Section 8: Domestic hot water

  1. Section 8

    Section 8

    8.1 Introduction

    This section gives guidance on specifying domestic hot water (DHW) systems for new and existing buildings to meet relevant energy efficiency requirements in the Building Regulations. It includes guidance on measures, such as additional controls, that can be used to gain heating efficiency credits to improve the heat generator seasonal efficiency.

    Note: Water quality can have a major impact on system efficiency. It is important that designers take appropriate measures to ensure that the system water is of good quality.

    As well as the Building Regulations, other regulations apply to the provision of domestic hot water. Energy-saving measures should not compromise the safety of people or the ability of the system to achieve approved regimes for the control of legionella.

    Domestic hot water systems are referred to as hot water service systems in SBEM.

    8.2 Scope of guidance

    The guidance in this section covers the conventional direct and indirect gas-fired, oil-fired and electrically-heated domestic hot water systems shown in Table 25.
    The recommended minimum standards set out in this section apply only to dedicated water heaters. Central heating boilers which provide space heating and domestic hot water should meet the minimum standards in Section 2; and heat pumps which provide domestic hot water should meet the minimum standards in Section 3.

    The guidance in this section applies to back-up gas and electric systems used with solar thermal hot water systems, but not to solar thermal systems themselves.

    For solar systems with a cylinder capacity of less than 440 litres or collector surface area less than 20 m2, consult the DCLG Domestic Building Services Compliance Guide, or for larger systems, the CIBSE Solar heating design and installation guide.

    ****Table 25   Types of hot water system****

    8.3 Key terms
    The heat generator seasonal efficiency is defined for each system type in Table 26.

    The effective heat generator seasonal efficiency is the heat generator seasonal efficiency plus heating efficiency credits gained by adopting additional measures from Table 31.

    ****Table 26 Definition of heat generator seasonal efficiency for DHW systems****

    8.4 Domestic hot water systems in new and existing buildings

    Domestic hot water systems in new and existing buildings should meet the recommended minimum standards for:

    a. heat losses from DHW storage vessels in Table 27, or maintenance consumption values in BS EN 89:2000

    b. heat generator efficiency (gross calorific value) in Table 28

    c. controls in Tables 29 and 30.

    ****Table 27  Recommended maximum heat losses from DHW storage   vessels****

    ****Table 28   Recommended minimum thermal efficiencies for domestic hot water    systems****

    ****Table 29 Recommended minimum controls package for gas and oil-fired domestic hot****

    ****Table 30 Recommended minimum controls package for electrically-heated domestic hot water systems****

    8.5 Supplementary information on electric water heaters

     ****Table****

    8.6 Heating efficiency credits for domestic hot water systems in new and existing buildings

    Heating efficiency credits are available for gas and oil-fired systems for the additional measures listed in Table 31. If these measures are adopted, heating efficiency credits can be added to the heat generator seasonal efficiency to give the effective heat generator seasonal efficiency, which is the value entered into the accredited NCM tool in order to calculate the carbon dioxide emission rate for a building.

    Effective heat generator seasonal efficiencyheat generator seasonal efficiencyheating efficiency credits where the heat generator seasonal efficiency is:

    • the thermal efficiency for direct-fired systems

    • the boiler seasonal efficiency for indirect-fired systems.

    ****Table 31 Heating efficiency credits for additional measures applicable to domestic hot water systems****

    Example: Using heating efficiency credits to improve the heat generator seasonal efficiency for a direct- fired circulator system

    Step 1: Calculating thermal efficiency of direct-fired DHW system

    • recovery rate of heater0.3830 litres/s
    • gross input rate of heater83.5 kW
    • specific heat capacity of water4.186 kJ/(kg.K)
    • temperature rise of water inside heater50°C. Using Equation 9:

    Heater outputrecovery rate of heater in litres/secondspecific heat capacity of watertemperature rise of the water

    0.38304.1865080.16 kW

    Using Equation 8:

    Thermal efficiency (gross)output of the heater / gross input
    80.16 / 83.50.96 (96%)

    Step 2: Adding heating efficiency credits for additional measures

    The heater has been sized to closely match the system demand by using the manufacturer’s sizing guide and has fully automatic controls.

    Table 32 shows how credits would be assigned in this example.

    ****Table 32 Example to illustrate allocation of heating efficiency credits for a DHW system****

    Effective heat generator seasonal efficiencythermal efficiency (gross)heating efficiency credits

    962.598.5%

    In this example, the value 98.5% would be entered in the NCM tool.