Water Usage Calculations and Examples

With a city population of 3 million people using this simple cost effective water saving device, a minimum saving of between 1,398 million and 2,073 million gallons of water can be made per year.

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The daily human requirement of water for healthy living is 2.5 litres for drinking and 9 litres for hygiene per day and costs 0.86p (UK), 0.38p (USA), 4.20p (Tanzania), 2.70p (Uganda), 1.15p (Pakistan). In the UK 8 litres is spent on flushing the toilet, 80 litres taking a shower, 90 litres using the garden hose for 10 minutes

- Source: Extract from The Guardian newspaper (2001), repeated by 3 Valleys Water' UK (2005).

Primarily use is in the home and on boats for saving the cold water that arrives when running the hot to warm. When using kitchen sinks and showers, the cold water is normally drained away and wasted. In the home, this water can be saved in a water butt, used for watering the garden or filling the toilet cistern. In boats it can be diverted back to the storage tank to extend the water supply.

The water that can be saved each time depends on:-

  • the ambient air temperature
  • the length of the pipe run from the hot tap to the hot water storage tank or combination boiler
  • the current temperature of the water in the pipe (for second and subsequent operations). On combination boilers (immediate heating ones that are fed directly from the incoming water main supply) the savings can be very great due to the speed and amount of water passing through the pipe under the high mains supply pressures (before the boiler has reached temperature) and its location in the house

The following sections are suggested water savings achieved through a 15 mm pipe from a hot water storage tank or combination boiler, taken from typical houses in the UK:-

Single personal yearly water usage = 11.5 ltr x 365 days = 4,197 litres

Technical details:-

A pipe of external diameter 15mm, internal diameter 13mm and length 1 meter holds 0.133 litres of water.
1 cubic meter = 1,000 litres = 220 gallons
1 litre = 0.22 gallon

Cost of water = 0.7533 / 1,000 ltr = 0.07533 pence / ltr (3 Valleys Water published data 2005)

HOT WATER TANK SUPPLY (STORAGE)

The values in this table are empirically calculated from the pipe arrangement. This table also assumes that the cold water pipes do not cool the hot water as it passes through (which of course they will). This shows a best case scenario. Therefore the actual water passing or saved will be slightly greater.

Hot water storage tank located above ceiling at the floor level indicated

Floor Where used Pipe run (meters) Water saved litres) ** Water saved gallons)
Ground Kitchen sink 4 0.5 0.11
  Shower 4 0.5 0.11
First Basin 9 1.2 0.26
  Shower 4 0.5 0.11
  Basin 14 1.9 0.42
Second Shower 4 0.5 0.14

 

Hot water storage tank on an average day using the Hydrosave unit in a typical 2 story house:-

  • Using the first floor basin operating 4 times saves 1.2 litres each time = 4.8 litres total saving
  • Usng the first floor shower 2 times saves 0.5 litres each time = 1 litre total saving
  • Total saving = 5.8 litres per day or 2,117 litres per year or 466 gallons per year
  • With a city population of 3 million people 1,398 million gallons of water can be saved per year

With a hot water storage tank supply, a saving of 2,117 litres per year is equivalent to 1/2 yearly single personal water usage and a yearly saving of £1.59 (at 0.07533p per litre).

COMBINATION BOILERS (IMMEDIATE HEATING)

With immediate heating boilers, it is not just the pipe run, but also the time it takes the water to heat from cold AND the pressure of the mains supply feeding the boiler. The higher the inlet water pressure, the more water passes over the heating plates in the boiler, causing a cooling function. A reasonable assumption is made that the water saved is the minimum that was recorded to reach the required temperature over a 2 meter pipe test run plus the initial volume of cold water lying in the remainder of the pipes at the start **. One way to overcome this large loss is to throttle back the amount of water being saved by closing the isolating tap to about 3/4 shut. There is a minimum pressure drop at the boiler outlet required to start the heating and this will determine the minimum throttle back setting of the isolating valve. This reduced cold flow rate over the heating plates in the boiler will also decrease the time to reach the required heat level and the amount of water passed to that point.

The values in this table are derived from an initially collected amount, in the first row, of 1 litre and for subsequent rows, then had added to it the empirically (calculated) values from the where used column item to the boiler.

Immediate heating combination boilers at ground floor level only 

Floor Where used Pipe run (meters) Water saved (litres) Water saved (gallons)
Ground Kitchen sink 2 1 litre 0.22
  Shower 10 1 + 1.33 = 2.33 0.51
First Basin 2 1 + 0.27 = 1.27 0.26
  Shower 10 1 + 1.33 = 2.33 0.51
  Basin 2 1 + 0.27 = 1.27 0.51
Second Shower 15 1 + 1.99 = 2.99 0.66

 

This table shows water saved with NO throttle back setting (i.e. worst case scenario where water saved could be less by a small percentage).

Combination boiler on an average day using the Hydrosave unit in a typical 2 story house:-

  • Using the first floor basin operating 4 times saves 1 litre each time = 4 litres total saving
  • Using the first floor shower 2 times saves 2.33 litres each time = 4.66 litres total saving
  • Total saving = 8.66 litres per day or 3,139 litres per year or 691 gallons per year
  • With a city population of 3 million people, a saving of 2,073 million gallons of water can be saved per year.

With a combination boiler, a saving of  3,139 litres per year is equivalent to 3/4 yearly single personal water usage and a yearly saving of £2.36 (at 0.07533p per litre).

Interestingly an alternative, but rather costly method of saving this cold water, is detailed here on an Amercian site under "Water savings". This method is used mainly in large buildings but it gives another way of calculating the 'cold before hot' water saving. Their calculations indicate savings far in excess of those stipulated above but of course their pipe runs and daily usages are different. If you think that their presentation is more appropriate for your home, rather than ours, then the Hydrosave could save up to a minimum of 9,855 gallons = 44,800 litres = £33.75 per year or a maximum of 55,115 gallons = 250,522 litres = £188 per year meaning that the Hydrosave will pay for itself between 1.5 years and 3 months. You decide what are the appropriate figures for your home. As water prices increase the payback time will come down - An interesting thought.

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