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Designing an HVAC solution for a building is not something that follows a pre-defined process. As with many elements of design, the decisions are complex, and affect things outside of the system being planned. One of the first choices to consider is whether to opt for a centralised or decentralised solution for air handling units. We will deep dive in the decentralised solution in this blog post by letting our expert, Mike Ward, elaborate on the benefits of this alternative.
Let us take a look in the rearview mirror. Back in history, buildings relied on natural ventilation in one way or another and it was not until the early 1900s that ducts where introduced. That as a result of taller buildings being constructed and ventilation no longer being able to depend on a temperature difference between the indoors and the outdoors.
Buildings with lower ceilings were built in the post-war era and that is when the demand for proper ventilation grew rapidly. At this point, mechanical supply and extract ventilation spread quickly across Europe. Ventilation has since then developed vastly and today, most new buildings and renovation projects design complete indoor climate solutions which start with carefully chosen air handling units (AHUs) and end with digital solutions for monitoring, control and predictive maintenance.
Worth to know about the often used centralised solution
AHUs take up floor space and affect a building construction no matter where they are placed. They also require a range of services to be connected to them, power heating and cooling to mention a few.
Where the ideal place is to locate them, depends primarily on the building, but also on the type of space it will serve. If the building is made up of large, open elements, such as open plan offices, then it is often the best choice to go with a central solution, using ductwork to distribute the ventilation to where it is actually needed in the space. However, this solution demands extensive ductwork which obviously has a cost, and the additional expense of installation and commissioning of a large complex system must be accounted for. It is also important to reflect upon the requirement of fan power to move air around the entire property, it implies a continuous energy cost throughout the life of the building.
Worth to know, is that if the plant area is planned for the basement, separate underground floors might have to be designed and constructed, and if planned for the rooftop, the entire construction has to be built to carry the weight – a cost during the building process either way. A rooftop placement in densely populated areas may imply certain casings or other sound attenuation to not cause disturbing noise. But again, this is a highly recommended alternative for certain buildings and indoor environments.
A decentralised solution has other advantages
In the case where the building is more divided, for example in a high-rise building, there is more opportunity to be flexible with the location of the air handling plant. With a decentralised solution, the units can be spread out on different floors throughout a building, or even multiple units can be provided each floor. The latter is a typical example of small separate demises such as apartments. Routes to the building envelope, to access fresh air, is shorter and energy use would likely be lower. Perhaps a side-note to this blog post, but the most decentralised option is where a wall placed air handling unit, positioned within a wall space, serves an apartment. It provides flexible ventilation and heat recovery to an entire dwelling with a minimum of disruption, ductwork and controls. See our BluMartin range for more information.
Going back to the decentralised solution of larger air handling units, and looking beyond the units themselves. A decentralised solution often require smaller duct work which first and foremost is favourable in terms of noise and responsiveness. Ductwork would usually have runs through central areas and risers of a building, when the size of it is reduced, premium central space can be allowed back to the designer and/or real estate owner.
Using Karlatornet in Gothenburg, Sweden, as an example, ventilation was designed with one/two GOLD air handling units to serve approximately 5 floors. When every five floors were ready for installation and commissioning, it was possible to complete the work and enable the real estate owner to do the finishing touches and allow for early move-in. Tenants were allowed to move in while the top part of the envelope was still under construction.
The decentralised solution also works favourably in a “reverse” scenario. Meaning, one ventilated section can be temporarily turned off for service and maintenance, and only five floors at the time are affected, not all 73 levels of the skyscraper.
In this case, the majority of the building has the same purpose – built for residential use. The decentralised solution is then not the game changer it would be, if it was a multi-use property. A building with offices, apartments, hotel space, restaurants and stores has vastly differing needs, demands and use scenarios, and it can undeniably be easier to adjust and finetune the air flow for each requirement with a decentralised ventilation solution.
As indicated, a decentralised solution requires smaller AHUs and smaller duct work which makes it more or less obvious that the construction process can be carried out with smaller measures. Going back to the case of Karlatornet, the two construction elevators were the only means to get people and materials up, and into the building. An HVAC solution of smaller units, ducts and parts was, from a logistics point of view, very welcomed. Small scale also makes things easier and quicker for the installer and technician – both during construction and throughout the many years of operation, service and maintenance.
In summary
The perhaps more known centralised solution for air handling units has a clear set of advantages, and this way of tackling the requirement of ventilation is in many cases the best way forward. However, we have shed light on the decentralised solution as it has a number of obvious benefits too – but in a context of another layout of the indoor space. Make sure to not let the complex decisions set you off. We are familiar with both centralised and decentralised solutions and we can for sure be of help when you are planning your next project. Learn more about Swegon and visit our reference case of Karlatornet here.
