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.

No. CO2 is primarily a useful indicator of occupancy and the degree of air renewal in a room where people are present. It does not tell you everything about, for example, particulate matter, volatile compounds, moisture or outdoor air quality. That is why base ventilation remains necessary, even when a CO2 sensor measures low occupancy.

A home is not used with equal intensity everywhere at the same time. A bedroom with two sleeping people, an empty study and a living room with visitors each have a different ventilation need at the same moment. With measurement and control per room, the system can respond to this in a targeted way, within the chosen base and maximum settings.

Not without limit. A low value usually indicates plenty of fresh air relative to occupancy, but additional ventilation also requires energy and can affect noise and comfort. The goal is therefore not ‘as low as possible’, but ventilating sufficiently and in good time for the desired quality in the room that is actually being used.

CO2 responds with a delay: once the value is high, it takes time for the concentration to fall again. A smart control system can therefore scale up earlier and gradually as CO2 rises, so the desired value is not unnecessarily exceeded. As the load decreases, the airflow can also be scaled back down smoothly.

Condensation does not start from an average figure for the room, but at a locally cold surface. If the temperature of that spot drops below the dew point of the indoor air, condensation can form there. An average RH in the living room therefore does not automatically mean that every detail of a wall, ceiling or window frame stays dry.

Absolute humidity is the actual amount of water vapour in the air. Relative humidity indicates how close the air is to saturation at the current temperature. Because warm air can hold more water vapour, the same absolute amount of moisture can produce a different RH at a different temperature.

These activities introduce a lot of moisture into the home in a short time. That moisture spreads through the air and can settle on cold surfaces if it is not sufficiently removed. Ventilating extra in good time helps limit the moisture load; extraction from wet rooms and the structural situation remain essential in this.

No. Ventilation can lower the amount of moisture in the indoor air and thereby reduce condensation risk. A serious thermal bridge, a moisture problem in the building envelope, or a leak, however, must be investigated and resolved structurally. The best result comes from a good building envelope combined with well-functioning ventilation.

Controlling on dew point means looking not only at RH, but at the relationship between the moisture content of the indoor air and the temperature of the coldest relevant interior surface. In the FHHS, this is used to explain when additional ventilation can be a sensible response. The practical settings must always match the actual home, its sensors and the chosen control strategy.

Demand-based ventilation adjusts the ventilation rate to what is happening in a room. Sensors or other signals can, for example, indicate that CO2 or moisture is rising. The system can then ventilate more and later return to a lower setting once the load decreases.

A fixed high setting can provide enough air at peak moments, but it also keeps running high when a room is empty or air quality is already good. That can cause unnecessary energy use, noise and air movement. Conversely, a fixed low setting can fall short during sleeping, visits, showering or cooking. Demand-based control tries to avoid both of these drawbacks.

The base setting provides continuous, controlled air renewal. The boost, or higher, setting is used temporarily when a sensor or a chosen control strategy determines that extra ventilation is needed. In a well-designed control system, the transition happens gradually, so the system does not keep switching abruptly on and off.

In a balanced system, the starting principle is that supply and extraction remain in equilibrium. In practice, wind pressure, resistance or building conditions can affect that balance. A suitable control system can therefore adjust a fan to maintain the balance. How this is implemented depends on the product, the installation and the control strategy.

A manual setting or boost function remains useful, for example during unexpected moisture or odour loads. The advantage of sensor-based control is that the system can also respond when a resident doesn't think of it, is asleep, or doesn't notice that CO2 and moisture are rising. Clear explanation and an easy-to-understand control option remain important.

HRV exchanges heat between the warm extract air leaving the home and the colder outdoor air being supplied. The two airstreams remain physically separated. The fresh air therefore still comes from outside, but is pre-warmed using heat that would otherwise be lost with the extract air.

No. HRV is precisely a way to supply fresh outdoor air with less ventilation-related heat loss. Used indoor air is extracted and replaced with outdoor air. HRV does not simply recirculate the same room air as ventilation air.

In cold weather, fresh outdoor air supplied via HRV can already be pre-warmed. This makes the temperature jump at the supply point smaller. The ultimate comfort also depends on placement, design airflow, air distribution, noise, commissioning and the building envelope.

Renewing more air generally requires more energy for fans and, during the heating season, for bringing supply air up to temperature. HRV limits the heat loss, but it does not make ventilation energy-free. That is precisely why ‘ventilating just enough’ is a good starting point: sufficient for air quality and moisture, without an unnecessarily high airflow.

Air quality, noise, maintenance, filters or the heat exchanger, condensate drainage, commissioning and the availability of the required capacity are at least as important as efficiency. A system is only truly good when it keeps ventilating sufficiently, quietly and reliably in everyday use.

Yes. A wood stove without a direct supply of combustion air draws air from the home. That air then leaves the home together with the flue gases via the chimney.

A Fresh-r Compac ventilates in a balanced way: the unit extracts roughly as much air as it supplies. As a result, it does not automatically compensate for the extra air a wood stove uses for combustion. With such a stove, a separate air supply provision is therefore needed, preferably close to the stove — for example a window trickle vent or a purpose-designed facade vent.

That additional supply also helps the stove perform better. There is enough oxygen for good combustion, the draught in the chimney stays stronger, and the chance of smoke entering the home is reduced.

Yes, in many cases it is. An extractor hood that vents air outside temporarily draws a lot of air out of the home while cooking. The Fresh-r Compac cannot replace that extra extraction, because the Compac keeps its supply and extract air in balance.

A window trickle vent or a dedicated facade vent in or near the kitchen gives the extractor hood enough replacement air. This allows the hood to better capture cooking fumes, moisture and — when cooking on gas — any combustion gases. Without sufficient supply air, the extractor hood can perform less well, and cooking fumes or moisture can linger in the home for longer.

No, not as a replacement for a separate provision. The Fresh-r Compac supplies fresh air for the balanced ventilation of the home or room, with heat recovery. In doing so, it also extracts a comparable amount of used air.

A wood stove without its own outdoor air connection and an extractor hood venting outside cause exactly this kind of extra, unbalanced extraction. A separate, sufficiently sized air supply must therefore be included in the design for this.

Preferably as close as possible to the point where the air is needed:

• for a wood stove: near the stove;
• for an extractor hood: in or near the kitchen.

This means the required air does not first have to be drawn through the home from bedrooms or other rooms. The exact execution and capacity should be determined in the design, depending among other things on the stove type, its power rating, the chimney, the extractor hood and the airtightness of the home.

No. For normal, balanced ventilation with a Fresh-r Compac, a window trickle vent is not the primary air supply. But for an appliance that independently extracts air from the home to the outside — such as a wood stove without its own outdoor air supply, or an extractor hood venting outside — a separate supply provision can indeed be necessary.