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 marketers 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 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.
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.
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 switch(es) 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.
Some shower pumps operate from Safety Extra Low Voltage (SELV), e.g. 24V, and are supplied with separate transformers which are installed well away from the water. This, in theory, reduces the risk of an electric shock from them. My opinion is that approved mains pumps will have been rigorously designed and tested to be very safe, and ***if installed properly*** will be as safe as SELV pumps. Naturally, pumped and instantaneous electric showers should be connected via an RCD (q.v.), and should only be installed by those who are confident in their abilities to produce safe electrical installations.
