Housing ventilation – central or decentralised?

Fan picture: © nikkytok / Shutterstock.com

Both central and decentralised systems ensure a consistently good indoor environment. In addition, they prevent mould formation and moisture damage by removing moisture. Systems with heat recovery also enable efficient heat utilisation. This saves homeowners heating costs on the one hand and reduces the burden on the environment on the other. However, there are some differences between central and decentralised ventilation systems. 

Design and function as important distinguishing features

When comparing central vs. decentralised ventilation systems, a distinction must first be made between design and function. This is because both features influence the area of application and are therefore also important criteria for homeowners.

Ventilation of the building via an air distribution system

A central ventilation system consists of a ventilation unit with fans and heat exchanger and the associated air distribution system. This ventilates the entire building. The ventilation unit draws in fresh outdoor air and passes it to supply air areas (living room, bedrooms or children's rooms) via the built-in heat exchanger and supply air ducts. At the same time, warm extract air from extract air areas (bathroom, kitchen or WC) is drawn into the ventilation unit via the extract air ducts and also passed through the heat exchanger. Heat from the extract air is transferred to the supply air and thus recovered. Cold extract air is then passed outdoors via the exhaust air ducts. 

Decentralised Vitovent 100-D mechanical ventilation system

Ventilation of individual rooms

A major difference between central and decentralised ventilation systems is that the latter usually do not have an air duct system. The units are installed directly in the external wall of individual rooms. Just like central systems, decentralised ventilation systems from Viessmann are also equipped with a heat exchanger and are able to extract heat from the extract air. There are two types of units.

Permanent fan: units with built-in cross-countercurrent heat exchanger. Two fans operate continuously. One fan always supplies fresh outdoor air into the room, the other removes extract air.

Pendulum fan: systems use only one fan and one thermal store. Two units communicate with each other for the air change rate. One unit draws in the warm indoor air via the storage core and blows it outside. The second unit draws in the fresh outdoor air and passes it into the room via the previously heated storage core. After about 70 seconds, the process reverses.

The section on decentralised mechanical ventilation explains what is important when using such a system.

Option of temperate heating as a distinguishing feature

In contrast to decentralised mechanical ventilation, the centralised version also offers the option of temperate room heating. All Viessmann central ventilation units have an automatic summer bypass. This is used to bypass the heat exchanger during warm summer nights. This allows fresh night air to enter the rooms and temper them to a certain extent.

Building characteristics necessitate different areas of application

Whether a ventilation system is installed centrally or decentrally depends largely on the building characteristics. Due to the almost invisible installation and the associated high planning effort, the central version is mostly used in energy efficient new buildings. A utility room, basement room or well-insulated attics are suitable installation rooms.

Since no air distribution system is necessary, decentralised systems, on the other hand, can also be retrofitted quite easily. Depending on the type of system, only a drilled hole through the external wall is required. This feature makes the decentralised ventilation system very attractive for modernisations of older and existing buildings. In addition, the units can be installed specifically in individual rooms where, for example, a humidity problem has occurred. However, whole residential units can also be ventilated with decentralised units. The wide range of possible applications and the relatively simple installation are thus two decisive advantages of decentralised mechanical ventilation.

Central systems can also be retrofitted

Depending on the building characteristics, a central mechanical ventilation system can also be retrofitted. This usually requires lowering the ceiling. Ceiling mounted or wall mounted flat ventilation units such as Vitovent 200-C or Vitovent 300-C are therefore designed to have a low overall height. 

Performance comparison: central vs. decentralised ventilation system

The comparison of central and decentralised ventilation systems makes it clear that the former convey a larger air flow. This is because the entire air flow of the system is provided by a single appliance. Central systems are designed to ventilate an entire house. The required air volumes for individual rooms are calculated beforehand and adjusted at the valves. The total air volume is set on the ventilation unit itself. With a control unit, the user also has the option of adjusting individual ventilation levels as required. Only two external wall diffusers for fresh air and exhaust air are required. As already mentioned, decentralised ventilation units only ventilate individual rooms. The air flow rate is therefore lower. Several decentralised units must be included in the system. Each decentralised unit requires an external wall diffuser. 

Constant flow rate fans as an advantage of central systems

All Viessmann central ventilation units have constant flow rate fans. This makes it very easy to regulate the ventilation units. During commissioning, the required flow rate is set directly on the ventilation unit. By maintaining the constant flow rate, the ventilation unit can compensate for any filter contamination and always provide the required flow rate to individual rooms. The section on ventilation systems shows what you should look out for when searching for a suitable system. 

Central and decentralised ventilation systems have different noise levels

Depending on the design, there is another quite significant difference between central and decentralised ventilation systems: the noise level. By placing system components outside the living areas and installing silencers, the noise level of a central ventilation system is usually lower than that of a decentralised system. However, how high this also depends on the design, adjustment and positioning of the diffusers. If individual components are well matched to each other, there is no air noise.

All Viessmann ventilation units operate very quietly from the factory. If necessary, additional soundproofing measures can be implemented.

Costs depend on various factors

Whether a central or decentralised ventilation system is installed also influences the costs. However, it is not possible to make a blanket statement. Careful consideration of all factors is required here. Decentralised ventilation systems are more cost effective to retrofit because no distribution system is required. In new build, a central system is often the more cost effective choice, as the air distribution system is installed directly during construction.

Proper maintenance ensures hygienic operation

Whether a ventilation system is operated centrally or decentrally also influences its maintenance. Maintenance is absolutely essential for hygienic operation. All ventilation systems have filters that retain dirt. These must be cleaned or changed regularly. In the case of a central system, this concerns the filters in the ventilation unit and in the extract air valves. The outdoor air filter prevents dirt from entering from the outdoor air. For allergy sufferers, pollen filters can also be used here. The extract air filter protects the ventilation unit from contamination from indoors and also ensures a permanently high level of heat recovery. In contrast, the filters in decentralised systems are located in each individual ventilation unit.

All Viessmann ventilation units are designed so that filter maintenance can be carried out without tools and by the system user themselves. 

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