Sunday, 22 October 2017

How much less water do modern houses use

Cambridge is growing and more people means more water consumption, which is a concern in this area. Will the new homes use as much water as existing ones? Building regulations require that new homes are fitted with water efficient appliances so that an 'average' person will use no more than 125 litres/person/day (lpd). This is considerably less than the UK average 141 lpd. So what is an 'average' person? Using the water calculator specified in building regulations I have estimated water consumption for this mythical individual using water efficient appliances versus a traditional context - by which I mean one from a few decades ago. Based on this estimate, even if all new homes use the most efficient appliances that would not be enough to keep Cambridge's usage within the allowed limits.

Here are the headline figures: the efficient house comes in at 104 lpd compared to 146 for the traditional house, and 47% of the difference is due to savings from flushing the WC. This chart compares the two.
Total use Efficient home 104 lpd, Traditional 146 lpd. For assumptions and use factors see below.

You might like to compare your usage with the 'average' person. If you use more then you could probably save more too.

The water calculator specifies average use factors - for example flushing the WC 4.42 times per day - and you multiply this by the amount of water for each use, depending on the actual appliance fitted. In practice people vary hugely in their water consumption habits. For example you will flush the toilet more often if you are at home all day rather than at work, and more still if you are taking medicinal diuretics, or if you have a variety of medical conditions with implications in this area. The table at the end of this post summarises use factors for all the appliances.

Traditional house: 9 litres/flush (but the really old ones can use up to 13 litres).
Efficient house: 6 litres full flush, 4 litres part flush, mean 4.7 litres.
Saves: 19 lpd

The water calculator assumes you flush 4.42 times/day.

In the traditional house the WC is the biggest water use, at 27% of all water; in the efficient house this is down to 20% and cedes top place to showers.

I calculate savings from installing an efficient WC at £16/year, based on current Cambridge Water prices (0.23p/litre including sewerage as well as supply). If you go to the loo more often your savings will be more. However there are reasons to use less water other than financial savings.

You can reduce your toilet water consumption cheaply using a cistern displacement device such as a Save-a-flush or a HIPPO.

Bath and shower
Bath: 90 litres (no difference)
Traditional shower 8 l/min
Efficient shower 6 l/min
Saves: 9 lpd

I have assumed this house has both bath and shower and in these circumstances the water calculator says the average person apparently takes either a bath or a 5.6 minute shower each day - the bath is 1.5 times/week and the shower 5.5 times/week.

For the bath, I have assumed there is no difference between the traditional and efficient house because no-one likes a small bath.

For the shower, 8 l/min is reasonable for a gravity fed shower without special features. 6 l/min is possible with an efficient shower head. These savings are significant, as you can see in the chart. However they would be a lot more if you like to linger in the shower, or if you have a power shower (17 l/min on average) or if you shower more than once a day as some people do.

Traditional basin taps: 6 l/min
Efficient basin taps: 4 l/min
Traditional kitchen taps: 12 l/min
Efficient kitchen taps: 8 l/min
Saves: 5 lpd

Flow rates will vary from one house to another - these are reasonable averages. The calculator assumes you use the basin tap for 1.56 minutes/day (either you are turning it off while you brush your teeth or you are not brushing for the recommended 2 minutes!).

For the kitchen it assumes only 0.44 minutes which is probably just for rinsing. However it also adds a fixed quantity of 10.36 litres/day which is due to filling the sink and also for running cold water through the pipes while you wait for the hot to come through.

Washing machine

Traditional washing machine: 47 litres for 5 kg load (my 20 years old machine on eco setting)
Efficient washing machine : 38 litres for 6 kg (Indesit BWSC61252WUK , 6kg Load, A++)
Saves: 6 lpd

The water use of a washing machine does not influence the energy rating directly. However, less water means less energy to heat it so an efficient machine will tend to use less hot water.

The water calculator assumes you wash 2.1 kg clothes each day, or possibly less clothes on a partial load. For example, if the machines takes 6 kg it assumes you run it 0.35 times per day per person.

Traditional dishwasher: 18 litres for 14 place settings (my 20 years old machine on medium wash)
Efficient dishwasher (9.5 litres for 13 place settings) (Bosch SMS50M18GB, 13 settings A++)
Saves: 2 lpd

As with the washing machine, water use has no direct implications for energy but in practice more efficient machines must be heating less water.

It is hard to imagine using less water than this to wash by hand.

The water calculator expects you to run 3.6 place settings per person per day- or as with the washing machine, less than this but on a partial load. If you wash cooking and serving dishes as well as place settings this figure does not seem ridiculously large. If there are two of you at home and you have a 14 place settings dishwasher they estimate you would run it every two days.

Use factors summary

ApplianceUse factorNotes
Toilet4.42 flushes/day.
For a dual flush toilet: 1.46 full flushes and 2.96 part flushes 

Bath If no shower, assume 0.5 per day (ie. one bath that is half full) If you have a shower, assume 0.11 baths per day; this implies 1.5 baths/weekBath capacity is to the overflow and you are assumed to fill it half full.
Shower If no bath, assume 5.6 minutes per day. If you have a bath too, 4.37 minutes per day; this implies 5.5 showers/week
Basin taps1.58 minutes Excluding kitchen and bath taps
Kitchen taps0.46 minutes plus 10.36 litres Running water is for rinsing and the fixed use is for filling the sink and for running water through until the tap runs hot.
Dishwasher3.6 place settings 
Washing machine2.1 kg dry laundry 

It is easy to meet the required standard with water saving appliances.
The regulatory limit of 125 lpd is very easy to meet with modern water saving appliances, at least according to the water calculator. Whether this accurately reflects an average person is hard to say.

If someone wants to swap in a power shower there is nothing to stop them.
In any case, there is no law that says you have to live with the appliances you start with. For example replacing the shower with a power shower, would increase the 'average' person's consumption from 104 lpd to 152 lpd. The cost in terms of water bill is fairly minor (depending on your attitude) at an extra £40/year (current Cambridge prices) but, if everyone did this it would put a huge strain on our water resources.

Cambridge Water assumes existing households will also reduce consumption.
In Cambridge, our water comes mainly from aquifers and increasing supply has consequences for a number of water sources that are also fed by those aquifers. We are already extracting 80% of the maximum permitted by licences issued by the Environment Agency [6]. To allow for population growth, the Cambridge Water Resources Management Plan (WRMP) assumes that per person water use declines to less than 125 lpd by 2040 on average over all customers. As of 2014 they estimated 141 lpd (somewhat less in metered households and more in unmetered households. Adding in the anticipated 47000 new households, with 93,000 people, assuming 104 lpd this brings average consumption down to 132 lpd, not even close to the targat.

If we all install dual flush toilets that could be enough.
Existing households will need to make some changes too and the toilet is an obvious place to start. Have you replaced your installed a low flush toilet yet? I have. If half of existing households haven't yet and they all do the switch that would be enough to bring the average consumption down to 125 lpd (according to the calculator).

[1] Building regulations part G Sanitation, Hot water safety and Water Efficiency (

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