Notes applicable to CHeSS HR2, HC2, HR1, HC1 (year 2000)

The specifications list only the principal components of a heating system affecting energy efficiency. Other components will be required, such as radiators, circulating pumps, cisterns, and motorised valves. All components must be selected and sized correctly.

Heating systems should be designed and installed in accordance with relevant safety regulations, manufacturers' instructions, the Benchmark scheme (Ref 3), Building Regulations (Refs 5,6,7), and British Standards (Refs 9, 11). More detailed advice on domestic wet central heating systems is given in the government's Energy Efficiency Best Practice Programme Good Practice Guides (Refs 1, 2) and reference 14.

A regular boiler does not have the capability to provide domestic hot water directly, though it may do so indirectly via a separate hot water store.

A combination (combi) boiler does have the capability to provide domestic hot water directly, and some models contain an internal hot water store.

A combined primary storage unit (CPSU) is a boiler with a burner that heats a thermal store directly. Each of these may be either a condensing or non-condensing boiler :condensing boilers are always more efficient.

For further definitions of boiler types see appendix D of reference 12

SEDBUK is a measure of the seasonal efficiency of a boiler installed in typical domestic conditions in the UK. The SEDBUK efficiency of most current and obsolete boilers can be found on the website www.boilers.org.uk.

Although SEDBUK is expressed as a percentage, an A to G scale of percentage bands has also been defined in the following table:

A hot water cylinder may be regarded as meeting or exceeding the insulation requirements of the relevant British Standard if the manufacturer confirms that the standing heat loss is not greater than 2.3 x (0.02 x 0.051 V 0.666) kWh per 24 hours, where V is the capacity in litres. This is equivalent to about 1 watt per litre for the popular 117 litre cylinder.

A high performance hot water cylinder may either be vented or unvented. The manufacturer must confirm that the heat exchanger and insulation properties exceed the requirements of the relevant British Standards (Refs 8, 10) as follows:

1. The standing heat loss must not exceed 2.0 x (0.02 x 0.051 V 0.666) kWh per 24 hours, where V is the capacity in litres. This is equivalent to about 0.85 watts per litre for the popular 117 litre cylinder.
2. The heat exchanger surface area must be at least 40% greater, or the performance 40% better, than specified in the relevant standard.
3. For a vented cylinder of nominal capacity 120 litres the time to re-heat from 15 deg.C to 60 deg.C should not exceed 20 minutes, where the primary flow rate is 15 litres per minute at a flow temperature of 82 deg.C. Differently sized vented cylinders should have a re-heat time in proportion to capacity (eg; not more than 10 minutes for a 60 litre cylinder).

A high performance thermal (primary) storage system must have insulation properties exceeding by at least 15% those given in the WMA Performance Specification for Thermal Stores (Ref 4), and comply with the Specification in other respects.

Systems with regular boilers must have separately controlled circuits to the hot water cylinder and radiators, and both circuits must have pumped circulation. Large properties should be divided into zones not exceeding 150 m² floor area, so that heating in each zone can be timed independently.

A time switch is an electrical switch operated by a clock to control either space heating or hot water, or both together but not independently.

A full programmer allows the time settings for space heating and hot water to be fully independent.

A room thermostat measures the air temperature within the building and switches on and off the space heating. A single target temperature may be set by the user.

A programmable room thermostat is a combined time switch and room thermostat which allows the user to set different periods with different target temperatures for space heating, usually in a daily or weekly cycle. Some models also allow time control of hot water, so can replace a full programmer.

A cylinder thermostat measures the temperature of the hot water cylinder and switches on and off the water heating.

A TRV (thermostatic radiator valve) has an air temperature sensor which is used to control the heat output from the radiator by adjusting water flow.

For further definitions and explanations of heating controls see Ref 2.

Wireless controls should be designed with a satisfactory level of immunity to blocking by other radio transmissions. Otherwise they may become unreliable, or cease to work, as nearby radio frequency bands become increasingly heavily used from year 2002 onwards. Products bearing the new "Radiomark" symbol have been certified to meet this requirement concerning quality, fitness for purpose, and traceability (see website www.radiomark.org).

For products not bearing the Radiomark, the manufacturer should confirm that the switching range (and preferably alignment range) do not include any frequencies below 430 MHz, and that in regard to EN 300 220-1 v1.3.1 (July 2000) the receiver classification (clause 4.1.1) is either Class 1 or Class 2, and the device is marked in accordance with clause 4.3.4.

Compliance with the essential requirements under article 3.2 of the EC Radio & Telecommunications Terminal Equipment Directive 1999/5/EC is not sufficient, as the directive is designed only to ensure that wireless products do not cause harmful interference to other transmissions.

Boiler interlock is not a physical device but an arrangement of the system controls (room thermostats, programmable room thermostats, cylinder thermostats, programmers and time switches) so as to ensure that the boiler does not fire when there is no demand for heat. In a system with a combi boiler it can be achieved by fitting a room thermostat.

In a system with a regular boiler it can be achieved by correct wiring interconnection of the room thermostat, cylinder thermostat, and motorised valve(s). It may also be achieved by more advanced controls, such as a boiler energy manager. TRVs alone are not sufficient for boiler interlock.

An automatic bypass valve controls water flow in accordance with the water pressure across it, and is used to maintain a minimum flow rate through the boiler and to limit circulation pressure when alternative water paths are closed.

A bypass circuit must be installed if the boiler manufacturer requires one, or specifies that a minimum flow rate has to be maintained while the boiler is firing. The installed bypass circuit must then include an automatic bypass valve (not a fixed position valve).