Low water-usage calculation
The demand for domestic hot water (DHW) is based on the SAP methodology (SAP 10). It is derived from the dwelling floor area and calculated taking account of heating, storage, and distribution losses as well as several other parameters such as:
- Number of electric showers (using electricity directly rather than from the water heating system);
- Mixer showers, fed from the water heating system;
- The presence of flow restrictors on the mixer shower. Flow restrictors1 must be permanently fitted (in other words cannot be removed without the use of tools).
- Is the mixer shower unvented?
- Is the mixer shower vented (and does it have a pump)?
- Is the flow rate (L/min) known for the mixer shower2?
- Is there an instantaneous wastewater heat recovery unit (WWHR) listed on the SAP PCDB fitted to the mixer shower? The WWHR utilisation factor and efficiency are entered based on PCDB 2012 figures.
- Instantaneous WWHR systems use a heat exchanger to recover heat from waste warm water as it flows through the waste plumbing system to pre-heat the cold-water feed of a shower and a combi boiler or mains pressure hot water system (thermal store or unvented cylinder). The calculation in DEAP deals with instantaneous WWHRS linked to thermostatic mixer showers (where the shower water is a mixture of cold feed and that from the combi boiler or mains pressure hot water system). It is assumed that the WWHRS is located inside the heated envelope. A WWHRS linked to an instantaneous electric shower (IES) is not covered by this procedure.
- The presence of one or more baths. Bath taps fitted with temporary/portable shower head that is not fixed in place, is entered as a bath and not as a shower in DEAP. Bath taps fitted with fixed/permanent shower head is entered as a bath and shower in DEAP.
- Water usage (hot and cold) per person, per day. The methodology to determine the water usage in the dwelling is detailed in water efficiency calculation section.
Consequent heat gains from storage cylinders (within the heated space), distribution pipe work, electric showers and combi boilers are estimated and used in the calculation of space heating requirements. A distinction is made between instantaneous water heating and water heating that relies on hot water storage in a cylinder, tank, or thermal store. Primary and cylinder losses are not used for instantaneous heaters.
1 Flow restrictors may also be identified on the Unified Water Label Association product database. (This was formerly known as the European Water Label Database).
2 (i). Flow rate of each shower at the outlet using cold water 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).
(ii) A minimum of 6L/Min is applied by DEAP to all mixer showers.