These showers are connected directly to the mains water supply, as a branch from the rising main as it heads up to the cold water storage tank in the loft.
They’re very good for temperature control, because they take the cold water from the main pipe and heat it within the unit, so you don’t have to spend ages fiddling with the hot and cold taps. A thermostatic control ensures that the water doesn’t suddenly get too hot if there’s a sudden reduction in water pressure due to heavy demand on the water main elsewhere, for instance if someone turns on the washing machine or flushes a toilet. If the flow of water gets too weak, the heater may even shut down altogether rather than risk you getting a scalding.
Some instantaneous showers run on for a short time after you turn off the unit, to ensure that anyone using the shower immediately afterwards doesn’t get a scalding.
Because of the heavy demand the water heater makes on the electricity supply, and to ensure that the circuit breaker works properly in case of an electrical fault, instant showers have to have a dedicated circuit from the consumer unit. The main switch for the appliance will be a double-pole switch in a ceiling mounting, usually with some kind of indicator (a light or a flag) to show when the power’s on.
Often a single mixer cabinet mounted on the shower wall contains the necessary connections, both plumbing and electrical. But it’s also possible to buy split-unit showers, so that the control panel on the wall of the shower is connected to the heater unit installed somewhere out of sight, but not too far away; suitable locations include under the bath, in an airing cupboard or even in the loft above the shower cubicle.
To enable you to have the shower unit serviced easily and conveniently, it’s a good idea to fit a stopcock or an isolating valve in the water supply pipe.