Pumped/"Power" Showers

The second word is in quotes because it is misleading: one could argue that category (2) is a power shower as it uses electricity, and it is also being abused by the marketeers because "power shower" seems to imply "excellent shower" in the minds of the public. What we are talking about here is a shower containing an electric pump, to boost the pressure, and therefore, if the supply is capable of it, the flow rate. Power showers are easier to wire than type (2) because they only need a low current supply for the motor, which consumes perhaps 500W or less.

Pumped showers MUST be fed from a cistern - i.e. you cannot use them with combination boilers, Megaflos, multipoints, etc. Quite apart from the fact that it is against water regulations to pump from the water main, you are unlikely to achieve much by trying to do this, because if its resistance. Pumped showers are usually only really needed when a cistern is employed anyway.

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 chanber 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.

Cavitation and air in pumped showers

The nearer the pump is to the supplies, the better it will operate (and obviously a two-chamber pump can be put nearer to the supply). This is because a pump may be capable of producing a very high pressure, but can only "suck" at one atmosphere before a vacuum is created and performance will not increase. Even before this, "cavitation" (tiny vacuum or dissolved air bubbles) will start at the impeller blades, and this is very bad for the pump. Water from the rising main contains dissolved air, and heating it up encourages it to liberate this. It is therefore a good idea to connect a shower pump to a hot water cylinder with a "Surrey flange" or an "Essex flange", which has a short dip-tube to avoid trapping the liberated air rolling up the sides of the cylinder. (A Surrey flange fits into the top of the cylinder and has an additional output for the existing connections an Essex flange is a dip-tube only and goes into a new hole made in the cylinder.) Surrey/Essex flanges will also help to avoid interaction between the pump and other hot water consumers.

A shower pump should not be installed at a high point in the system: trapped air will be difficult to expel, and, in a worst case, the pump may not operate at all as it is not self-priming: i.e. it cannot pump air.