Let’s get the facts straight on centralised and decentralised residential ventilation

Essentially, a centralised system refers to one unit ventilating the entire building, while a decentralised system refers to one unit for each apartment or for each room in case of a family house. However, the definition of centralised and decentralised systems also depends on the context and use of products.

In the context of multi-family homes, a centralised system typically refers to big AHUs, which operate as a central unit, providing and distributing air in each flat. In a decentralised system, each flat will have its own individual ventilation system. The advantage of a decentralised system in a multi-family house is that every customer is the master of the indoor air quality (IAQ) in his flat. So they can increase air volume, change the temperature, and decide how often they want to have the filters changed. Or it can be a small unit ventilating only one room.

In the context of a single-family home, a centralised unit ventilates the entire development. A decentralised unit in a single room can be in the form of a push-pull or alternating system.

The benefits of centralised and decentralised systems greatly vary from the perspective of the investor, building maintenance, and the tenants.

For single-family houses, it is always better to have a centralised ventilation system. There are important differences between centralised systems and push pull units.

The instances where a push pull system should be used are rare, however it is not entirely uncommon. This can include existing buildings where a duct system would not be suitable or cannot be installed, in single room flats, such as students’ homes, or in refurbishments of older buildings.

Push pull systems should only be considered as an emergency solution and installed if there is no other option. As it is only a last resort alternative, push pull systems are generally poor solutions owing to the following:

• Disturbing sound level in the rooms from the fan and from the outside. This means that the system is especially disruptive and noisy when used in sleeping rooms, where operation is most critical.

• Bad filtration of the outdoor air because only one filter is used in both directions and a high-class filter would highly reduce the air flow. Thus, the unit needs to be cleaned frequently due to dust inside.

• Highly influenced by wind outside because of the weak fan used inside. Thus, it is not a controlled mechanical ventilation because rooms at the luv side might get only an undefined amount of supply air and the rooms at the lee side only extract. Short circuit of air is also a permanent issue.

• Limited air volumes.

• It needs an outside wall for installation.

• It should never be used along main roads or cities with high traffic and in regions with high air pollution or pollen concentration because of poor filtration.

• Although the product seems to be cheaper at first sight, a proper installation would require several units, generally a pair of them in each room, otherwise only half of the air volume would be available. This will in turn increase overall cost.

• The holes in the outside walls reduce the insulation level of the building.

• The exchanged air is much higher than in other systems.

For single-family houses and new residential buildings, it is always better to have a centralised ventilation system. The use of a central unit for a single-family and multi-family house offers several advantages, such as the following:

• Maintenance of only one unit

• Need to only change two filters for the complete building

• Better and controlled IAQ

• Flexible air exchange rates

• Less air needed due to double use of air

• Higher heat recovery

• Flexibility in placing supply and extract valves

• High comfort due to optimised filtration, sound (silencers) and efficiency, as well as possible air treatment

A centralised system in multi-family homes poses its own disadvantages, such as the following:

• Users in the different flats have limited possibilities to influence the ventilation system.

• Fire protection can be more difficult and more expensive. This is because there will be central ducts going through the building from the central unit, and it will always cross different fire safety areas. Each time it crosses a ceiling or a wall, you must place a fire damper, which can be costly, and will require regular maintenance.

• The need to go into each flat if you must do maintenance.

Centralised vs decentralised

The ideal system for any given project depends on the customers’ priorities. Some investors have decided to use only centralised systems, while others are more in favour of decentralised systems.

Officially, there is no universally right system, however, push pull, decentralised units are not the best option in view of available solutions in the market. The ideal solution is the use of a residential unit especially designed for the application. Such units are suitable for single family houses and for multi-family houses, to be used in each flat or apartment, which will be the most optimum in terms of energy efficiency, IAQ and the comfort of the tenants.

The complexity in the discussion related to the advantages and disadvantages of the different systems underscores the need for a unified standard for residential ventilation systems in Europe and elsewhere. There has been work in this direction, but it can be challenging owing to the different demands and requirements of each country.

Currently, most countries have local standards, which regulates only air exchange rates and, in some cases, the filter class, which are the most important issues. Yet, there is no system standard valid in most countries, only products. The customers develop the system by choosing the unit, ducts, valve, or dealing with it room by room.

Ideally, a regional standard should have a standard calculation of air exchange rate, considering different calculation approaches and different demands of fresh air, either for a person or for an area. Some calculate according to area or living space, some calculate according to number of persons, and some calculate air volume.

What to consider when developing a common standard

In the development of a common standard, there is also a need to align the definition of the required amount of air, which can vary among countries. Once this has been aligned, a decision needs to be made on air distribution for ducts, air velocity and pressure drop. That is partly regulated in the EU Ecodesign Directive. Then the unified standard would have to determine appropriate sound level as well as consider air velocity inside the room to avoid draft.

Filter standards must also be addressed. In some countries it may not be recommended to even have a coarse filter, in others a fine filter is needed. There are many different levels of filtration and this falls under the IAQ aspect of any standard. Likewise, there is a need for greater alignment in terms of demand of power consumption, energy recovery efficiency and comfort (IAQ).

Despite these challenges, there continues to be a proactive move to find a common European solution. This would not only be helpful for manufacturers working in all European countries, in being able to better assess the countries demands and requirements, it will also help raise the quality of air in homes across the region.