AIR HANDLING UNITS A
s buildings grow in complexity, so does the need for intelligent and adaptable HVAC strategies. One of the core design decisions
for any building’s ventilation system is which of these two options to implement. A tale of two air handling units It’s important to start with understanding the
difference between centralised and decentralised solutions. A centralised AHU system can comprise of a singular unit that conditions air for the entire building or in some circumstances multiple units which each look after large areas or multiple floors. The conditioned air is then distributed through an extensive network of ducts to the various zones. In contrast decentralised systems use multiple
smaller AHUs or even Mechanical Ventilation Heat Recovery Units (MVHRs) which are located throughout the building and focus on specific areas. For example, each office within a building may have its own unit. This approach can necessitate additional utility connections and upkeep at multiple locations, but it also provides certain advantages.
Key considerations for system selection
So, what are the key points to consider when selecting an AHU system? Put simply, the solution should be informed by the building’s size, use-case, occupancy patterns, retrofit potential, and energy goals. But it’s critical to also consider:
1. Budget
Cost is often a primary factor in choosing between centralised and decentralised AHU systems. Centralised systems tend to offer lower upfront capital costs in large-scale new builds because a single or limited number of units can service the entire facility. Economies of scale, reduced duplication of other plant, and centralised control infrastructure all contribute to lower overall system costs. However, decentralised systems, while typically more expensive initially due to the higher number of units and localised controls, may be more cost-effective in retrofit scenarios or smaller buildings where running extensive ductwork would be prohibitively complex or expensive. Additionally, the ability to scale decentralised systems gradually may offer more flexibility in managing capital expenditure over time.
2. Energy efficiency Energy efficiency is growing in importance thanks to environmental concerns and Government legislation. Decentralised systems often provide better energy efficiency in buildings with varied usage, as individual zones can be set to operate only when needed, minimising unnecessary energy consumption. This zoning capability is especially beneficial in facilities with fluctuating occupancy, such as schools, offices, or healthcare environments. On the other hand, centralised systems when paired with modern building management systems and variable speed drives can offer precise control across the whole building, and this can be beneficial in always on
14 September 2025 Decentralised AHU
environments. However, the overall energy profile of either approach must consider duct losses, system cycling, and the ability to recover waste heat.
3. Flexibility If flexibility is a major driver, then decentralised AHU systems are often the preferred route. We’ve seen this especially in dynamic or multi-use buildings, where the system allows each zone or area to be controlled independently, allowing facility managers to fine-tune indoor air quality, temperature, and ventilation based on the occupiers and the work which is being undertaken in a facility. This is particularly advantageous in hospitals or research facilities where some spaces may have specific environmental requirements.
4. Maintenance: Similar to budgetary considerations, maintenance is a deciding factor for facilities teams. While both centralised and decentralised systems need maintaining the specific requirements vary significantly between the two system types. Centralised systems consolidate equipment into a single or minimum number of locations, making it easier for maintenance personnel to access and service components. Routine tasks like filter changes, motor inspections, or control diagnostics are streamlined due to equipment proximity and standardised components. Conversely, decentralised systems disperse these tasks across multiple locations, which can be time-consuming and harder to manage, especially in larger facilities. However, with modern building management systems and
Right: Centralised AHU
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Choosing the right air handling strategy
Mathew Hopwood, sales director at Mansfield Pollard outlines the critical differences between centralised or decentralised air handling units (AHUs) and explores the key considerations which should inform your choice between them
smart maintenance protocols this can be mitigated. It is important therefore to consider how you will approach maintaining the equipment to ensure that you see the benefits of a decentralised solution, and not just potential higher operational costs. 5. Environmental considerations: Environmental considerations are key when picking a unit, especially if the occupiers require a quiet or silent operation. In these instances, centralised AHUs, located away from occupied areas, often on rooftops or in mechanical plant rooms, offer a natural advantage in minimising acoustic disruption. This can be further improved through duct silencers and acoustic insulation. In contrast, decentralised units installed within or near occupied spaces can pose acoustic challenges. New isolation and noise attenuation solutions are continually being developed but it’s crucial that you understand the acoustic performance of any equipment and build acoustic design into your project.
6. Ductwork:
Ductwork is a defining difference between centralised and decentralised systems. Centralised AHUs require an extensive duct network to distribute conditioned air throughout the building, which can take up significant ceiling or wall space and may restrict architectural design options. This infrastructure can also add to installation costs and
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