Part L2 Additional information


The carbon emissions calculation methodology required when generating the BER, TER and the compliance checklist, is implemented in the Simplified Building Energy Model (SBEM) with its user interface - iSBEM – freely obtainable from the National Calculation Methodology web site. Local authorities are authorised to accept calculations only from persons deemed by Building Control Bodies as suitably competent. Several dynamic thermal modelling software tools have also been approved for the purpose of assessment and are commercially available, and offer additional functionality.

Note that it is required to provide Building Control with the design based TER / BER calculations, together with details of the building specifications upon which they have been based, prior to the commencement of construction works.

At the end of construction, calculations based on the completed design, and details of changes in construction method or specification must also be submitted.



Because of the large amount of technical data and information that is now required to prove compliance with the Building Regulations, and the relative inflexibility of the Approved document as a design tool, it has become necessary to refer to additional documentation for much of the technical detail. These associated documents again offer guidance on requirements without necessarily providing absolute limits of performance. In many cases, reference is made to a wide variety of relevant harmonised European technical standards that exist to support the important EU directives such as the Energy performance in Buildings Directive, and the Energy Using / Energy Related Products Directive.

For building services products, the Non Domestic Building Services Compliance Guide describes performance standards for the majority of equipment types that are encountered in current designs, including heating, cooling, hot water, pipe and duct insulation, lighting, pumping and, of most relevance to Nuaire products – Section 10: Air Distribution systems.

This guidance considers the following system types :-

Central air conditioning systems, central mechanical ventilation systems with heating, cooling or heat recovery, all “other” central systems, zonal supply and extract systems where the fan is remote from the zone and ducting is required – for example in a ceiling void or on the roof, and local systems serving one room directly (un-ducted) such as a window mounted fans.

The principal means of efficiency regulation for ventilation systems is the use of limiting Specific Fan Power (SFP).

As a straight-forward, easily calculated location and duty specific measure, this single figure reference effectively combines the actual efficiency rating of the product, and the adequacy of the actual system design. Calculated as the mains delivered power (W) divided by the system delivered airflow (l/s), the guide provides

SFP limits for each system type. The variations shown are related to assumptions made about the efficiencies of the available impellers / motors / drives employed, and the aerodynamic resistances of the internal components –   for example coils and filters – within air handling units, and of the distribution ductwork etc.

Additional SFP allowances are given for various system elements. Some of the application classes defined are extremely tightly specified in this regard, and this leads to a situation where fan types that may have practical benefits (for example in acoustic properties) cannot be used.

There are practical limits to the efficiency of commercially available components, and this will ultimately constrain the types of distribution systems that may be used in terms of the operational system resistance.

The latest edition of the Scottish Technical handbook Section 6, includes limiting values for external system resistance (this was also planned for the England and Wales version, but deleted late on in the process). The guide very clearly states (10.3 Key Terms) that the specific fan power of an air distribution system should be calculated according to the procedure given in BS EN 13779 2007 Annex D:- Ventilation for Non-residentialbuildings – Performance requirements for ventilation and room conditioning systems.

This important and informative standard describes in Annex D: - the SFP value for the entire building, defined as the weighted average of the SFPE values of individual units and fans. For variable air volume flow systems, the SFPE shall be defined at a partial operating air flow and related external pressure drop - specified for the fan or air handling unit.  If the operating profile (of flow rate with respect to time) of the unit is known, then this can be used to establish the partial flow rate, and if not known, then a default value of 65% of design maximum is suggested as “a realistic mean annual value” for normal comfort ventilation.

Additional design criteria are defined for the equipment: - for SFP at 25% of maximum duty to be no more than that at full speed; provision of variable speed drives for fans with motors of over 1100 W; and for the enclosure leakage grade to be L2 as defined in BS EN 1886:2007 Ventilation for buildings – Airhandling units - Mechanical performance.

Ducting leakage standards are also defined. Minimum control packages are suggested for each system type, and it is stated that Heat recovery devices should be included where supply and extract ventilation systems include  heating and/or cooling.