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Wednesday, November 14th, 2018
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Water Usage-Savings
Showers and baths

In new houses showers and baths account for around 45 per cent of the total water used.

Summary

In new houses, showers and baths now account for around 45 per cent of the water used. Modern plumbing, en-suite bathrooms and changes in lifestyle are all contributing to the trend towards using significantly more water for bathing and showering.
Showers can be a water-saving alternative to baths but people tend to take them more frequently. Recent trends with 'power showers' and mains pressure systems, however, have increased flow rates to the point where a long shower can use more water than a bath.
The fact remains that a shower can use about a third of the water of an ordinary bath. Also, as bathing water is heated there is an associated energy and carbon cost. In an efficient home, hot water typically uses more energy than space heating.

Retrofit options

For older houses with electric showers or simple gravity-fed mixer showers, there is little scope to save water. Homes with pumped or mains pressure showers can be fitted with simple flow regulators or 'water saver shower heads' to limit the maximum flow rate to below nine litres per minute.
Before spending money on a new showerhead it is worth measuring the flow of your existing one. The simplest way to do this is with a watch and a small bucket or large jug. Set the shower to the highest flow rate and direct it into the bucket for 10 seconds. Measure this volume with a measuring jug or kitchen scales (1 litre = 1kg) and multiply by six for litres per minute. Table 3 shows typical flow rates of different types of showers.
You can use the same test to check that an instantaneous water heater, for example a combi or multipoint boiler will work at low flows. For example, if you are considering buying a six litre per minute shower head, set the flow rate of the existing shower to six litres per minute and check that the temperature is stable at this flow rate. Some combi boilers are unable to operate at low flows and may not cut in or the temperature might be erratic.
Obviously even high flow rate showers can be turned down but the shower may not perform as well as a purpose-designed low flow showerhead. Also, some people might not be as conscious of saving water, which is why limiting the maximum flow rate can bring about savings, rather than just relying on people to adjust their behaviour.
Showers In the UK there is no agreed definition of a water-efficient showerhead. Also, the amount of water a shower uses depends very much on the person using it, for example what flow setting they are using and how long they shower. A quick three minute shower with the flow adjusted to a comfortable 5 litres per minute uses only 15 litres of water, whilst 10 minutes at 15 litres per minute will use ten times as much water and energy without making us any cleaner.

How much flow do we like?
Many showerheads have no flow regulation and so the maximum flow rate is only limited by the available water pressure. Trials by the Building Research Establishment (BRE) suggest that most people find flow rates of less than three litres per minute in a shower unacceptable. Currently there are no upper limits on flows in the UK, while in the US the maximum flow rate is 9.5 litres per minute.     Bathers are interested in comfort and only a handful of people have any idea how much water their shower uses. However, it is commonly assumed that flow equals comfort and so most manufacturers are hesitant to make water efficiency claims for their products and indeed boast of high flow rates.
'Water-saver' showerheads usually work by creating finer drops or by incorporating air into the flow. Typically, these showerheads require a pressure of at least one bar, which is available from mains pressure and pumped systems but rarely from gravity-feed hot water systems. These water-saver showers typically work at a flow rate of between four and nine litres per minute, and the effect is usually perceived as a 'power shower' but with perhaps half the flow rate.
The perceived performance of a showerhead is influenced by the quality as well as the flow rate. For more information on typical flow rates from different shower designs, see Table  below. Another important consideration for many buyers will be how a showerhead looks. Happily, water efficient shower heads are now available in a range of styles and need not look any different to a standard shower head. Showerheads can provide a gentle rain effect, an aerated champagne flow with almost no splashing, or a more invigorating massage effect with high skin pressure. Showerheads that create fine droplets can lead to cold feet since smaller water droplets cool quickly. Other showerheads can be quite noisy adding to the illusion of 'power' and flow. All this makes performance testing and recommendations difficult since user perceptions of what makes a good shower vary so widely.
In trials by Liverpool John Moores University, fitting an aerated showerhead was effective in reducing flow-rate by 28 per cent (3.2 litres per minute) on average, whilst improving or only marginally reducing customer satisfaction with the shower performance. Despite the reduced flow rate, eight of the nine households where an aerated showerhead was fitted asked to keep it.
It can be dangerous to use water-saver showerheads and restrictors with electric showers. Check with the equipment manufacturer.

Water use in showers depends on a number of factors:
Heating mechanism:

  •     Combination-boiler warm-up
  •     Pipe dead-leg (time for water to run hot)

Fixed / adjustable controls:

  •     Separate flow and temperature controls
  •     Stability of combination-boiler temperature control
  •     Stability of plumbing system pressures

Flow rate:

  •     Pressure and spray pattern influence perception of flow
  •     Small flow reductions may not be noticed
  •     Position of header tank/mains pressure/ pumped pressure

Water efficient showers are important for saving water, energy and carbon. They also allow a comfortable shower without using up all the hot water when storage capacity is limited.

Mixer valves
The choice of mixer valve will also influence comfort and volume of water wasted during showering. Simple hot and cold tap controls mean both taps have to be adjusted with an infinite number of possible combinations in order to achieve the desired flow and temperature. As this must be done by feel, the valves will have to be adjusted as the hot water starts to reach the mixer. This presents an increased risk of scalding.
Thermostatic mixers usually have a calibrated dial, so the temperature can be set from experience. The flow is adjusted with a separate control so that reducing or interrupting the flow, for example to apply shampoo, is simple.

Table 1. Shower flow rates   

 


4 litre /

minute

7.2kW

electric

 9.8kW

electric

  6 litre / minute

water saver"

9.5 litre / minute

water saver"

Power shower
 Flow litre /

   minute

  4 l/min   3.5 l/min 30°C

     temp rise

 4.7 l/min 30°C

     temp rise

 6 l/min regulated

      flow

 9.5 l/min regulated

        flow

 Typically 12

     + l/min

 Notes  Can be effective

but Probably 

for the lower

limit for most 

people

especially if the 

bathroom is cold

May be

perceived as

poor performance 

particularly

in winter

Perceived

by many as 

adequate

 A 'good shower'

by traditional

UK standards

 Maximum flow

rate permitted

in the USA

 Might not be

used at full

flow

 Plumbing

system 

compatibility

 Combi boiler

and some

thermostatic

mixer valves

unlikely to 

work at s

uch a low

flow rate

 Fed from

mains pressure

cold water

 Fed from

mains pressure 

cold water

 Mains pressure

or pumped

hot water.Some

combi

boilers might 

not work at

this flow rate

 Mains pressure

hot water or 

pumped


 Water use for

5 minute 

shower

         20          17.5          23.5        30 litres           47.5       60+
 As a % of 70

litre bath

         29           25           39          43          68       86+
 Kg CO2 Gas boiler     0.07-0.27      0.34 direct 

       electric

   0.45 direct

    electric

       0.27-0.4        0.42-0.63    0.53-0.8+


Baths
The main variables which determine how much water is used to fill a bath are the volume and shape. Tapered or peanut-shaped baths may provide more space for bathing with less water.
Bath volumes are usually given in promotional literature and are specified to the centre of the overflow but other manufacturers consider Archimedes' principle and subtract the volume of an adult (about 70 litres), so make sure that you compare like with like. Very few modern baths hold less than 130 litres, which is about 60 litres of water with a submerged adult. Some larger baths hold more than 300 litres, equivalent to the average volume of water two people use each day.
Under the Water Regulations in some countries you have to let your water supply company know if you plan to install a bath with a volume of more than 230 litres.