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For occupants, the most important planning aspects are health and comfort. Excellent air quality is especially essential and can only be achieved if “used” air is regularly replaced by fresh air. Opening windows twice a day is not enough (see Purge ventilation through windows). Comfort ventilation based on the requirements for fresh air is therefore indispensable in every Passive House. A regular, guaranteed and adequate exchange of air in winter is only possible by means of comfort ventilation – this also applies for ordinary new buildings. The issue here is not energy efficiency, but the health of the building's occupants; Indoor Air Quality (IAQ) has a much higher priority than energy conservation - but it turns out that there is no conflict at all, if efficient components are used.
Gap ventilation through leaks is not adequate in the heating period at all (see also Airtight construction):
|Wind and weather fluctuate - so does the air exchange in “free”
ventilation. If this is sufficent on wind-free days, the heat losses
during strong winds will be intolerably high. Gap ventilation is
therefore no longer accepted by occupants in colder climates.
Without comfort ventilation, adequate air exchange in new buildings can only take place by means of regular purge ventilation. In order to achieve an air exchange of about 0.33 ach (air change per hour), one would have to open the windows wide for 5 to 10 minutes every three hours – even at night! This is rarely done in practice.
Accordingly, the air quality is usually poor and there is an increased risk of high air humidity. Because we cannot perceive the indoor air quality ourselves and it is not possible for us to estimate the amount of fresh air actually supplied through open windows, it is difficult, even for an expert, to achieve “just the right” amount of air exchange.
⇒ One of the reasons for home ventilation is to reduce the air humidity in the home slightly, because a high level of moisture in the air often causes building damage. However, the air should not be too dry either. You can find more information on the page about Ventilation and Humidity ("Air volumes" page).
The right level of air humidity is not the only requirement for an adequate exchange of air. Pollution of indoor air, due for example to the radioactive inert gas Radon, must be reduced to safe levels by adding fresh air. 1)
It's quite simple:
There is no doubt that 0.1 air exchanges per hour is insufficient for good health and comfort (see following illustration).
|This diagram illustrates why sufficient ventilation is so important:
excessive humidity in a bedroom of an old building without thermal insulation.
Moisture is continuously being released into the room, especially at night.
Conclusion: The overall air exchange achieved for this measurement is inadequate, more ventilation is required in order to remove the moisture in the room air.
Note 1: the situation depicted in the graph above is not an isolated case, in fact it is the general rule in lots of existing buildings in Central Europe - and we have heard about this from other Winter cold climates in the world, too. This explains why there is so much damage due to moisture in homes in Germany. Behind a cabinet or a picture, the external wall is even colder, here the relative humidity increases – the same happens in areas with thermal bridging. This problem is not “caused” by thermal protection at all, on the contrary, well insulated external walls have warmer and therefore more dry internal surfaces and are less susceptible for mould (see also Insulation prevents structural damage - Evidence no.4 Measurements in a retrofit - let us add, that these facts are not new at all, have been known in building physics for longer than five decades but nevertheless neglected by lots of designers).
Note 2: The effects of insufficient air exchange shown in the graph were based on the concentration of water vapour (humidity). Water vapour is not the only nor even the main impurity in indoor air. Radon, volatile organic substances, dust, and many other toxic substances are also present in indoor air. An insufficient supply of fresh air increases the concentration of these substances to unnecessarily high levels. An adequate exchange of air is not only a question of comfort but is also a prerequisite for healthy living conditions.
Well, the answer to this is difficult: the general conditions, size of windows, location of the house etc. vary in individual cases. The best solution is a ventilation system which always ensures an adequate supply of fresh air.
Because most people don't yet have comfort ventilation in their homes, we have also studied minimum ventilation through opened windows. This was done in a systematic scientific analysis which has been published in the Protocol Volume for the Working Group Number 23. This analysis showed that for adequate air exchange in a house without a ventilation system, windows have to be opened at least 4 times a day for purge ventilation - and at the largest possible time intervals, preferably in 6-hour intervals ([Feist 2003] ). That is our recommendation for all the users of homes which do not yet have a ventilation system.
This doesn't apply to residents of Passive Houses: they don't have to bother about opening the windows at the right time; of course they may open the windows if they want to – but they don't have to remember to do it regularly.
The function of comfort ventilation is to supply fresh air in “just the right” quantities to the living space. The simplest solution is an exhaust fan system, that extracts the stale and humid air from the kitchen, bathroom and toilet. At the same time, fresh air (cold air in winter) is drawn in through outdoor air inlets into the living areas.
These simple systems are now standard in France; exhaust systems have been used in Sweden for more than 50 years and since 1980 it has become obligatory to have home ventilation. In Germany this could be an effective solution for new constructions built to EnEV standard and for refurbishment of existing buildings (which have now become more airtight), but unfortunately this has not been made compulsory.
For the Passive House, however, this simple system can't be considered because the incoming air is cold, the ventilation losses will therefore be too high (see thermographic image). For one thing, a correspondingly high output heat supply near the inlet will then be necessary and for another, the annual heating demand will be at least double that of a Passive House. Less ventilation doesn't come into question because energy conservation should not mean less hygienic conditions ore worse indoor air quality.
|Thermographic image of an external air inlet of an exhaust
system. The minimum investment for indoor air quality that is indis-
pensable for every new building and for every modernisation of
existing buildings. An acceptable solution for a low-energy house
if the heater is located under the inlet. For a Passive House the
cold incoming air is not acceptable - and also the high heat los-
ses are inacceptable.
(Photograph and thermographic image: ebök)
Systematic examination of homes has shown that proper distribution of fresh air in all rooms and safe dehumidification of kitchens and bathrooms is possible through controlled ventilation.
|Ventilation only functions 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. (diagram: PHI)
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 95 %. 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 fresh) 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
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 sensible 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.
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.
Passive Houses always have an integrated home ventilation system with heat recovery, and often this is the central component of the complete building services. Only high quality ventilation technology is suitable for the Passive House. The Passive House Institute has summarised these (in German) in the requirements for central ventilation units: Apart from a high heat recovery rate, low electricity consumption, and hygienically faultless and very quiet operation must be guaranteed.
[Feist 2003] Feist, Wolfgang: Empfehlungen zur Lüftungsstrategie, in Protokollband 23 des Arbeitskreises kostengünstige Passivhäuser, Passivhaus Institut, 2003