Aqua Focus

Water treatment supplies, services and consultancy


2, Bradman Drive, Shrewsbury

                                                                                                Shropshire, SY3 5FZ

                                                                                                United Kingdom

25thSeptember 2018

                                                                                                Tel: +44 (0) 1743 632143

                                                                                                Mobile: +44 (0) 7870 596930



BS 7593 – Urgent Need for Reaction to Proposed Revision

The current, proposed revision by the drafting group of Cii62 for BS7593 (Code of practice for treatment of domestic hot water central heating system) contains the requirement in clause 6.2, that unsoftened water should be used in the primary circuit of central heating systems. This is based on the supposition that softened water is more corrosive. Due to a sustained objection by one member of the drafting group, the version currently issued for public consultation contains 3 options. In order to avoid the unjustified implicit criticism of water softeners, we would urge you to access the draft: (

and respond in favour of option 2. 

Our opposition to option 1 (and 3) is based on:

  1. There is no substantiated evidence, field, laboratory or valid science, that confirms that base exchange softened water is more corrosive than the unsoftened source.Quite the contrary: the evidence POSITIVELY DEMONSTRATES that softened water is NOT more corrosive than the unsoftened source. This includes a comprehensive study which was undertaken in conjunction with BSi(see attached document – Softened Water and Central Heating Systems – for more detail)
  2. The allegation stems from the chemical inhibitor manufacturing industry whose interests are challenged by softened water, both in the need for the inhibitor to prevent hardness scale, and in the suitability of some chemical inhibitors for use with softened water.
  3. The chemical inhibitor manufacturers represent the vast majority of the BSi committee Cii62, and therefore dominate in the preparation and revision of BS7593. This standard has influenced some boiler manufacturers who then cite that standard and exclude the use of softened water for their boilers.
  4. But some boiler manufacturers recommend the use of softened water above a certain hardness level.
  5. Some inhibitor manufacturers state that their inhibitor is suitable for softened water
  6. The Building Regulations Compliance Guide for Domestic central Heating Systems includes the option of softening when the water supply hardness level is greater than 200 mg/l.


The exclusion of softened water, in the current revision of BS7593, from the primary circuit is therefore:

  • Technically incorrect

  • Reflects intent and interests of one market sector

  • Is prescriptive in that it excludes the advice of some boiler manufacturers, the claims of some inhibitor manufacturers and Building Regulations compliance

  • inhibits innovation

In conflict with the basic principles of BSi as a national standards body and as laid out in BS 0.


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