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The convenient solution: supply and exhaust air systems with heat recovery

Ventilation will only work properly if used air is continuously being removed from the kitchen, bathroom, toilet and other rooms with high pollution and humidity. In return, fresh, unused external air is supplied to the living room bedrooms and functional rooms.

The principle behind convenient home ventilation: used air (brown) is continuously being removed from the rooms with high levels of pollution and humidity. Fresh air (light blue) is supplied to the living areas. Good quality air is an important prerequisite for a healthy and comfortable living climate.

Just the right quantities of fresh air that are required for the good health and comfort of the occupants are supplied. Only untreated air enters the living areas, there is no recirculated air, thus providing a hygienic air quality.

Ventilation can also take place if a simple exhaust air system and external air inlets are used. The external air inlets let fresh (cold) air in the required amounts into the rooms. However, for the Passive House the ventilation heat losses that would be caused by the disposal of the unused extract air would be much too high. It would only be possible to adjust the energy balance with a high heating output.

In Central Europe, Passive Houses only work if a highly efficient heat recovery system is also present This recovers the heat from the exhaust air and using a heat exchanger, transfers it back into the supply air without mixing the air flows. Today, modern ventilation technology allows a heat recovery rate of between 75 and 90 %. This is possible due to counterflow heat exchangers and special energy-efficient fans (with so-called EC motors with a particularly high effectiveness), so that the recovered heat is 8 to 15 times the electricity consumed.

This is how a heat exchanger works:
The stale extract air (red) flows through a duct and transfers its
heat to the plates above and below. It cools down and exits as
exhaust air (orange). Unused fresh air streams in through separate
ducts on the other side of the plates. It takes up the heat and is
available as warm (but still unused) supply air (light turquoise).
The counterflow principle makes up for almost 100% of the tem-
perature difference. Saving energy by using heat recovery is not
only cost-effective and environmentally friendly but also healthy
– fresh air is provided constantly without having to keep
opening the windows. This applies for all buildings, not just for
Passive Houses.

Due to this principle of directed air flow, the fresh air is optimally utilised: it provides high quality air in the living areas, removes any bad air from the transferred air zones (e.g. odours from clothes), and finally dehumidifies the humid areas.

Supply air and exhaust air ducts allow the heat from the extracted used air to be recovered. The ventilation heat loss without heat recovery is between 20 and 30 kWh/(m²a) in apartments with adequate ventilation. This is very high in comparison with all other heat flows in the well insulated Passive House.

This highly efficient heat recovery system was specially developed for use in Passive Houses. The devices ensure the separation of exhaust air and supply air, don't consume much electricity and are very silent.

With such a heat recovery system, the remaining ventilation losses are insignificant: they are only between 2 and 7 kWh/(m²a), which is a good prerequisite for a functioning Passive House.

Thus,due to the heat recovery, the temperature of the supply air is raised to near room air temperature, therefore the air entering the room is not “cold” any more. Together with very good insulation of the building envelope and the windows, it is possible to get along with very little heating power and also reduce the effort for installation.

An exclusive advantage of the Passive House is that heating using the supply air is possible. Because the fresh air is supplied to the living room, bedrooms and workrooms in any case, this air can also be used to provide warmth. Because it is fresh air (not recirculated air), the quantity of this fresh air is limited (because otherwise the air will become excessively dry), and as its temperature may not be increased too much, the supply air heating method functions only for houses with a very small heating demand – i.e. Passive Houses. Therefore, it is possible to provide very elegant and space-saving building services solutions, like the compact ventilation unit.

Thermographic image of an opened counterflow heat recovery unit. The actual heat exchanger can be seen as a hexagon. It recovers more than 75% of the perceptible heat from the extract air. (Image: PHI).

The highly efficient ventilation units developed for the Passive House have also proved to be effective in modernisations of existing buildings. Here they contribute to the improvement of the air quality, and ensure that mould growth does not occur at weak points in external building components, as well as helping to save energy.

An additional possibility for improving the efficiency of ventilation systems is offered by the subsoil heat exchanger: on average, the ground is warmer in winter than the surrounding air, and colder in summer. Fresh air can therefore be pre-heated or pre-cooled using the earth. This can take place directly through air ducts (air-to-soil heat exchanger) or indirectly by means of a hydraulic system (brine-carrying subsoil heat exchanger).

In hot climates, air-to-air counterflow heat exchangers can also help to recover “cool temperature” from the exhaust air and to reduce the temperature of the supply air, if the fresh air is uncomfortably hot. But this requires low energy fans in order to reduce heat loads caused by the ventilators. Humidity recovery, which is possible with special types of heat exchangers, is another highly efficient option in extreme cold and/or extreme hot and humid climates.































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