Decentralised ventilation requires no false ceiling and means that the height of multi-storey buildings can be reduced. But the benefits don't stop there, says Trox UK's Barry Trewhitt.
For many European specifiers of air handling equipment, decentralised ventilation has provided the optimum solution to the demands of today's new build and refurbishment projects. From prestigious new builds such as Capricorn House in Düsseldorf, to more modest projects, like refurbishment of Franziska-Hager school in Prien, decentralised ventilation has been the technology of choice.

It is easy to see why this might be. Decentralised ventilation requires no central AHU or ductwork and, as it does not require a false ceiling void, the architect can considerably reduce the height (and construction costs) of multi-storey buildings.


Pros and cons

But before moving away from tried and tested alternatives such as fan coils and chilled beams, the system designer needs to study all aspects of the technology. In this article we focus on its key benefits and disadvantages, and highlight factors to be taken into consideration.

It is very common for decentralised ventilation and natural ventilation systems to be confused with one another. So, to clarify, this article discusses mechanical, perimeter systems such as the Trox FSL range, designed to carry out functions such as ventilation, heating, cooling, dehumidification and/or heat recovery.

Key advantages of decentralised ventilation include reduced constructional cost and complexity. There is no requirement for air distribution ducts, or fire protection dampers that would otherwise be fitted in ductwork. Risers are not required either as the building services equipment is typically installed in fascias/perimeters. Footprint/cost for the central plant room is also saved.

By eliminating the need for a false ceiling void, and facilitating a smaller slab to slab height, the overall height of a new building can be reduced whilst maintaining the same lettable floor area. As the construction costs increase linearly with building height, the saving, in the case of an office block of 30 occupied floors, could be approximately 20 per cent compared with centralised systems. See the graph.

The installation of units at the perimeter, e.g. at sill level, also provides maximum flexibility for interior space utilisation.

For commercial buildings with multiple tenants, decentralised ventilation can be shut down in non-occupied spaces. It can also suit zones with different hours of occupation (eg a 24/7 call centre next to an office occupied 8am to 6pm).

Decentralised ventilation can also prove popular with building occupants as individuals have local control and can change temperature easily.

When refurbishing, decentralised ventilation gives the ability to improve air quality on a 'room by room' basis, without making changes to the central AHU.

As pressure on public spending intensifies, budgets for centralised system projects may be withdrawn, whilst the worst-affected parts of a building (eg a classroom, hospital ward or lecture hall) remain in urgent need of attention.

Depending on the age of a building, decentralised ventilation may be the only feasible option for introducing ventilation and comfort cooling in buildings designed without air conditioning in mind.

Self-contained, scaleable, decentralised ventilation often provides a solution. The specific challenges of the technology must be factored in however. Some of these are marginal, or easily overcome, whilst others need serious consideration at design stage.
System designers in the UK have often rejected decentralised technology because the requirement for cooling or heating was too high to be met by these units. But perhaps its time to take another look.

The downward pressure on energy consumption in buildings is likely to drive down building loads, bringing cooling and heating requirements of buildings within levels that are more achievable using decentralised ventilation approaches.

Secondly, the latest generation of units can carry out additional functions and provide greater capability. Traditional units were used for ventilation and heating, but modern units can also deliver cooling, dehumidification, air filtration and/or heat recovery.

In terms of energy consumption, assuming that the air intake for centralised systems is at roof level (compared to decentralised units installed in the façade), the system designer needs to factor in the potential for higher energy requirement for conditioning of the intake air in summer conditions with a decentralised system. This could, however, be compensated by heating energy savings in the winter period.

When considering comfort conditions, it needs to be recognised that, as the supply air discharge is typically sill mounted, the supply air temperatures could lead to levels of discomfort for individuals sat nearest to the units where there are very high cooling capacities. In addition, due to the numerous decentralised ventilation openings in the façade of the building, the effects of the wind pressure on the building cannot be ignored. Control systems for perimeter ventilation units need to avoid the generation of draughts and compensate by shifting the fan operating point.

Trox can help customers investigate these issues at design stage using computer simulation and/or full scale tests in our laboratories.

From the viewpoint of fire protection, whilst fire dampers can be omitted, the system designer still needs to consider the avoidance of fire transmission around the face of the building.

Finally, careful consideration needs to be given to condensation removal, maintenance and control.

As the perimeter ventilation units are not usually integrated into the central building management system, controlling the units will involve higher cost. And distribution of many units around a building can increase maintenance requirements. In addition, removal of condensate is not so simple with decentralised units and will require an individual solution.

A realistic assessment of decentralised ventilation, however, will offset these factors against the significant advantages (already outlined) of greater flexibility for space utilisation, reduced construction complexity, and so on.