One of the major challenges is that of noise entering the ventilated space, whether the system uses local ventilation units or centralised air handling plant serving a ductwork distribution system. Local air handling units (AHUs) with heat recovery have been shown to offer significant advantages over traditional ductwork systems. However, many specifiers have concerns that the introduction of fans directly to the ventilated space would result in increased noise levels. In fact this needn’t be the case, as there are now decentralised AHUs that are capable of delivering noise levels that are remarkably low.

The principle benefit of moving away from centralised ductwork systems to local, direct solutions is that the units are mounted in the space to be ventilated and connected directly to the outside through the wall or roof. This enables the unit to be very responsive to local changes in IAQ (e.g. a rise in CO₂levels) and deliver trueC. An integral cross-flow heat exchanger recovers at least 84% (dry bulb) of the extracted heat – or 91% of the heat energy at 89% relative humidity.

Getting the noise down

With all of these systems the acoustic design is crucial. In a distributed system attenuation is achieved with acoustic insulation in the central AHU and ductwork; these measures are relatively effective but add to the overall cost of the system.

With decentralised AHUs  noise reduction measures are focused. on the unit itself. In most cases there is no ductwork to insulate, though occasionally there may be a short length of intermediate ducting between the unit and the outside wall. Attenuation of casing breakout noise is achieved by acoustic insulation fitted to the internal casing and positioning of fans on the external side of the heat exchanger.

The result is a casing breakout noise level of no more than 35dB(A) at 1 metre at full throughput, reducing to 30dB(A) 1 metre at 80% fan speed. In addition, attenuation through the unit of outside noise, such as from a road is a hefty  49dB, which means that transmitted noise is effectively insignificant.

When specifying such units it is also important to ensure they use EC type drive motors that will allow modulation to the lowest acceptable fan speed.

It is also worth noting that designing for low noise, even if there is no specific requirement at the time, helps to ‘future proof’ against changes in the use of the space(s). For example, low noise levels are particularly beneficial for teaching children with special educational needs (SEN). However, many children with SEN attend mainstream schools and are taught in general classrooms, so aiming for the best noise levels suits all applications.

Keeping the noise down

Having selected a low noise unit it’s important to ensure that its noise profile be maintained. One problem experienced with many MVHR (mechanical ventilation with heat recovery) units is that they become noisier over time. This is usually the result of deterioration of the heat exchanger caused by condensation and/or premature failure of filters, resulting in higher fan power to overcome the increased air resistance.

This general deterioration in noise levels can be avoided by using units fitted with aluminium heat exchangers that comply with EN308, which are not susceptible to condensation damage. Filters to at least class M5 are essential to keep the heat exchanger surfaces clean, thus maintaining transfer efficiency over the long term.

The best of all worlds

In addition to the noise issues and the ease of response to local IAQ changes, there are many other benefits to using direct AHUs when compared to central plant. By  eliminating the need for complex ductwork systems, a decentralised approach reduces capital and installation costs. As there is no penetration of fire compartments, it is not necessary to install fire dampers or intumescent seals, as would be the case with a traditional ductwork system.

Furthermore, decentralisation helps to avoid the wasted energy associated with over-ventilation. This is because ductwork systems are designed typically to allow for maximum, rather than actual, space usage. Although commissioning is assumed to result in a well-balanced system, this is seldom the case.

Consequently there are compelling reasons for considering the use of decentralised AHUs, as direct units are well equipped to meet real concerns relating to noise, energy efficiency, cost and responsiveness.

Lars Fabricius is Managing Director of SAV Systems