15.—(1) Subject to the following provisions of this paragraph, every water system shall contain an adequate device or devices for preventing backflow of fluid from any appliance, fitting or process from occurring.
(2) Paragraph (1) does not apply to–
(a)a water heater where the expanded water is permitted to flow back into a supply pipe, or
(b)a vented water storage vessel supplied from a storage cistern,
where the temperature of the water in the supply pipe or the cistern does not exceed 25°C.
(3) The device used to prevent backflow shall be appropriate to the highest applicable fluid category to which the fitting is subject downstream before the next such device.
(4) Backflow prevention shall be provided on any supply pipe or distributing pipe–
(a)where it is necessary to prevent backflow between separately occupied premises, or
(b)where the water undertaker has given notice for the purposes of this Schedule that such prevention is needed for the whole or part of any premises.
(5) A backflow prevention device is adequate for the purposes of paragraph (1) if it is in accordance with a specification approved by the regulator for the purposes of this Schedule.