The arrival of the new Part L this year and the emphasis it places on energy usage of building services equipment means that fan electrical power is now one of the key factors in plant selection.

What used to be a relatively straightforward process has now become much more complicated, with careful consideration required from the designer regarding the static pressures throughout the air system. However, with the use of the most appropriate fan, the energy savings should be considerable.

In the case of AHUs, fan electrical power is now a maximum of 2.5 W/l/s. The optimum AHU design solution involves the selection of several key factors: air volume, heating/cooling requirements, space availability and noise levels – consequently, the final design will be the product of a complex balancing act involving performance issues, space requirements and capital cost.

In this cost model, the key design criteria are examined, with selection of components to ensure that the air handling unit meets Part L requirements. However, it is worth noting that the comparisons given here refer to the AHUs only, and do not cover the complete ventilation system.