Ian Thomas, Product Manager, TROX UK
In the majority of projects, installing an air handling unit (AHU) provides the central plant upon which the site’s ventilation system can be built. But what if it is not feasible, affordable or desirable to install an AHU? Or what if there is already one in place but, in specific zones across the site, the air quality is dropping below the required levels, resulting in complaints from building occupants?
There are many types of building in which high-levels of occupancy increase the risk of poor air quality, such as schools, offices, care homes and gyms. But in many cases it can be difficult to provide good air quality for these sites with traditional ventilation approaches. If air quality is poor, building occupants tend to introduce their own measures, by opening windows or using low-efficiency fans. This leads to energy wastage and makes it impossible for the energy manager to retain control over the building’s environmental performance.
In these ‘problem spaces’, a decentralised approach to ventilation could provide an effective alternative. Decentralised units work by delivering fresh air, heating, cooling, and heat recovery directly within the room – eliminating the need for long duct runs or central air handling plant. A key benefit of these units is that they are self-contained. They operate by delivering fresh air into the space from the perimeter or fascia of the building, or from the ceiling void, using high-efficiency EC fans. In addition, as the units have integrated cross-flow or rotary heat exchangers, they enable heat from the room air to be used. Heat is recovered via the heat exchangers as the air passes through the units, providing free heating of the air in winter or free cooling in summer.
When deciding whether a decentralised approach is appropriate for the building, the design configurations of the units are an important consideration. As each unit will require access to a fresh air source, it is important to find a range of units offering as many configurations as possible, to ensure that they can be installed in many different physical locations, to suit the site. The TROX range, for example, includes units for vertical or horizontal installation next to windows, for placement under window sills of external walls, or for installation directly under the ceiling or actually fitted into the suspended ceiling. See Figure 1.
Benefits of decentralised ventilation
This approach can deliver a number of benefits. Firstly, air quality levels are measured by the unit itself, enabling fresh air supply to adapt automatically to demands, as levels of occupancy, and types of usage of the room, change during the day. An integrated air quality sensor in the TROX units, for example, calculates and adjusts to demand. Filters (of at least ISO ePM1 60%, formerly F7, in the case of TROX units) reduce fine dust, pollen and pathogens in the room air for better health and hygiene.
Secondly, decentralised ventilation units can minimise planning and loss of space, whilst ranges designed for installation in multiple locations within the space offer greater flexibility and design freedom. All components for filtration, heat recovery, thermaltreatment of the air and control are already integrated in the units. Even though the units are perfectly capable of operating independently, the TROX range for example, can also be connected to the central building management system due to the integrated FSL Control III control, for management and monitoring.
Lastly, there could also be valuable energy efficiency advantages of decentralised ventilation. Heat recovery is available all year round, enabling heating to be reduced during the winter, and creating a cooler indoor environment in warmer weather. Best-in-class units are those incorporating rotary heat exchangers which operate regeneratively. The thermal energy is temporarily stored in the storage mass, a solid, slowly rotating wheel, and then, as the wheel comes into contact with the other airflow, is transferred to this other airflow. This process results in much higher efficiency levels.
To conclude, there are many buildings and spaces for which traditional systems involving AHUs are simply not feasible or desirable, but that does not mean that occupants have to suffer (and complain about) poor levels of air quality.
For more information on either centralised or decentralised ventilation options contact TROX on tel: 01842 754545 or email: sales@troxuk.co.uk
This article appeared in the June 2026 issue of Energy Manager magazine. Subscribe here.
