This article is based on a paper published in the 62nd volume of the Research Group for cost-effective Passive House buildings on the topic of “ Cost-Effective Refurbishments”. The original article by Rainer Pfluger is availabe in German in the proceedings, which can be accessed here.

Ventilation with heat recovery has now become widely accepted in new buildings and is a core component of Passive House building technology. In particular, a supply air and extract air system with heat recovery (hereafter referred to as “comfort ventilation”) should always be retrofitted whenever an existing building is modernised. The advantages (healthy indoor air through the removal of pollutants, reduced ventilation heat losses) are basically the same as in new buildings, but the prevention of structural damage is even more important in existing buildings. In many cases, airtightness cannot be achieved quite as perfectly as in new buildings, and surface temperatures may also be lower than in new buildings due to remaining thermal bridges. The dehumidifying effect of controlled ventilation can minimise the resulting risks of moisture and mould damage.

Despite these far-reaching and obvious advantages, just a fraction of existing buildings have been equipped with comfort ventilation so far. According to a study by dena (German Energy Agency), 50-60% of new buildings already have this, but estimates suggest that only around 5-10% of existing buildings have been retrofitted. Overall, sales figures for both centralised and decentralised heat recovery have actually declined in recent years [dena 2025].

The following section therefore combines an analysis of the obstacles with proposals for overcoming these as far as possible.

The most common argument for not installing comfort ventilation in existing buildings is the cost – both in terms of investment and operation. This article aims to present solutions, methods and variants for cost-effective residential ventilation, and also to remove other barriers and refute prejudices as far as possible. The table below lists both the relevant obstacles and proposed solutions.  

The following sections explore the topic of costs in terms of investment, operation and maintenance. Even though the topic of costs is not the sole deciding factor when it comes to retrofitting comfort ventilation systems in existing buildings, building renovation practice has shown that ventilation is often omitted from the renovation planning for budget reasons, even though relatively high subsidies are available in this area as well. Unfortunately, subsidies often drive up prices and are therefore counterproductive for the customers. A more promising approach here is to develop the mass market in the renovation sector, i.e. reduced costs through high volumes. Numerous manufacturers have recognised this market segment. The following section therefore describes cost-effective devices specifically for retrofitting in existing buildings.

The development of residential ventilation with heat recovery by manufacturers is still largely driven by new building construction. Here, the space demand is not quite as critical because the systems can be integrated into the floor plan at an early stage. For integration into existing buildings, extremely space-saving devices are frequently needed due to the lack of space. Many manufacturers are responding to this by offering compact and usually relatively flat devices for integration into the façade. The options available here are explained in the following section. In terms of nominal volume flows, apartment buildings often require lower air volumes per residential unit than single-family homes. This not only reduces the required output (and thus also noise emissions), but also the required size of the device housing. If each residential unit in an apartment building is supplied by one unit, a nominal volume flow of 40 to 120 m³/h will be required depending on the size of the dwelling. Especially units with a low volume flow range weren't widely available on the market previously; in particular, depiction of the partial load range was barely possible. A number of products are now available that can also be used in this range without any problems. According to the PHI certification criteria [PHI 2018], this lower range (see component database on the PHI homepage) is also reliably determined by an independent testing institute (i.e. not by the manufacturers, who often only specify the upper limit but not the lower limit). Regardless of the compact design, care should of course be taken to ensure that the device housing is as well insulated as possible. In addition to the insulation thickness, the absence of thermal bridges in the device housing also plays a role here. Modern devices are often manufactured with a polypropylene inner lining and thus without thermal bridges caused by metal profiles.

However, in renovation projects space can be saved not only through the size of the device housing but also through integration in the façade as described below.

In principle, it is advantageous to place the unit as near to the thermally insulated envelope as possible, as this minimises the outdoor air and exhaust air duct lengths. These ducts carry cold air and must therefore be insulated in a diffusion-tight manner. This also requires space and is cost-intensive. Long and inadequately insulated ducts have a negative effect on system efficiency.

With the small and flat units mentioned above, it is possible to install these on the surface and connect them to the outdoor air/exhaust air ducts through the wall over the shortest distance. As an alternative to two separate core drill holes, a coaxial or centrally divided pipe can be used through a larger core drill hole. The unit can be installed at virtually any point on the wall or next to or below the window. If, for example, underfloor heating is used, a radiator recess can be used if necessary.

Figure 1: Types of installation for integrating ventilation systems into existing facades (surface mounted: unit is placed on the wall; flush mounted: unit is recessed into the wall; wall-integrated: unit passes through the wall and insulation; wall-integrated at 45°: core drill hole is drilled at a 45° angle)

Compared to surface mounting, complete integration into the wall can save even more space. However, a recess in the exterior wall is necessary for this. With appropriate planning, this is easily possible in new buildings, both in masonry and concrete constructions. For retrofitting during a building renovation, structural considerations will be necessary for the creation of this recess. It is therefore worth considering whether the ventilation system, which is often installed together with new windows, could simply be installed directly next to a window. For this purpose, the new window must be made narrower by the width of the unit. The following illustration shows this concept using a 1:1 mock-up, which was exhibited within the framework of the Passive House Conference. Operation, maintenance and filter replacement can be carried out conveniently from the inside, and the supply and extract air ducts can be installed in the insulation on the surface of the existing wall – another way of saving space and thus also improving cost efficiency.

Figure 2: Cost-effective window integration of an apartment-by-apartment ventilation system; view from the inside (left), view from the outside (centre), view of the reveal (right)

If none of the above installation methods are possible or desirable, there is still the option of installing the entire unit in a core drill hole. In order to gain additional space for the whole length of the unit, the hole should be drilled at a 45° angle (see Figure 1, right).

As part of a building renovation using prefabricated modules (serial renovation or renovation on an industrial scale), ventilation units can already be installed in the façade module in the factory. In this case, the ventilation ducts (supply/extract air ducts) can also already be installed in the module (on the warm side, i.e. on the room side). The only remaining challenge is to ensure that these ducts are airtightly connected at the module joints.

Compact, short ductwork is always desirable, as this saves investment costs (materials and installation) as well as space. For many years, the use of jet nozzles utilising the Coanda effect has proven highly effective. In contrast to the use of classic poppet valves as supply air outlets, where a duct must be routed up to the opposite wall to avoid air flow short-circuiting, a jet nozzle can be installed directly above the door, approximately 15 cm below the ceiling. The air then flows along the ceiling without causing draughts. This saves many metres of ducting and encasing.

Further innovative air flow concepts can significantly reduce the length of the supply air ductwork. Shorter ducts offer many advantages – both in terms of operation (electricity for fans, cleaning, etc.) and in terms of investment and installation costs. The following concepts are possible here:

• 1. Cascade ventilation: Air is supplied to the supply air rooms (living areas and bedrooms), from where it enters the air transfer zones (e.g. hallway) and then flows into the extract air rooms (bathroom, - kitchen, WC). In this way, the duct lengths required can be significantly reduced; furthermore, the required air quantities are also reduced, because the air can be used “twice”, as it were.
  

• 2. Extended cascade ventilation: If the concept of cascade ventilation is consistently followed through, living areas (provided the floor plan is suitable) and hallways can also be considered air transfer zones. This was first proven in [Fräfel 2009] through measurement and simulation. This achieves more reductions in duct lengths, air volume and supply air elements. One can check whether this air flow concept is suitable for the respective floor plan of an existing building using the online tool [Doppelnutzen 2013], which was devised in the dissertation [Sibille 2015].
  

• 3. Not all floor plans are suitable for extended cascade ventilation. However, in renovation projects in particular, one has to work with the existing situation. There remains the possibility of introducing the air into the hallway (air distribution zone) via the shortest route. From there, the air is transferred into the supply air rooms via (fan-driven) active air transfer devices. From there, the air then flows back into the hallway via passive air transfer grilles. In order to avoid overpressure and thus potential moisture damage in existing buildings in particular, the reverse direction (active air extraction in the hallway and passive air transfer into the supply air rooms) is also possible. In both cases, however, routing of the extract air must be planned conventionally with air extracted from the bathroom, kitchen and WC. This method was developed in Switzerland under the name “Verbundlüfter” (connector fan). It has since spread to other countries, and is used not only in residential construction, but also in school and office buildings.
  

• 4. When room doors are kept open, an active air transfer device is not necessary because convection alone can cause several hundred cubic metres of air per hour to be transferred through the open door across the large cross-section. If living habits are accordingly adapted and doors are left open most of the time, the air flow concept described for concept number 3 can also be implemented without any active air transfer devices. However, this requirement must be clearly communicated to the residents, because supply air rooms will not be adequately ventilated if room doors are kept closed for longer periods of time.
  

Positive experience with this (concept number 4) type of extremely cost-effective ventilation has already been gained over many years in a demonstration building in Innsbruck (NHT, IN42; EU project Sinfonia) [Sinfonia 2022].

Figure 3: Cost-effective ventilation for a building renovation, based on the example of the IN42 deep retrofit project (NHT, EU project SINFONIA). Originally, active air transfer devices were planned, but in the end the concept with passive air transfer through open doors was implemented (concept number 4).

In addition to the air routing concepts mentioned above, the air volume can also be reduced in a targeted manner with the aid of control measures. If the air volumes are adjusted according to demand, this is referred to as demand control, i.e. user-independent air volume control as required. A sensor to determine the actual value and an actuator to make the adjustment is necessary for a control loop. Air quality-dependent parameters (CO2, TVOC, particulates, etc.) are suitable as measured variables. Air volumes are then varied either by controlling the fan speed and/or by means of volume flow controllers (motor-driven flow control valves). This demand-based adjustment of air volumes not only makes sense for economic and ecological reasons, it also avoids the problem of low indoor humidity in the middle of winter due to “over-ventilation”.

Some devices or systems also have a switchover damper that can be used to divert the air volumes from the bedroom to the living room or vice versa (to some extent). In this way most of the air volume is used in the bedroom at night, switching to the living rooms during the daytime. However, basic air exchange is permanently available in both rooms in each case. The switchover damper can be installed either directly inside the device or externally in the ductwork.

Nevertheless, both demand control and switchover dampers constitute a further complexity and therefore are prone to faults and need maintenance. It is therefore important to carefully weigh up the advantages and disadvantages of these concepts. If sensors are not checked and maintained regularly, a more complex control system may actually deliver worse long-term results than robust, simple solutions.

Highly efficient counterflow heat exchangers cool the extract air to a temperature close to that of the outside air. In doing so, the temperature often falls below the dew point (depending on the humidity of the extract air), leading to condensation. This is normally drained into a condensate drip tray and from there into a waste water pipe via a siphon. If the outside temperature drops significantly below freezing point, the heat exchanger may ice over. This should be prevented because the spaces between the plates of the heat exchanger will then be blocked by ice or frost. In the worst case, this may damage the heat exchanger and even cause leaks due to bursting of the plate joints. But it is not only icing up that can lead to increased pressure drops and even blockage in the heat exchanger; even droplets that get stuck between the plates due to capillary action may cause some blockage. This is particularly likely to occur if the device is installed horizontally (e.g. for ceiling integration), as the condensate cannot simply flow downwards on account of gravity. The air flow alone can hardly overcome capillary forces.

Due to the problems mentioned above – and especially in the case of renovation, also the challenge of controlled condensate drainage – the question arises as to whether there are ways of entirely eliminating the need for condensate drainage. If moisture-transferring recuperators or regenerators are used, the moisture in the extract air is partially recovered as water vapour and fed back into the supply air. Depending on the outdoor climate and extract air conditions, condensate drainage can be dispensed with in most cases because no liquid water is produced. Under extreme conditions (high extract air humidity, extremely cold outdoor air temperatures), problems may still arise, which will then have to be staved off through control measures (e.g. temporary shutdown).

Despite the slightly higher costs for moisture recovery systems, investment and operating costs are saved overall, because frost protection heating and condensate drainage can be dispensed with – at least if control measures are taken when extremely low outside temperatures (below approx. – 10°C) prevail. The cost of condensate drainage and maintenance alone is a good reason to opt for a moisture recovery system. If integrated near the window, it is possible to drain condensate onto the external window sill, but this carries the risk of icicles forming, as the amounts of water involved can be considerable.

For practical reasons alone, it is therefore advantageous to opt for moisture recovery systems. In terms of indoor air humidity (prevention of dry air during the main winter months), moisture recovery has a particularly positive effect in locations with a pronounced heating period [Pfluger 2011], and dispensing with direct electric frost protection heating not only reduces operating costs but also lowers output-related connection costs.

In the past, the often significantly lower heat recovery efficiency of enthalpy exchangers compared to purely heat exchangers was a counterargument in terms of energy efficiency. Today, the latest generation of exchangers are also competitive in this respect. This feature is not only important for saving heating energy; the resulting increased supply air temperatures also contribute to improved thermal comfort due to the supply air being introduced without any draughts.

A significant share of the investment costs goes into developing the air duct network. While this accounts for a major part of the costs in centralised building systems, it is significantly less in the case of devices on an apartment-by-apartment or even room-by-room basis [Fehrenberg 2013].

In building renovation, however, it is not only the investment costs that play a decisive role, but also the space demand. Space is particularly valuable in small residential units, so any additional negative impacts due to duct routing and panelling (encasing in plaster, etc.) should be avoided as far as possible. As shown in the section on innovative air routing, at least the supply air duct network can be largely minimised or eliminated by using the cascade principle. However, depending on the floor plans, this is not always possible. In these cases, it may be helpful to use alternative routes for the ductwork. Disused chimney flues can be used for vertical ducting if necessary. New ducts can then be installed in these or they can be lined with so-called inliners to make them smooth-walled and cleanable.

For horizontal ducting, i.e. for the distribution of supply air and extract air, ceiling suspension in a hallway is normally used, which is an easy and practicable solution also for retrofitting in building renovations – provided that the clear height in the hallway is sufficient. Silencers and duct intersections (e.g. of supply air and extract air) can cause problems with regard to height. Rectangular or flat duct silencers can help here, but even then, the clear room height may still be insufficient. Two other alternatives are therefore suggested below.

Supply and extract air ducts carry air at close to room temperature. They should therefore be installed within the thermally insulated building envelope. Only in exceptional cases can they also be installed in cold areas – and then only with huge insulation thicknesses. Unheated attic areas would actually be suitable for lateral routing – but the cost of insulation would hardly be justifiable. If the insulation is installed on the top floor ceiling, it makes sense to install the ducts below it, i.e. covered by the thermal insulation. This is especially unproblematic with loose-fill or blown-in insulation (e.g. cellulose).

Also, on storeys that are not adjacent to the attic it may make sense to route ducts on the outside in order to save space on the inside. If the ducts are routed as part of the exterior wall insulation, then supply and extract air ducts can be routed under the thermal insulation, so that these are positioned directly on the existing wall and are covered by the thermal insulation. With Passive House or EnerPHit insulation thicknesses, even round ducts with small diameters will still be sufficiently covered to ensure that at this point the thermal bridge effect caused by the reduced insulation thickness remains acceptable. This type of air duct routing is particularly advantageous for integration of the units in windows, because here the ducts can be directly routed horizontally to the adjacent rooms.

Figure 4: Duct routing under the thermal insulation on an existing wall (left), round, oval and flat ducts (right)

However, vertical routing is also possible in the case of central units installed in the roof area. The duct is then usually horizontally laid in the insulation of the top floor ceiling.

The third innovative method, which is still in the testing phase, is what is known as the laminar flow distributor. This is an extremely flat duct that is dimensioned in such a way that design volume flows pass through it in a laminar rather than a turbulent manner, as is normally the case. This reduces the pressure loss and air can be distributed with extremely low installation heights. Particularly good sound insulation can be achieved by covering with absorber foams (insertion loss). This principle was investigated in theory and laboratory tested on a 1:1 scale at the University of Innsbruck within the scope of a master's theses. However, a marketable system is not yet available. Its functioning has already been tested in practice in a DIY project in a small residential unit.

In thermodynamic terms, heat exchangers achieve maximum efficiency when the heat capacity flows are exactly the same on the primary and secondary sides. In this state, the temperature courses over the heat exchanger length represent two parallel straight lines (outside air/supply air and extract air/exhaust air); the temperature difference across the exchanger plates is therefore the same at every point. If there is any deviation from this ideal condition, the efficiency will also decrease because the exchanger surface is not used optimally. In practice, disbalances of up to +/- 10% are tolerable because even buildings with a high level of airtightness still have a certain amount of residual leakage and thus infiltration air exchange. In the event of a disbalance, natural infiltration is replaced by forced infiltration, which initially has no significant effect in terms of energy. Ventilation heat losses increase only with higher disbalances. From a building physics perspective, slight negative pressure (excess extract air) is less harmful than positive pressure with reference to moisture damage, which can lead to condensation in leaks. However, significant negative pressure not only reduces efficiency but also causes discomfort when cold air enters through leaks.

In summary, the recommendation is that balancing of ventilation systems should be as well-adjusted as possible. This can be set manually during commissioning, but that is time-consuming and not permanent, as changes in the ventilation system (e.g. increasing contamination of the filters, etc.) may upset the adjustment again. For this reason, many devices today already have automatic balance adjustment in their control technology. For example, volume flow controlled fans ensure that the preset air volume of the supply air fan is also delivered by the extract air fan. However, depending on the fan type and control system, this does not always succeed in an ideal manner. Testing with measuring orifices or baffle plates also provides additional certainty in this regard. EBM offers fans with integrated impeller anemometers across the entire air cross-section. This also allows a very high degree of accuracy to be achieved.

With the use of enthalpy heat exchangers which, as already mentioned, do not have a condensation problem and in which the gaps between the plates with water droplets or frost therefore are not blocked, one can rely on pressure loss measurement via the heat exchanger without having to install separate measuring devices. In order to deliver accurate results, measuring orifices or baffle plates require relatively long upstream and downstream sections, which is at least 5 to 10 pipe diameters upstream and 3 to 5 pipe diameters downstream of the measuring orifice or baffle plate, depending on the type of flow (e.g. in the case of bends, changes in cross-sections, etc.). Such long, straight duct sections are hardly found in the confined space situations particularly in existing building renovations. For this reason, measurement via the heat exchanger in the outside air/supply air section and in the extract air/exhaust air section is preferable.

Figure 5: Air flow adjustment by means of pressure loss measurement via the heat exchanger in the outside air/supply air (ODA-SUP) and extract air/exhaust air (ETA-EHA) sections

This method was tested on a prototype at the University of Innsbruck. There are already device manufacturers who use this system in commercial devices (e.g. Zehnder ComfoAir series, Vitovent 300-W, SAVE VTR series). To do this, the characteristic curves of both measurement sections first had to be determined in the laboratory. The aim of the fan control is to achieve the same air volume in the extract air duct as that currently conveyed by the supply air duct. The control system should react quickly enough to adequately make up for any irregularities caused by wind fluctuations (overpressure on the windward side, underpressure on the leeward side). Smaller systems as well as individual room devices are more susceptible to wind due to their lower pressure drops. This problem has a particularly strong impact on heat recovery ventilators (push-pull devices). Their use in wind-exposed locations should therefore be called into question. The sensitivity of the air flow to pressure fluctuations is specified in % at +/- 20 Pa for non-ducted units in accordance with EN 13141-8:2014.

In summary, it can be said that flow rate adjustment is not only decisive for the efficiency of the system and comfort; robust and accurate balance control can even be decisive for economic efficiency and the prevention of structural damage.  

The aim of the energy transition in Europe is to decarbonise the entire energy system by 2050. This can only succeed if energy efficiency measures and the use of renewable energy are jointly strengthened. The energy deficit in winter due to lower solar energy production and simultaneously higher seasonal energy consumption for heating and lighting can only be covered through the increased expansion of seasonal energy storage facilities and flexible energy sources (biomass or geothermal energy). However, the latter are limited in scope. Seasonal energy storage is very expensive due to the low charging/discharging cycles, but cannot be entirely avoided. This means that all efficiency measures that are particularly effective in the main winter months are all the more important. The heating sector is predestined for this purpose as new Passive House constructions and EnerPHit building renovations specifically address this. One part of this concept is ventilation with heat recovery, which achieves the highest savings at the same time(ventilation heat losses in the main winter months). Heat recovery ventilation therefore not only has advantages in terms of healthy indoor air and energy savings, but also has strategic importance for the energy transition. Heat recovery reduces the heating load of buildings and with this the maximum demand in the heating sector. To date, peak load boilers with a high proportion of non-renewable primary energy still have to be used in district heating systems to cover the main winter period as well. Every kWh saved during this period therefore directly relieves the burden and advances decarbonisation. Economic efficiency here is particularly high because the capital costs are already covered by the savings. In the future energy system, there are the high energy prices in the main winter period in addition, increasing economic efficiency even further. Another positive effect with regard to economic efficiency is now becoming apparent with the increased switch from oil and gas to heat pump heating systems. The coefficients of performance of air source heat pumps are lowest in the coldest winter months because of the climatic conditions, so every heating load saving during this period has an impact in particular. Also, in terms of the dimensioning of heat pumps and heat distribution systems, ventilation system heat recovery also allows the use of smaller devices and systems – and thus lower investments. This is particularly noticeable in the case of heat transfer systems. Especially low flow temperatures are desirable for the highest possible coefficients of performance. After an energy retrofit (EnerPHit standard) combined with a ventilation system with heat recovery, the now oversized existing radiators are often sufficient. Otherwise, they would have to be replaced at considerable expense with new, larger radiators or even underfloor heating. This is a considerable advantage especially for building renovation.

The use of comfort ventilation for building renovations has been a niche market for many years and still is. In this article it was shown that besides other entirely surmountable obstacles and prejudices, costs (investment and operation) also play a decisive role. However, the concepts and modern devices mentioned above will make it easier to develop also this market in the future. This not only has positive economic effects, but also possesses a strategic component for the energy transition. By specifically reducing the heating load of buildings, the particularly problematic winter shortage (energy deficit) will be mitigated in the future, thereby reducing the need for costly seasonal storage.

[dena 2025] dena Building Report 2025 – Figures, data and facts on climate protection in the existing building stock, www.gebaeudeforum.de/gebaeudereport2025. Accessed on 08.07.2025

[Dual use 2013] https://passivhaus-austria.org/luftfuehrung/ Accessed on 08.07.2025

[Fehrenberg 2013] Fehrenberg, W.: Lüftungstechnik für Gebäude (ventilation technology for buildings). Springer Vieweg, 2013

[Pfluger 2011] Pfluger, R.: “Ventilation rates and indoor air humidity depending on local climates – simulation and measurements of 9 European countries.” Tight Vent conference Brussels 2011, 2011. [PHI 2018] Requirements and test procedures for the energy relevant and acoustic assessment of Passive House ventilation units <600 m³/h for certification as “Passive House suitable components”, PHI, 2013

[Sibille 2015] Sibille, E.: Optimised Integration of Ventilation with Heat Recovery in Residential Buildings Through the Implementation of Innovative Air Distribution Strategies and Pre-fabricated Components, Dissertation, University of Innsbruck, 2015

[Sinfonia 2022] Peper, S., Failla, M.-C.: Comparison of Measured and Calculated Energy Demand and Assessment of the Results, D5.22 ANNEX Task 5.7, 2022

Research Group 62 - Cost-Effective Refurbishments

Refurbishment with Passive House components

Ventilation