FAQ

Residents often close grilles when they experience draughts, cold air drop, traffic noise or a sense of heat loss. That is understandable behaviour, but it limits the supply of outdoor air. Especially after insulation work and new, airtight window frames, a ventilation solution is needed that stays comfortable in everyday use as well.

With too little outdoor air, CO2, moisture and odours can linger longer in the home. The consequences vary per home and usage pattern, but can include a stuffy bedroom, higher humidity and a greater risk of condensation on cold surfaces. Good ventilation is therefore not only a design choice but also a matter of everyday use.

No. A window trickle vent can be part of a working ventilation concept. The key question is whether residents actually leave it open in practice, and whether supply, extraction, comfort, noise and control work well together as a whole. In a major renovation, a balanced, decentralised solution can be an alternative when ducting or large-scale interventions are undesirable.

Decentralised HRV extracts used air and returns filtered outdoor air in a controlled way. In a balanced setup, air exchange does not primarily take place through an open window trickle vent. Heat recovery limits the heat loss caused by ventilation; the actual comfort result depends on design, commissioning, placement, noise and usage.

With decentralised ventilation, a ventilation unit is placed close to the room being ventilated. A version with heat recovery exchanges heat between extract and supply air. This allows fresh outdoor air to be supplied with less ventilation-related heat loss than with uncontrolled supply through gaps or an open window.

In renovation projects, space for air ducts is often limited, and opening up ceilings, shafts or homes can cause considerable disruption. A decentralised solution can then be attractive because ventilation is organised closer to the room itself. Whether it is the best choice depends on the floor plan, the number of rooms, wet rooms, facade options, noise requirements and maintenance strategy.

Not always. Many decentralised solutions greatly reduce the need for a central duct network. Some versions do use a short duct connection, for example for extraction from a wet room. Assess per home and per system which penetrations and connections are actually required.

That is possible, because fewer long ducts need to run through a building. The actual disruption, however, is determined by factors such as facade penetrations, electrical provisions, remedial work, accessibility and the chosen phasing. A thorough survey and execution planning remain essential.

No. Relative humidity (RH) indicates how saturated the air is with water vapour at the current temperature. The same 50% RH can represent a different amount of moisture in the air at a different air temperature. For condensation risk, it is not only RH that matters, but also the dew point temperature and the temperature of the coldest interior surface.

The dew point is the temperature at which the amount of water vapour present just starts to condense. If a surface cools below the dew point of the indoor air, condensation can form. On repeatedly damp, cold surfaces, the risk of mould growth increases.

Condensation occurs locally: wherever the surface is coldest. That can be at a thermal bridge, a connection detail, a window-frame edge or a damper part of the building envelope, for example. An average room temperature or a single RH value therefore does not, by itself, tell you whether every interior surface stays safe.

Ventilation removes moisture from cooking, showering, laundry drying and occupancy, and brings in drier outdoor air when the absolute moisture content outside is lower. Smart control can increase the ventilation rate when the moisture load rises. Ventilation does not replace a structural solution for serious thermal bridges or leaks, but it is an important layer in controlling the indoor climate.

Student rooms often combine a small space with varying occupancy, sleeping, studying, showering or cooking in close proximity, and little storage space for technical equipment. As a result, CO2 and moisture can rise quickly when ventilation is not sufficient or not consistently present. A solution therefore has to be technically sound and fit everyday use.

CO2, moisture, temperature, noise and the actual ventilation rate are particularly relevant. CO2 is a useful indicator of occupancy and the need for fresh air; moisture load can be locally and temporarily high. A proper assessment requires looking at the room, the usage pattern and the structural situation together.

Real-world projects reveal how a solution performs with actual residents, varying occupancy, maintenance, seasons and existing buildings. These experiences help improve installation, control, communication with residents and maintenance. Laboratory data remains valuable but does not answer every question about everyday use in a building.

Decentralised HRV can be interesting when renovation takes place in occupied buildings, when space for ducting is limited, or when rooms need to be tackled individually. Feasibility must be assessed per building, including facade, fire safety, noise, electrics, extraction from wet rooms, maintenance and management.

Central HRV typically serves multiple rooms from one central unit via a duct system. With smart decentralised HRV, ventilation sits closer to the room it serves. The unit can measure air quality there and adjust the airflow accordingly. The difference therefore lies not only in where the technology is located, but also in how far ventilation per room can respond to actual use, CO2 and moisture.

No. Central HRV can be a good choice, especially when there is enough space for ducts and shafts, the construction logistics allow it, and management is organised centrally. In existing buildings, factors such as ceiling height, fire compartmentation, asbestos, limited shaft space, occupied conditions or the desired phasing can make a central approach complicated. In that case, decentralised HRV is a serious alternative that needs to be assessed per project.

No. The regulations for a residential function set requirements for both occupied zones and individual occupied rooms. In new-build, the minimum capacity is linked, among other things, to floor area, with a minimum per room. For a central system, there is also a requirement for the total capacity of the connected occupied zones. The regulations thereby set a lower limit for capacity and availability; they do not require a system to continuously control based on measured IAQ per room.

Smart per-room control is not, in itself, an obstacle. The chosen system must demonstrably meet the applicable structural, ventilation, acoustic, fire-safety and any local requirements within the specific project. Sensor-based control can be applied on top of the required base ventilation to respond to varying occupancy and moisture load; the justification and commissioning remain project-specific.

Practice is largely built around familiar system types, standard specifications, fixed responsibilities and cost estimates. As a result, larger projects often start from a central system or from existing ventilation patterns by default. In addition, smart decentralised solutions have been known to many stakeholders for a shorter time and require a different design dialogue: not just ‘how much air does the home need?’, but also ‘which room needs which air quality, and when?’. Unfamiliarity is not a technical counter-argument, but it is a real barrier to specification and tendering.

No. Demand-based control is not a reason to omit the necessary base ventilation. A good design combines a controlled base level with temporary boosting whenever CO2, moisture or other relevant signals warrant it. This allows the system to run more quietly when the room is lightly loaded, and to scale up in time for sleeping, visitors, cooking or showering.

Start with the building and its use, not with a preferred product. Consider, among other things: available space for ducts and shafts; renovation in occupied conditions; facade options; connection of wet rooms; noise; fire safety; maintenance and management; the desired phasing; and the value of monitoring or room-by-room IAQ control. Only then is a fair technical and financial comparison possible.

Every sleeping person continuously exhales CO2. In a closed bedroom, the concentration rises when less fresh air is supplied than is needed to remove that CO2. How quickly this happens depends, among other things, on the number of people, the room volume, any infiltration, and the actual ventilation rate.