The most important requirement is that the hot and cold supply pipes to the shower must be under equal water pressure. With a cylinder storage hot water system, whether direct or indirect, hot water pressure comes from the main cold water storage cistern supplying the cylinder with water. The cold water supply to the shower must therefore also come from this cistern (or perhaps from a separate cistern at the same level); it must not be taken direct from the cold water main. It is, in fact, illegal to mix, in any plumbing appliance, water which comes direct from the main and water coming from a storage cistern. However, quite apart from the question of legality, it is impossible to mix streams of water satisfactorily under such differing pressures. The shower will inevitably run either very hot or very cold; depending on which stream is the high pressure one. The cold water storage cistern must also be high enough above the shower sprinkler to provide a satisfactory operating pressure. Best results will be obtained if the base of the cold water storage cistern is 1, 5m (5ft) or more above the sprinkler. However, provided that pipe runs are short and have only slight changes of direction, a reasonable shower can be obtained when the vertical distance between the base of the cistern and the shower sprinkler is as little as 1 m (39in). The level of the hot water storage tank in relation to the shower doesn’t matter in the least. It can be above, below or at the same level as the shower. It is the level of the cold water storage cistern that matters. There is yet another design requirement for conventional shower installation which sometimes applies. This is that the cold water supply to the shower should be a separate 15mm (1/2 in) branch direct from the cold water storage cistern, and not taken from the main bathroom distribution pipe.
This is a safety precaution. If the cold supply were taken as a branch from a main distribution pipe, then flushing a lavatory cistern would reduce the pressure on the cold side of the shower causing it to run dangerously hot. For the same reason it is best for the hot supply to be taken direct from the vent pipe immediately above the hot water storage cylinder and not as a branch from another distribution pipe, though this is rather less important. A reduction in the hot water pressure would result in the shower running cold. This would be highly unpleasant, although not dangerous.