Yet Another Plumbing and Heating engineer


Hard Water and Limescale

What it is

Hard water is basically water that contains salts (mainly Calcium Bicarbonate) that dissolve out of limestone (and other "soft" rocks) as rainwater makes its way through to our domestic water supply. When water evaporates all the salts it contains are left behind, on baths and basins, tiles and shower curtains. Also, when water is heated it cannot hold as much of some hardness-causing salts (including the main one, Calcium Bicarbonate) as it contained when it was cold. The salts then precipitate out as solid deposits on kettle elements and in hot water cylinders and the heating elements in washing machines, dishwashers and other appliances.

Soft water is water that does not contain hardness-causing salts. Naturally-occurring soft water tends not to contain much of any sorts of salts.

Where it is

The Drinking Water Inspectorate's online Information Leaflet on Water Hardness contains a map of approximate water hardness in England and Wales. (An older version of the leaflet has the same map in monochrome but with clearer lettering.) The DWI leaflet/website omits to mention two appliances particularly affected by hard water: combi boilers and electrically-heated showers. These appliances frequently require descaling or replacement due to scaling-up.

Reading's water is quite hard. Thames Water quotes 277ppm (parts per million - other measures are 19.4 Degrees Clarke, 15.5 Degrees German (DH) and 27.7 Degrees French). Hardness and other water quality measures for other areas can be found from their web site following the link "Water quality and hardness" (on the right hand side of the page) or by calling them on 0845 9200 800 for a written report, which is free for the report for your area.

What to do about it

There are three main approaches to dealing with hard water:

  1. Ignore it.
  2. Prevent (inhibit) scale from depositing.
  3. Artificially soften the water.

The first approach - ignoring it - is appropriate in many circumstances. Hard water doesn't do any harm to drink or cook with; indeed there is some evidence that it may actually be healthier for men's hearts. (On the other hand it may be bad for children suffering from eczema: a current research project at Portsmouth University is looking at whether artificially-softened water is better for such children.) Hard water reduces the lather obtained from soaps so more soap is necessary for the same effect, but otherwise is OK for washing and cleaning. Dishwashers are generally set up for the hardness of their water supply and cope with hard water on their own (using their supply of salt which one must top up as required). Washing machines may suffer from scaling up if run at high wash temperatures, especially from cold fill. With modern low temperature washes and the occasional hot wash scaling shouldn't be a problem, though the machine can be run from a scale inhibitor just to be sure. When washing windows they should be squeegeed dry rather than left for the water to evaporate (specialist window cleaning companies who use spray/brush lances which allow glass to air dry use purified water to avoid scale deposition on the glass). Hard water is also fine for other uses such as plant watering, WC flushing and (generally) in central heating systems.

Scale Inhibitors

There are various devices on the market which claim to prevent limescale from depositing out of water when it is heated, including

Naturally the suppliers of magnetic, electrolytic and electromagnetic types all claim that their devices are effective. Some suppliers give scientific-sounding explanations of how their devices are supposed to work. I recall that Which? magazine once tested magnetic inhibitors and found that they worked in some cases. In my own experience I have had to de-scale several boilers fitted with electrolytic and electromagnetic inhibitors which leads me to the opinion that these devices are not always - if ever - effective.

My own preference is for chemical (phosphor-dosing) inhibitors. Of these there are a few different designs, including

A device I came across in June 2010 is called the NoCalc. From the company's brochure it seems to be a straightforward (if expensive — £249.99) phosphor-dosing system, with the exception of the claims made for it, which I found a bit surprising and prompted me to follow up.

Water Softeners

These operate on a chemical principle, turning Calcium Carbonate (and other scale-producing salts) into Sodium Carbonate (and other Sodium salts). Sodium salts do not precipitate out or affect the lathering of soaps as Calcium salts do, so give the feel and effects of soft water. Their disadvantages are that they