Showers

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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.

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).

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.