Introduction Changes to the UK's Model Water Byelaws will permit, subject to their adoption by the regional Water Authorities, the use of hot water storage systems of more than 50 litres capacity which are connected directly to the mains water supply (storage systems of less than 50 litres are exempt from the Building Regulations), removing the need for a cold water storage cistern. Systems of this type have been in widespread use throughout the world for many years but have not been allowed in the UK. The new Building Regulations for England and Wales, published in November 1985, include safety requirements for these unvented systems. The Approved Documents supporting the Building Regulations state that such systems should be covered by an Agrment Certificate and should be fitted only by BBA Approved Installers, as defined in the Certificate. Similar requirements are likely to be introduced in due course into the Building Regulations for Scotland and Northern Ireland.