Air handling units are often seen as basic, almost commodity, items but, with more thought and a flexible approach, they can be made to work harder for their money, suggests Neil Oliver of Dunham Bush
DESPITE a lot of evidence to the contrary, you still hear people saying things like 'air handling units, there's nothing to them, they're just a box with a fan and a filter'.
Thankfully, though, that attitude is changing as people come to recognise the key role of air handling units and how much more can be achieved with their performance given the right approach.
An open-minded approach is vital in achieving the best from air handling plant, combined with a willingness to 'think outside the box'.
In a project for a new conference facility in Wales, for example, there is a large function space that can be partitioned into six smaller spaces.
The ventilation system, therefore, will need to accommodate very variable levels of occupancy in each of the spaces.
Of course, there's nothing unusual about this scenario or the challenges it presents but the traditional approach of installing small, individuallycontrolled air handlers to serve each space isn't necessarily the best solution.
Apart from higher capital and installation costs, it takes up more plant room space and increases on going maintenance requirements.
Working with consulting engineer R W Gregory, Dunham Bush has been exploring a better solution based on using large, central air handling plant and controlling it in a more flexible way. Achieving this requires high efficiency at low fan speeds and, with the forward curved centrifugal fans conventionally used in air handlers, efficiency falls dramatically at lower speeds.
Maintain efficiencies
For that reason, in a break from tradition, the air handlers will use backward curved or plug fans that maintain efficiencies at all speeds.
As a result, the ventilation rates will be precisely controlled to match the needs of each space, resulting in lower energy consumption by the fans, as well as less air being heated or cooled at times of low occupancy.
And, of course, capital and installation costs will be reduced and less space will be dedicated to plant. It is not rocket science but it does represent how a more elegant solution can be achieved through a more flexible approach.
Another significant change that impinges on the specification of air handlers is the higher level of interest from end-clients, be they building occupiers/operators or developers.
Not so long ago, these people would rarely have given a second thought to the air handling plant. However, with drivers such as Part L of the Building Regulations and the forthcoming Energy Performance of Buildings Directive (EPBD) everyone involved in designing and operating a building is being forced to sit up and take notice.
Part L 2006, for example, places a great deal more emphasis on the air tightness of a building and this impacts on the ventilation in two ways. First, a tighter building needs more effective ventilation because you can no longer rely on unplanned leaks making up the fresh air content.
Secondly, any testing for air tightness needs to encompass the air handling plant and the ductwork as well as the building itself, or the purpose of this regulation is defeated. For this reason, the quality of construction of the air handling units becomes even more important.
In fact, now that 50mm of insulation and elimination of cold bridges have now become standard features of most modern air handlers, the air tightness is probably one of the key differentiating factors between a good unit and a unit that fails to deliver the required energy performance.
While Part L makes its presence felt largely in the stages from design through to commissioning, the EPBD will set ratings for the operational performance of buildings through their life. So building operators who want to achieve and maintain a good energy rating are starting to provide more input at the design stage with a view to influencing the on going energy performance of the plant.
Increased need
Even in a speculative development, where many developers have traditionally gone for the lowest cost option they could get away with, there is now an increasing need to provide prospective tenants with information on the energy performance of the building.
At the same time, building operators are much more aware of their health and safety responsibilities and are very keen to ensure that any plant installed will minimise risk.
In the case of air handing units, risk is minimised by ensuring they are designed for ease of maintenance. They should be easy to clean and the filters should be easy to replace. Such considerations should be obvious, but it's surprising how often these factors are compromised through lack of thought at the design or installation stage.
All of this means that specifiers and installers are likely to find themselves working more closely with their end clients to arrive at a solution that suits everyone's needs. So even the under-appreciated air handling unit can look forward to a higher profile.