A conventional shower is just a means of mixing hot and cold water, and spraying it at you. It is simple and cheap, but its performance is highly dependent on the water supplies to it.
If upstairs in a house with a conventional cistern-fed hot water supply, there will be very little pressure, and hence the shower may be very poor. By reducing the restriction of the shower head as much as possible, you may be able to get quite a lot of water pouring out from the large reservoir of the cistern, but with little force. This is an example of low pressure, but high flow rate. There may also be temperature fluctuations caused by water being drawn off elsewhere, and a thermostatic valve can be used to compensate for this.
If the hot water is at high pressure, such as from a combination boiler (q.v.) or multipoint or a Megaflo (q.v.), then the shower will be better, but may still be limited by the resistance of the incoming rising main. A thermostatic valve is recommended for combi boilers, because they sometimes produce rapid fluctuations in water temperature.
Conventional showers must be fed with hot and cold water of the same pressure, with the exception of a special valve which uses low pressure hot and high pressure cold. This also claims to boost the flow rate of the hot water by taking advantage of the pressure of the cold water.
Electric /Instantaneous Showers
These are available with built-in pumps for low pressure stored supplies, but normally they take high pressure cold water only, and heat it up instantaneously with an electric element. They are simple to install plumbing-wise, and allow you to take showers continuously, because they require no hot water supply. In the simpler devices, water temperature is controlled by a choice of two powers (plus no heat at all!), and by varying the flow rate. On the snazzier models there is electronic control of the heating element, giving variable output.
The main disadvantage with instantaneous electric showers is the power output. The largest I have seen is 10kW, which not only requires hefty cable from a separate fuse-way in the consumer unit, but is also less than half the power of even a low-end combination boiler: if you want a decent temperature, especially in the winter when the incoming water is colder, the flow rate will be quite low. They may also be quite expensive to run, as they are electrically operated and won’t be on at cheap rate unless you shower in the small hours. However, this may be offset by not having to store hot water with consequent losses.
A characteristic of these showers is small but strong jets. This is an example of high pressure, but low flow rate. The shower head is designed in conjunction with the flow rate adjuster, and so the head supplied should be used.
