Pumped showers are less likely to suffer from temperature variations than conventional showers. They can produce copious amounts of water with a lot of force: high pressure, and high flow rate. They often come with shower heads that can produce varying spray patterns and mix air with the water. They can be extremely wasteful: it is drummed into one that a shower takes less water than a bath, and it is possible to be blissfully unaware that this may no longer be the case with such a beast!

The simplest pumped shower is a device which you screw to the wall and connect between your existing mixer and shower head with flexible hoses. It has an on/off switch, and the shower will still function with it switched off, as the pump chamber does not present much resistance.

More sophisticated models include a mixer within the case, and are plumbed permanently into low pressure hot and cold supplies. They usually have a combined on-off switch and mechanical flow rate control, and may vary the speed of the motor.

More sophisticated still are the separate pumps. The cheap ones connect to the mixed water the expensive ones have two chambers which connect to the hot and cold supplies. They may be used with manual or thermostatic valves.

Switching of separate pumps
Separate pumps have automatic switches to operate them. Positive head pumps are used where there is still some flow when the pump is off, i.e. when the bottom of the cistern (this is used as the datum, as it is a worst case) is significantly above the level of the shower head. The flow switches activate the pump as soon as the valve is opened and water starts to flow, and deactivates it as soon as the valve is closed and flow ceases. Another reason to keep air out of a shower pump is that it might cause these switches to oscillate, in the following manner: 

1) a sudden increase in pressure (such as water hammer from a valve being closed quickly) pushes water through the pump as it compresses an air bubble in the system

2) the water flow switches on the pump, causing more water to flow as the bubble is compressed further

3) the air pressure reaches the pump pressure, the water stops, and the pump switches off

4) the air bubble pushes the water back again

5) the momentum of the water causes the air to expand beyond equilibrium

6) the water eventually stops, and then flows back again, as the air pressure is lower than it, and the cycle starts again.

Negative head pumps allow a shower head to be higher than the bottom of the supply cistern, and are useful in flats and for bathrooms in loft conversions. These require pressure vessels, non-return valves and pressure switches, and are therefore rather more expensive than positive head pumps. The pressure vessels are on the outlets of the pump, and are monitored by the pressure switches. They contain an air chamber and a diaphragm, rather like for a sealed heating system (q.v.), and allow a significant amount of water, which is essentially incompressible, to flow in or out for a given change in pressure.

The pressure switches are normally closed, causing the pump to charge the pressure vessels. This will open the switches, the pump will stop, and the non-return valves will ensure that the vessels remain charged. There will now be enough pressure in the system to allow water to flow out of the shower, and as soon as this happens, the pump will operate. Leaking shower valves will be obvious by the annoying intermittent operation of the pump that they cause!

Other considerations with pumped showers
Pumped showers can be noisy. There is not much that can be done about this in the case of the ones mounted in the showering area, but separate pumps should be supplied with flexible couplings to reduce the amount of vibration being transmitted around the house through the pipework, and they can be supported on rubber mats to reduce noise transmission through the house structure.

Pumped showers may require more sophisticated measures to avoid water escaping from the showering area: some screens, etc., are not recommended for use with them.