This section sets out the water efficiency calculation methodology for assessing the whole house potable water consumption in the dwellings. The DEAP water efficiency calculation tool is to be used to assess the water use within the dwelling. It is not a design tool for water supply and drainage systems. Behaviour and changing behaviour can also affect the amount of potable water used throughout a home.
Hot water usage in DEAP is reduced by 5% if the dwelling is designed to achieve a water use target of not more than 125 litres per person per day (all water use, hot and cold).
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:
- 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 ‘Water efficiency calculation tool: consumption from multiple fittings’.
- Bidets are excluded from the water efficiency calculator for 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.
- 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 (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 (such as 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 ‘Water efficiency calculation tool: consumption from multiple fittings’.
- 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.
- 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 ‘Water efficiency calculation tool: consumption from multiple fittings’.
- 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.
- Litres per place setting derived from the value quoted on the EU energy label, in other words annual water use divided by (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 ‘Water efficiency calculation tool: consumption from multiple fittings’.
- Litres per kilogram of dry load derived from the value quoted on the EU energy label, in other words annual water use divided by (220 x capacity in kg).
- Where no washing machine is to be provided and therefore consumption figures are unknown, a figure of8.17 litres per kg must be assumed.
- Where multiple washing machines are specified with various consumptions, the average must be used as set out in ‘Water efficiency calculation tool: consumption from multiple fittings’.
- 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 (EN 1112:2008, Sanitary tapware. Shower outlets for sanitary tapware for water supply systems type 1 and 2. General technical specifications).
- Where multiple showers are specified with various flow rates, the average must be used as set out in ‘Water efficiency calculation tool: consumption from multiple fittings’.
- Percentage of total capacity used per regeneration cycle.
- Water consumed per regeneration cycle (litres).
- Average number of regeneration cycles per day.
- Number of occupants (based on two occupants in the first bedroom and one occupant per additional bedroom assuming two occupants in studio flats).
- Water softeners that do not have a water consumption such as electromagnetic types, are not included in the calculation.
- Where present, a standard consumption of 3.08 litres per person per day must be assumed.
- 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
¶ Greywater and rainwater
Where rainwater harvesting and greywater recycling is used as a means of reducing water consumption to achieve higher water efficiency performance levels, the calculation is as follows:
Calculations should be carried out in accordance with standards BS 8525 or EN 16941-2:
- Manufacturer or system designer details on the percentage of used water to be recycled, taking into account the storage capacity of the system.
- The volume of recycled water collected from waste bath, shower and wash hand basin, dishwasher and washing machine usage, with the volume collected calculated in accordance with the DEAP water efficiency calculation tool, available from the SEAI website.
- The consumption of fittings where greywater is to be used in accordance with water efficiency calculation tool which can include WCs and washing machines or Tables 1 and 2 within the tool where greywater is just being used in a proportion of fittings.
Calculation according to the standard includes accounting for the following aspects:
Collection area
Yield co-efficient and hydraulic filter efficiency.
Rainfall (average mm/year)
Daily non-potable water demand
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.