Showers

Showers are increasingly popular in British homes: what was rather exotic and luxurious in the day's of "Barry Bucknell's Do It Yourself" is now a standard fitting in new houses. However, since much of Britain's housing stock still has nothing more sophisticated than a Boots' rubber hose hairwashing attachment pushed onto the bath taps, putting in a 'proper' shower is a popular DIY project.

Showers may range from a bath/shower mixer tap with a shower head on a peg on the wall over the bath, to a multi-jet full-body massage shower with drencher heads and diverter valves in a walk-in enclosure in a wet room.

This article is about designing and installing showers so that they give satisfactory performance to the users. It is not about the aesthetics of showers, except inasmuch as aesthetic choices cannot be made in isolation from technical considerations e.g. how much water is required to supply that dinner-plate-sized drencher shower head and multi-body-jet system.

Shower performance

The main factors affecting how satisfactory a shower's performance is are:

Pressure

The force with which water comes out of the shower head (or whether water comes out at all!) depends on the pressure of the hot and cold supplies: whichever has the lower pressure will be the limiting factor. If the pressure available from the existing water supplies is too low it may be possible to boost it with a pump.

Flow

The rate at which water comes out of the shower is determined by the pressure and other factors. In particular it may be limited by whatever is heating the water. Higher flow rates are better for washing (e.g. rinsing off soap or shampoo) and warming (or cooling) the body, but consume more water and energy. The energy conveyed by water at high pressure and high flow rates has a physical effect on the skin which some users may find stimulating but others may find uncomfortable.

Temperature

How well the temperature is controlled depends on the design of the shower valve and the characteristics of the hot and cold water supplies. Thermostatic valves make it easier to set a comfortable temperature, and the temperature tends to remain constant despite changes in temperature and pressure of the water supplies. However some types of valve are better than others at keeping the temperature constant during brief fluctuations such as when other taps connected to the supplies are turned on or off.

Shower Types

Conventional Showers

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 or pressure balancing 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.

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 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, multi-points, 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.

Hot and cold water supplies

As indicated above, for a shower to give good performance the type of hot and cold supplies must be known and taken into consideration in choosing the type of shower to be installed.

Types of hot water systems

(See also Domestic Hot Water Systems)

For the purposes of shower design an important distinction is between systems supplying hot water at mains pressure those supplying at low pressures.

Mains pressure systems

In most circumstances the pressure available from the mains is more than enough for satisfactory showers. However the flow - which may be limited by the heating source (and other factors) - may be less satisfactory. The main types of mains pressure hot water systems are:

Gas instantaneous

Typically from a combi boiler, or a multipoint ('Ascot' type) water heater

The flow rate from these sources is satisfactory for normal showers (but may not be for whole-body, multi jet or drencher types). However when other users also draw hot water the supply to the shower will be reduced to some extent. To what extent depends on the power rating of the boiler and on the relative resistances of the pipework to the shower and to other outlets. A 40kW combi will supply about 66% more hot water than a 24kW unit, but neither may be satisfactory if another outlet (e.g. a bath tap) is capable of drawing the entire output of the combi. Conversely even the smaller combi may be satisfactory if the resistance to other outlets is high enough (or made artificially so by partly closing service isolation valves) for the shower always to receive good pressure.

Another problem which may occur with these systems is that the boiler or water heater may cut in and out at low flow rates, which can result in the shower going hot and cold. This tends to be a problem more with older boilers which cannot modulate their output down to low rates. To test whether this is likely to be a problem before installing a shower onto such a system one can run a hot water tap at low rates and feel whether the water alternates hot and cold.

Stored

These are found as unvented cylinders (e.g. "Megaflow") and thermal stores or heat banks (e.g. "Pandora" or "Boilermate" units)

The flow rate from these is practically unlimited (although the total quantity of hot water available before the store runs cold is limited to, typically, several showers in succession). They are usually found in more upmarket installations designed to supply multiple baths and/or showers simultaneously.

"Storage combis" have a small built-in storage vessel and typically can supply high flow rates, like other stored systems, but for shorter times. Thus they perform better than plain combis when short demands for additional DHW are made (e.g. filling a sink) and they may be able to run two showers more or less continuously. However when their internal store is depleted (e.g. when filling a bath) the output reverts to that of a standard combi, thus they are not ideal for installations serving baths and showers simultaneously.

Electric showers

These are a special case of mains pressure hot water supply as the water is heated inside the shower unit itself (or occasionally in a separate unit connected to the shower head and controls). Although the entire output of the heater is dedicated to the shower the heat input is limited by electrical considerations and is typically one third to one half that of even the smallest combi boiler. The amount of hot water they give may be found unsatisfactory by those used to showers heated by other methods, especially in winter when the flow is lowest due to the low temperature of the incoming cold water.

Low pressure systems

Traditional, conventional systems with a hot water cylinder (other than "Megaflow" etc as described above) give water at low pressure, which is usually insufficient for a shower unless boosted in some way (see discussion of head). They may be part of a CH system and/or heated by electric immersion elements (including night storage "Economy 7"-type systems).

Head

Without augmentation by a pump the pressure (head) of water available is determined by the difference in height between water level in the storage tank and the height of the shower head. The position of the hot water cylinder itself is immaterial (although if the tank and the cylinder are part of a single package they will obviously be closely related). The horizontal distance between CW tank and shower does not affect the pressure either (though long pipe runs, and the size of pipework and other constrictions will affect the possible flow rate).

If the water level in the tank is lower than the shower head the pressure will be negative and shower simply will not work without a pump.

As a rule of thumb a head of less than 1-2 metres is unlikely to give a satisfactory shower (depending on the expectations of the user), even with short runs of wide-bore pipework and appropriate valves and shower head. Over 3 metres gravity-fed showers may be satisfactory (e.g. with a tank in the attic and a shower on the floor below the top floor).

Pumps

Whether it is possible to fit a pump, and what type, depends on whether the cold water storage is:

separate - with a large (rectangular or cylindrical) tank, usually in the attic

it is usually possible to fit a pump to this type of system with both hot and cold supplies derived from the tank and boosted by the pump, allowing a choice of packaged "power shower" or standalone pump.

packaged - with a cuboid CW tank above the HW cylinder usually in a frame

depending on the construction of the package it may not be possible to arrange a cold feed from the tank so a shower which can accept unequal pressures (with the cold supplied directly from the mains) will be required.

combined - ("'Fortic" type) with a small cylindrical CW storage vessel directly on top of the HW cylinder

these systems have very small cold water storage tanks so any pump fitted may be subject to running dry; also as for packaged systems a low-pressure cold water supply will not be available. Venturi-type showers can be used. If an electric pump on the hot feed is used the float valve to the cylinder's header tank can be replaced by an equilibrium type (e.g. Torbeck) for quicker filling and the flow to the shower restricted to ensure that the tank can refill as fast as water is drawn for the shower.

When a pump is required and a cold feed from the storage tank is available (see above) the choices are an integral "power shower" or a separate pump and a normal shower mixer valve.

Power shower

This contains a pump and a mixer valve (thermostatic or manual) in a combined unit. This may be a box mounted on the wall (looking similar to an electrically-heated shower unit) or outside the shower area, connected to the shower head and a control (e.g. Aqualisa Quartz)

Separate pump

Standalone pumps may be

single ended

pumping just one supply (usually the hot)

double ended

pumping hot and cold

There are also choices of

mains voltage

these may not be installed in certain locations within bath or shower rooms due to electrical regulations

low voltage

these are run from a transformer mounted remote from the pump so the latter may be located within restricted zones in the bath/shower room.

Integral power showers may also have separate low voltage transformers.

Pumps are usually quite intolerant of being run dry so it is important to prevent this happening.

Negative head

Where there is no (or negative) head of water to the shower and a pump is to be used the options are

• A pump with a manually operated switch to start it running

These usually have an air-pressure operated switch at the pump connected by plastic tubing (about 6mm diameter) to a push-button which can be mounted inside the shower enclosure. The tubing can be many tens of metres long. Once the push-button-operated switch has started the pump its built-in flow-sense switches keep it running until the shower valve is closed, when it stops running.

• A negative head pump

These maintain a boosted pressure at their outlets at all times. When no water is being drawn they do not need to run and a small expansion vessel built-into the pump maintains the pressure until a demand for water causes the pressure to drop and the pump to run again.

Cold supply to pump

A double-ended pump or power shower (in which both hot and cold water supplies are pumped) requires a supply of cold water at the same pressure as the hot. Where the hot water is supplied from a cold water storage tank in the attic the shower's cold water supply is taken from the same tank. Sometimes there is already a tapping on the tank for supplying the bath cold tap and possibly other services. If there is not (or the existing tapping is unsuitable) a new tapping can be made. This should be lower than the tapping supplying the hot water system so that if the tank empties the hot feed will be lost first, to avoid scalding the shower user. (Where a second tapping would be too low on the tank a new tapping can sometimes be made higher up and used to supply the existing DHW system with the existing tapping used for the pump cold feed.)

Where the hot water system has a pre-packaged tank and cylinder with a sizeable tank it may be feasible to make a second tapping on the tank for the pump. However 'Fortic'-type combination cylinders have very small tanks and are not suitable for taking additional tappings off. Even supplying only DHW these types are liable to quickly run dry if supplying heavy demands, so if being used with a shower pump care should be taken to ensure that the tank's filling valve can refill the tank as quickly as the shower draws from it.

Unequal supply pressures

Some shower valves are suitable for use with hot and cold supplies at differing pressures. In this case a single ended shower pump (or double-ended with both sides plumbed in parallel) can be used to boost the hot water pressure, with cold water fed from the mains.

Venturi shower mixers

A special case of unequal pressure shower valve, the venturi shower uses the energy of mains pressure cold water to boost the flow from a low pressure hot supply to provide a reasonable pressure and flow of shower. It is a power shower which does not require an electricity supply.

Boosting supplies to other outlets

Where a pump is necessary to supply a shower it is often desirable to also boost the supply to other outlets. For example where a bath or shower room has been created in an attic there may be insufficient pressure to give satisfactory flow at basin taps, particularly with modern monobloc mixers designed for high pressure supplies. In situations such as this the hot supply to the basin mixer (and possibly also to bath taps) can be taken from the shower pump. However this may give unsatisfactory operation with standard flow-actuated pumps since, as the tap is opened, there will at first be a slow flow at the un-boosted pressure and then, when the flow is sufficient to trigger the pump, a sudden surge at the boosted pressure. If the user is trying to get a comfortably mixed flow from a basin mixer, or a modest flow from a separate tap, the pump's switching in and out may cause problems. In these situations a negative head pump - which acts to present a constantly boosted pressure at its outlets - may be better.

Home booster pump

An alternative where boosted pressure is required for other outlets besides showers is the Grundfos Home Booster. This is similar to a central heating circulator (pump) but designed for pumping potable water, with a flow-operated switch like shower pumps. It is in effect a single-ended pump. It is quieter and probably longer lasting in applications requiring frequent service (e.g. where services such as kitchen taps will be supplied).

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.

Hose & Head

With internal bore from 5mm to 11mm, the shower hose can have a fairly big effect on flow rate when the hot feed is fed by a header tank. Heads also vary a fair bit in the number of outlet holes they provide. Debris lodged in the hose or head can also take its toll.