23.—(1) A temperature relief valve or combined temperature and pressure relief valve shall be provided on every unvented hot water storage vessel with a capacity greater than 15 litres.
(2) the valve shall–
(a)be located directly on the vessel in an appropriate location, and have a sufficient discharge capacity, to ensure that the temperature of the stored water does not exceed 100°C; and
(b)only discharge water at below its operating temperature when subjected to a pressure of not less than 0.5 bar (50 kPa) in excess of the greater of the following–
(i)the maximum working pressure in the vessel in which it is fitted, or
(ii)the operating pressure of the expansion valve.
(3) In this paragraph “unvented hot water storage vessel” means a hot water storage vessel that does not have a vent pipe to the atmosphere.