions.

• A whole life cost analysis based on project specific information is essential for a realistic best value appraisal.

First published in Building 2006


Further information

BLP provides latent defect warranties for buildings www.blpinsurance.com

Further information contact peter.mayer@blpinsurance.com or telephone: 020 7204 2450