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Office buildings can be treated by means of the whole-office carbon performance rating method. In principle this is the same as the CPR method for assessing the ACMV systems of office buildings (described in section 3.3.2.13.1 above), and which is part of the elemental method. However, when used as part of the elemental method, the CPR calculation deals only with the building's ACMV systems, whereas in the whole-building method, the CPR calculation is expanded to include lighting and space heating. Full details are to be found in BRE Digest No. 457 [44]. There are several assumptions implicit in using the method:

• The proposed design of the building and its energy consuming systems are capable of creating internal environmental conditions which are normal and acceptable for the occupants and the functional requirements of the building. Otherwise, it would be possible to meet the relevant criteria by deliberately undersizing the installed equipment.

• The design of the ACMV, space heating and lighting systems are conventional and do not require the use of novel equipment. This means that the heating system is likely to be supplied from a conventional boiler, and that cooling will be provided by conventional refrigeration plant.

• The CPR method applies only to the energy consuming systems themselves. Therefore, while it provides the flexibility for adjustment between the energies consumed by the ACMV, the heating system and the lighting system, the method is not intended to provide an excuse for poor fabric design.

• The CPR calculation is intended to be a relatively simple method for assessing a building and its systems for compliance. If certain features of the design are likely to make the calculation unduly complex, one of the alternative methods should be used.

The question of whether or not particular equipment may be considered conventional and within the scope of the method must be carefully considered. In some cases, it depends on the detail:

• Heat recovery can be included if the effect is to reduce the size of the heat generator. It can also be included, though not so easily, if the effect is to reduce the hours of use of the heat generator.

• Thermal storage can be included for ice thermal storage which is part of a cooling/refrigeration system. For thermal stores whose purpose is to even out the peaks in demand on heat generators, the CPR method is not recommended.

• Space heating using heat pumps is possible but not advised. The designer could not use the factors given as part of the method, and would have to make his own calculation of the carbon emissions.

• Combined heat and power is also possible but not advised. The calculation of the carbon emissions could be excessively complex.

Renewable energy is possible in some cases. The recommended technique is to calculate the carbon emissions assuming all the energy is supplied from conventional sources, and then to calculate the saving due to the renewable energy source. The amount saved is then subtracted from the initial result.

The calculation procedure is described in Chapter 12. When the CPR value of an office building has been found, it can be considered to comply if it meets all the following:

• A whole-office CPR which is no worse than the relevant maximum in Table 3.10

• The requirements for avoiding thermal bridging at junctions and around openings (as described previously)

• The requirements for meeting air leakage standards

• The upper limits for U-values, i.e for parts of a roof U < 0.35 W/m2K, and for parts of an exposed wall or floor U < 0.70 W/m2K.

Table 3.10 Maximum whole-office CPR.

Office building type

Maximum allowable CPR, kgC/m2/year

New office

Refurbished office

Naturally ventilated

7.1

7.8

Mechanically ventilated

10.0

11.0

Air-conditioned

18.5

20.4

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