Water Efficiency Calculator for New Dwellings (2009)

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The Government’s national calculation methodology for assessing water efficiency in new dwellings in support of:

The Code for Sustainable Homes, May 2009 and subsequent versions The Building Regulations 2000 (as amended)

The Building (Approved Inspector etc) Regulations 2000 (as amended)

The water calculator methodology

1.1 The following document sets out the water calculation methodology for assessing the whole house potable water consumption in new dwellings. The calculation method is to be used to assess compliance against the water performance targets in Building Regulations 17.K and the Code for Sustainable Homes (referred to in this document as the Code) as set out below. It is not a design tool for water supply and drainage systems. It is also not capable of calculating the actual potable water consumption

of a new dwelling. Behaviour and changing behaviour can also have an effect on the amount of potable water used throughout a home.

The calculation method requires the use of water consumption figures provided from manufacturers product details. Before the assessment can be carried out, figures
will need to be collected from manufacturers product information to determine the consumption of each terminal fitting, including:

a. WCs

  1. Flushing capacity for the WC suite including consumption at full and part flush for dual flush WCs.
  2. Where multiple WCs are specified with various flushing capacities, the average effective flushing volume must be used as set out in section 2.5.

b. Bidets

i. Bidets are excluded from the Water Efficiency Calculator for new dwellings due to their minimal water consumption, and although there is insufficient research to quantify this consumption, anecdotal evidence shows that there is evidence that bidets often displace other water consumption rather than increase consumption.

c. Taps

  1. Flow rate of each tap, at full flow rate in litres per minute measured at a dynamic pressure of 3±0.2 bar (0.3±0.02 MPa) for high pressure (Type 1) taps, or at a dynamic pressure of 0.1±0.02 bar (0.01±0.002 MPa) for low pressure (Type 2) taps (BS EN 200:2008, sanitary tapware, single taps and combination taps for supply systems of type 1 and 2. General technical specifications) including any reductions achieved with flow restrictions.
  2. Where multiple taps are to be provided (e.g. separate hot and cold taps) the flow rate of each tap will be needed in order to calculate an average flow rate in accordance with section 2.5.
  3. For ‘click taps’ and other taps with a ‘water break’, the manufacturer’s stated full flow rate should be used to perform calculations (measured as described above). Do not use the flow rate at the break point. A factor for percentage of flow rate is already assumed within the use factor for taps. There is currently no research to provide a separate use factor for ‘click taps’ so a standard use factor is applied.
  4. Taps on baths should not be included in the calculation as the water consumption from bath taps is taken account of in the use factor for baths.

d. Baths

  1. Total capacity of the bath to overflow, in litres (excluding displacement, this is already included in the use factor for baths).
  2. Where multiple baths are specified with various capacities, the average must be used as set out in section 2.5.
  3. Jacuzzis are not included in the water efficiency calculator as they are generally not filled on a daily basis and their water consumption over a year is minimal.

e. Dishwashers

  1. Litres per place setting derived from the figures quoted on the EU Energy Label.
  2. Where no dishwasher is to be provided and therefore consumption figures are unknown, a figure of 1.25 litres per place setting must be assumed.
  3. Where multiple dishwashers are specified with various consumptions, the average must be used as set out in section 2.5.