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Professional development

The CIBSE Journal CPD Programme

Members of the Chartered Institution of Building Services Engineers (CIBSE) and other professional bodies are required to maintain their professional competence throughout their careers.

Continuing professional development (CPD) means the systematic maintenance, improvement and broadening of your knowledge and skills, and is therefore a long-term commitment to enhancing your competence. CPD is a requirement of both CIBSE and the Register of the Engineering Council (UK).

CIBSE Journal is pleased to offer this module in its CPD programme. The programme is free and can be used by any reader. This module will help you to meet CIBSE’s requirement for CPD. It will equally assist members of other institutions, who should record CPD activities in accordance with their institution’s guidance.

Simply study the module and complete the questionnaire on the final page, following the instructions for its submission. Modules will be available online at while the information they contain remains current.

You can also complete the questionnaire online, and receive your results by return email. The performance of fans in ducted air systems

This module considers how the efficiency of fans in ducted air systems can be compromised by the ‘system effect’

When a ducted air system is designed, the air flow rate and pressure drop is calculated and a fan is selected to match the system requirements. The calculated design fan power must not exceed the requirements of the local regulations; however, this calculated power may not reflect the actual power consumed when the fan is installed. (This module is part of a series of CIBSE

Journal CPD articles (August, October, November and December 2011) that are all available from Please note that in the series we have variously

used both ‘Q’ and ‘qv’ to denote air volume flowrate – preferably, ‘qv’ should be used.)

Regulation and standardisation of fan power for buildings The Building Regulations1

Specific fan power W/(litre·s-1 Ventilation system type

Central mechanical ventilation system including heating and cooling Central mechanical ventilation system including heating only All other central mechanical ventilation systems

2.2 1.6 1.8


New buildings Existing buildings 1.8 1.6 1.4

Additional allowances are added for heat recovery and other specialist components

Table 1: Maximum specific fan powers in air distribution systems in buildings to comply with England and Wales Building Regulations 2010

applications (the full document also includes values for distributed fan-powered systems). These required values are far lower than those traditionally found in buildings2

and in the UK limit

the installed power that may be consumed by fans in ventilation systems. In an attempt to capture the associated parasitic power consumption, the limits are given in terms of specific fan power (SFP). The SFP of an air distribution system is the sum of the design watts of the system supply and extract fans, including losses through switchgear and controls (such as inverters), divided by the design air flow rate through that system, and has the unit of W/(litre·s-1

). Table 1 provides an extract relating to the

requirements for centralised fan systems from the regulations for non-domestic

so cannot simply be considered ‘business as normal’. Meeting the requirement in the Building Regulations is dependent on both the design of the distribution network and the choice of the fan (and its associated motor and controls). The underlying British Standard, BS EN13779:2007 Ventilation for non-residential buildings – Performance requirements for ventilation and room- conditioning systems, provides extensive and accessible information on the determination of SFP. The design of the ducting, dampers, diffusers and so on is, of course, the primary route to reducing excessive energy use (as discussed in the October 2011 CPD in CIBSE Journal). However, without applying the most effective fan technology, those efforts to reduce air power used in the system will be somewhat wasted. In terms of whole life cost (and total life carbon emissions), there is

a limit to how low the SFP can go before the capital cost of the equipment outweighs the operational savings in lower pressure drops – this is discussed at length in CIBSE TM30 Improve life cycle performance of mechanical ventilation systems. The first phase of the EU ecodesign requirements for fans3

(the ErP directive),

which will be implemented in January 2013 (with more stringent requirements following in 2015), provides a standardised methodology to determine a fan’s expected performance relative to benchmarks, and specifies limiting minimum fan efficiencies based on the type of fan. This includes the fan sizes that would be typical in HVAC applications (with an electric input power between 125 W and 500 kW). The development of these requirements has also encouraged the creation of ISO 12759:2010 – Efficiency Classification for Fans, which categorises the performance of fans into banded efficiency ratings (Fan Efficiency Grade), with the aim of providing

February 2012 CIBSE Journal 57

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