The water efficiency calculation methodology
A1
This appendix sets out the water efficiency calculation methodology for assessing the whole house potable water consumption in new dwellings. The calculation methodology is to be used to assess compliance against the water performance targets in Regulation 36 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.
Performance target/Maximum calculated consumption of potable water (litres/person/day)
Regulation 36 para (2)a/125
Regulation 36 optional requirement para (2)b/110
A2
The calculation methodology 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
Flushing capacity for the WC suite including consumption at full and part flush for dual flush WCs.
Where multiple WCs are specified with various flushing capacities, the average effective flushing volume must be used as set out in paragraphs A8 and A11.
b. Bidets
- 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
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.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 paragraphs A8 to A10.
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.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
Total capacity of the bath to overflow, in litres (excluding displacement, this is already included in the use factor for baths).
Where multiple baths are specified with various capacities, the average must be used as set out in paragraphs A8 to A10.
Spa hot tubs 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
Litres per place setting derived from the value quoted on the EU Energy Label, i.e. annual water use / (280 x number of place settings).
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.
Where multiple dishwashers are specified with various consumptions, the average must be used as set out in paragraphs A8 to A10.
f. Washing machines
- Litres per kilogram of dry load derived from the value quoted on the EU Energy Label, i.e. annual water use / (220 x capacity in kg).
ii. Where no washing machine is to be provided and therefore consumption figures are unknown, a figure of 8.17 litres per kilogram must be assumed.
iii. Where multiple washing machines are specified with various consumptions, the average must be used as set out in paragraphs A8 to A10.
g. Showers
i. Flow rate of each shower at the outlet using cold water (T 30˚C), in litres per minute measured at a dynamic pressure of 3±0.2 bar (0.3±0.02 MPa) for high pressure (Type 1) supply systems, or at a dynamic pressure of 0.1±0.05 bar (0.01±0.005 MPa) for low pressure (Type 2) supply systems (BS EN 1112:2008, Sanitary tapware. Shower outlets for sanitary tapware for water supply systems type 1 and 2. General technical specifications).
ii. Where multiple showers are specified with various flow rates, the average must be used as set out in paragraphs A8 to A10.
h. Water softeners (where present)
i. Percentage of total capacity used per regeneration cycle.
ii. Water consumed per regeneration cycle (litres).
iii. Average number of regeneration cycles per day.
iv. Number of occupants (based on two occupants in the first bedroom and one occupant per additional bedroom
assuming two occupants in studio flats).
v. Water softeners that do not have a water consumption such as electromagnetic types, are not included in the calculation.
i. Waste disposal units (where present)
ii. Where present, a standard consumption of 3.08 litres per person per day must be assumed.
j. External taps
i. Flow rates of external taps are not included in the calculation as a fixed allowance of five litres per person per day is assumed for external water use. A3 In some cases rainwater harvesting and greywater recycling may be used as a means of reducing water consumption to achieve higher water efficiency performance levels. This may be needed where options for improving the efficiency of terminal fittings (taps, WCs etc.) have been maximised and further savings are still needed:
a. Greywater (in accordance with BS 8525)
i. Manufacturer or system designer details on the percentage of used water to be recycled, taking into account the storage capacity of the system.
ii. The volume of recycled water collected from waste bath, shower and washhand basin, dishwasher and washing machine usage, with the volume collected calculated in accordance with Table A1 or Tables A4.3, A4.4 and A4.5.
iii. The consumption of fittings where greywater is to be used in accordance with Table A1 which can include WCs and
washing machines or Tables A4.1 and A4.2 where greywater is just being used in a proportion of fittings.
b. Rainwater (in accordance with BS 8515)
i. Collection area
ii. Yield co-efficient and hydraulic filter efficiency
iii. Rainfall (average mm/year)
iv. Daily non-potable water demand A4 Large water consuming installations such as swimming pools and spa hot tubs where the water is replaced over a greater time interval do not need to be included as part of the water calculations.
Calculation tables
A5 Figures from manufacturers’ product details should be entered into Table A1 to calculate the consumption of each fitting in litres per person per day. Where there are multiple fittings of the same type that have various flow rates or capacities (e.g. hot and cold taps with different flow rates), Tables A2.1 to A2.7 should be used to determine the average flow rate or capacity of such fittings. The consumption of water softeners in litres per person per day is calculated using Table A3. All values throughout the water efficiency calculator should be rounded to two decimal places with the exception of the total water consumption figures, which should be rounded to one decimal place.
A6 The total calculated use, resulting from Table A1, is the total consumption of all water consuming fittings per person. To calculate the litres of water consumed per person per day, any savings from grey or rainwater need to be deducted from the total calculated use using figures from Tables A4.6 and A5.5. The litres/person/day figure is then multiplied by a normalisation factor to determine the total water consumption per person.
A7 To calculate the total water consumption, an additional allowance for external water use is added on to the total water consumption. This figure is set at 5 litres/person/day.
Table A1: The water efficiency calculator
Consumption from multiple fittings
A8 Where terminal fittings with varying flow rates and capacities are specified (e.g. hot and cold taps with different flow rates, two types of shower etc.), the average consumption should be calculated as set out in Tables A2.1 to A2.7:
a) Enter the full flow rate or volume of each type of fitting into column (a) of the relevant table.
b) For taps, where there are separate hot and cold water taps, the flow rate of each tap should be entered separately as two tap types to calculate the average flow rate.
c) Calculate the total consumption per fitting type.
d) Calculate the average flow rate/volume of the fittings detailed.
e) Enter the flow rate/volume of the fitting with the highest flow rate/volume into box (f) with the exception of WCs, where this step is not relevant.
f) Calculate the proportionate flow rate/volume by multiplying the highest flow rate/volume by a factor of 0.7 with the exception of WCs, where this step is not relevant.
A9 Where the average flow rate/volume is lower than the proportionate flow rate/volume, the proportionate figure must be entered into Table A1. The proportionate figure limits the flow rate/volume that can be specified to a proportion equal to 70 per cent of the highest flow rate/volume. This reduces the benefit of specifying ultra low fittings to bring the average flow rate/volume down, where such ultra low fittings may not be acceptable to dwellings occupants.
A10 The figure which is the greater of the average or proportionate flow rate/volume should be used. This is so that, where the average flow rate/volume is significantly lower than the highest flow rate/volume specified, the calculation sets a limitation for what figure can be assumed.
Table A2.1: Consumption calculator for multiple taps (excluding kitchen sink taps)
Table A2.2: Consumption calculator for multiple baths
Table A2.3: Consumption calculator for multiple taps (kitchen/utility room sink)
Table A2.4: Consumption calculator for multiple dishwashers
Table A2.5: Consumption calculator for multiple washing machines
Table A2.6: Consumption calculator for multiple showers
Table A2.7: Consumption calculator for multiple WCs
A11 Where more than one type of WC is provided, the average effective flushing volume is calculated using Table A2.7 below. The average effective flush volume should then be entered into Table A1 in the row ‘WCs (multiple fittings)’.
Ion exchange water softener
A12 Ion exchange water softeners use water in order to clean the resin that is used to absorb the mineral content of the dwelling’s water supply. This cleaning process is referred to as the regeneration cycle, which occurs on a frequency dependent on the type of water softener specified and the hardness of the water. The water efficiency calculator looks at the water consumed per regeneration cycle that is beyond a level of good practice. The good practice level has been determined at a level of water consumption as a percentage of the water softener’s total capacity which is set at 4 per cent.
A13 The figure entered into the calculator is the volume of water consumed beyond this level of good practice to promote the use of more efficient water softeners. Where the water softener achieves a percentage that is equal to, or lower than this good practice benchmark figure, zero can be entered into Table A1 of the calculator for water softeners. The following formula is used to determine the litres of water consumed per person per day that is beyond the good practice level of 4 per cent.
A14 Litres of water consumed per person per day beyond the 4 per cent good practice level:
= [1 – (4 / (a))] ((b) (c))
Where:
(a) = % of total capacity* used per regeneration
(b) = Litres of water consumed per regeneration
(c) = Average number of regeneration cycles per day
*the total capacity is the volume of water that flows through the water softener between regeneration cycles. This volume is dependent on the hardness of the water and the total capacity used in this calculation needs to reflect the hardness of water specific to the geographic location of the specific development. This figure should be determined from manufacturer’s product details.
A15 To calculate the litres of water consumed per person per day beyond the 4 per cent good practice level, enter details of the water softener into Table A3. Where the result indicates zero or a negative figure, zero should be entered into Table A1 for water softeners. The number of occupants entered into the table should be based on two in the first bedroom and one in each additional room. Studio flats should assume for two occupants.
Table A3: Water softener consumption calculation
Table A4.1: Greywater demand calculations – WCs
Table A4.2: Greywater demand calculations – washing machines
Greywater calculations
Greywater demand calculation
A16 Where all WCs and/or washing machines are being supplied with greywater, the consumption values should be copied from Column 4 of Table A1 and entered into Table A4.6 to calculate the greywater savings.
A17 Where greywater is only being supplied to a proportion of fittings such as just to one WC or washing machine, the proportion is calculated by entering details into Tables A4.1 and A4.2.
Greywater collection calculations
A18 Where greywater is to be collected from all fittings including the shower, bath and wash hand basin taps, the total water consumption of the fittings calculated in Table A1 represents the total greywater collected, the sum of the consumption figures for fittings from which greywater is collected (from column 4 of Table A1) should be entered into Table A4.6. Where greywater is only being collected from a proportion of fittings, such as just some of the taps, the calculations in Tables A4.3 to A4.5 should be followed and the results entered into Table A4.6.
Greywater savings calculations
A19 Where greywater is to be reused within the dwelling, the savings from greywater can be calculated by entering the following details into Table A4.6:
a) Calculate the water to be recycled from Table A1 and/or using the method set out in section A18 where just a proportion of fittings are being collected from.
b) Determine the percentage of greywater collected to be recycled based upon manufacturer or system designer details of the system specified.
c) Determine the water demand of the fittings to be provided with greywater which can include WCs and washing machines depending on the quality of the treated water. This is determined from the WC and washing machine consumption from Table A1 or Tables A4.1 and A4.2 in paragraphs A16 and A17.
d) Multiply the volume of water to be recycled with the percentage of recycled water (determined in b. above) which will determine the actual volume of greywater available.
Where the greywater supply is greater than the demand, the greywater savings are equal to the demand. Where the demand is greater than the greywater supply, the savings are equal to the supply.
e) Enter the greywater saving figure from Table A4.6 into Table A1.
Table A4.3: Greywater collection calculations – taps
Table A4.4: Greywater collection calculations – showers
Table A4.5: Greywater collection calculations – baths
Table A4.6: Greywater collection calculation
A20 Where a communal greywater system is to be provided supplying more than one home, Tables A4.1 to A4.5 can be used in the same way. The figures entered into Table A4.6 need to be entered on an individual dwelling basis and not using figures to reflect the communal system as a whole. The percentage collected figure will, however, need to be based on manufacturer or system designer details of the communal system specified.
Rainwater calculations
Rainwater collection calculations
A21 Where rainwater is to be used, the following calculation method should be followed by entering the relevant details into Table A5.1 or Table A5.2 to calculate the rainwater collection volume.
A22 For Table A5.1 using the intermediate approach from BS 8515:
a) Calculate the volume of water collected using the collection area, yield coefficient and hydraulic filter efficiency and average rainfall with guidance from BS 8515.
b) Calculate the daily rainwater collection in box (d) using the collection area, yield coefficient, hydraulic filter efficiency and rainfall.
c) Enter the number of occupants into box (e), which can be based on two occupants in the first bedroom and one occupant in each additional bedroom. A studio flat should assume two occupants.
d) Where a communal rainwater system is to be provided supplying more than one home, Table A5.1 can be used in the same way calculating the total volume collected for the communal system and dividing it by the total number of occupants served by the system. This figure should then be entered in Table A5.5.
A23 For Table A5.2 using the detailed approach as described in BS 8515, enter details of the total daily rainwater collection (litres) and the number of occupants to calculate the daily rainwater per person (litres) and enter into
Table A5.5.
A24 The calculation detailed above in Table A5.2 is sufficient for evaluating the principles of the proposed system in the proposed development. However, for sizing of storage capacity and all other design and installation details, BS 8515 should be followed.
Rainwater demand calculations
A25 Where all WCs and/or washing machines are being supplied with rainwater, the consumption should be taken from Table A1 and entered into Table A5.5 to calculate the rainwater savings.
A26 Where rainwater is only being supplied to a proportion of fittings, such as just to one WC or washing machine, the proportion is calculated using Table A5.3 and A5.4. This rainwater demand can then be entered into Table A5.5 to calculate the rainwater savings.
Rainwater saving calculations
A27 Enter the total volume of rainwater collected per person per day from Table A5.1 or Table A5.2 depending on the BS 8515 approach followed. Enter the total consumption of fittings using rainwater (demand) from column 4 of Table A1, where rainwater is to be used in all WCs and/or washing machines. Where rainwater is only being used in a proportion of fittings, enter the total demand of WCs and washing machines from Table A5.3 and Table A5.4. This figure should then be entered into Table A1 to calculate the internal water consumption.
Table A5.1: Rainwater collection calculation – BS 8515 intermediate approach
Table A5.2: Rainwater collection calculation – BS 8515 detailed approach
Table A5.3: Rainwater demand calculations – WCs
Table A5.4: Rainwater demand calculations – washing machines
Table A5.5: Rainwater saving calculations for new dwellings
Fittings approach
A28 The fittings approach given in G2 uses the methodology described in this appendix to calculate the water consumption of ranges of fittings that meet the performance targets.
