An improvement on this very simple arrangement, as far as showers over baths are concerned, is the bath/shower mixer. This resembles an ordinary mixer tap on a bath, except that a flexible metal hose rises from the centre of the mixer to a spray head which can be fixed at varying heights on the wall above the bath.
Again the water is mixed by adjusting the hot and cold taps, and at this stage it will be coming out of the spout of the tap. When the required temperature has been reached you pull up a lever on the body of the tap and this diverts the water upwards to the spray head. Nowadays, showers in cubicles normally have what’s known as a manual mixing valve. This has two inlets, one for the hot and another for the cold supply; but the temperature is regulated by turning just one mixer knob. The flow may also be adjusted by turning another knob which is set round the outside of the temperature control. In this way you can control the water itiore quickly and positively than you could do if you had to adjust two separate taps (which tends to be a bit of a juggling act). Shower mixers are constantly being improved so that they are more convenient and safer to use. With one modern manual mixing valve, for example, the temperature of the water is controlled by turning a knurled knob, not unlike the handle of a tap. And the flow and on/off control is worked by pushing in or pulling out this knob You can therefore control the flow and temperature of the water in one movement. Another advantage of this kind of control is that the shower can be stopped instantly if the pressure on the cold side falls (as a result of a toilet being flushed or cold water being drawn off elsewhere in the house, for example). If this happened the shower would suddenly run very hot, but by flicking the control knob downwards the flow ceases. It’s not so serious if the pressure falls on the hot side, because the shower would just run cold. But again, to prevent discomfort the flow can be stopped quickly by flicking the control knob. However, prevention is better than cure and there are ways of organising the plumbing so that this problem can’t arise. To alleviate the danger it’s best to run the 15mm (1/2in) cold water supply pipe to the shower direct from the cold water storage cistern and not as a branch from the 22mm (¾in) distribution pipe to the bathroom. This will supply a continuous volume of cold water provided the cistern is working properly. Thermostatic valves Of course it may mean too much of an upheaval to lay in a new pipe run, but instead you could install a special thermostatic mixing valve. This enables you to pre-set the temperature of the shower water and this will remain constant despite fluctuations of pressure in the hot and cold supplies. And apart from this, thermostatic mixers provide just that extra margin of safety and assurance against discomfort. Before buying a thermostatic mixing valve, it’s important that you recognize its limitations as well as its advantages. These valves can deal with relatively minor fluctuations in pressure that can result from water being drawn off from one or other of the supply pipes. They can’t accommodate the great differences in pressure between a hot water supply under pressure from a storage cylinder and a cold supply taken direct from the main (in any case, you should never arrange your shower plumbing in this way).
Some thermostatic valves even require a greater working hydraulic head’ (the vertical distance between the cold water cistern and the shower rose) than the 1 m (3ft) minimum that is usual for manual mixers. So it’s a good idea to check on these points and on the ‘head’ available before you buy one of them.