# The Water Supply (Water Fittings) Regulations 1999, Paragraph 17

Extent: E+W

(1) Every unvented water heater, not being an instantaneous water heater with a capacity not greater than 15 litres, and every secondary coil contained in a primary system shall–
(a) be fitted with a temperature control device and either a temperature relief valve or a combined temperature and pressure relief valve; or
(b) be capable of accommodating expansion within the secondary hot water system.
(2) An expansion valve shall be fitted with provision to ensure that water is discharged in a correct manner in the event of a malfunction of the expansion vessel or system.

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Source: https://www.legislation.gov.uk/uksi/1999/1148/schedule/2/paragraph/17 (the official version)
Page: https://openlegislation.uk/water-supply-water-fittings-regulations-1999/schedule-2/paragraph-17
Contains public sector information licensed under the Open Government Licence v3.0. Source: legislation.gov.uk, The National Archives.