17.—(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 vent pipe, a temperature control device, a temperature relief device and a combined temperature pressure and 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.
18. Appropriate vent pipes, temperature control devices and combined temperature pressure and relief valves shall be provided to prevent the temperature of the water within a secondary hot water system from exceeding 100°C.
19. Discharges from temperature relief valves, combined temperature pressure and relief valves and expansion valves shall be made in a safe and conspicuous manner.
20.—(1) No vent pipe from a primary circuit shall determine over a storage cistern containing wholesome water for domestic supply or for supplying water to a secondary system.
(2) No vent pipe from a secondary circuit shall terminate over any combined feed and expansion cistern connection to a primary circuit.
21. Every expansion cistern or expansion vessel, and every cold water combined feed and expansion cistern connected to a primary circuit, shall be such as to accommodate any expansion water from that circuit during normal operation.
22.—(1) Every expansion valve, temperature relief valve or combined temperature and pressure relief valve connected to any fitting or appliance shall close automatically after a discharge of water.
(2) Every expansion valve shall–
(a)be fitted on the supply pipe close to the hot water vessel and without any intervening valves; and
(b)only discharge water when subjected to a water pressure of not less than 0.5 bar (50 kPa) above the pressure to which the hot water vessel is, or is likely to be, subjected in normal operation.
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.
24. No supply pipe or secondary circuit shall be permanently connected to a closed circuit for filling a heating system unless it incorporates a backflow prevention device in accordance with a specification approved by the regulator for the purposes of this schedule.