The weather conditions are fluctuating - and so is the air exchange rate using natural ventilation. If ventilation is just about sufficient during calm days, the infiltration losses during draughty periods will be unacceptably high.

Not that simple, ventilation by opening windows at regular intervals...

Infrared picture of an inlet air valve for fresh air used with exhaust fan ventilation. That could be a solution acceptable for low energy houses, if a radiator is placed underneath the inlet (graphik by: ebök). In a passive house the cold air flow is not acceptable - and neither is the resulting high ventilation heat loss.

Cross-flow-ventilation of a dwelling - this design is recommended to ensure good indoor air quality by small air flows (Section: PHI)

Infrared photo of an opened heat exchanger (counterflow). The heat exchanger is the hexagon in the center. The heat recovery rate is higher than 75% of the sensible heat (Photo PHI).

These are examples of central units of ventilation systems suitable for passive houses. These and more units will be shown at the exhibition.

It is essential that the ventilation system in a passive house can provide high quality indoor air for the following reasons: A continuous exchange of sufficient air volume has to be provided even in the cold season in any new building – and that will only work using a mechanical system.

• Uncontrolled infiltration through cracks in the building envelope is inadequate (see also "airtightness“): Wind and temperature driven stack ventilation fluctuates very significantly in Central European climate (and that will hold for almost any climate). A building, that is not completely airtight might allow for sufficient infilration during calm weather but will have unacceptable drafts during periods with high wind speeds (see the first diagram left hand side). All new buildings constructed in Germany after 1984 are built such that infiltration of air will not be sufficient at all. This also holds for the refurbishment of existing buildings, if new windows have been installed.
• It should also be noted that if air can leak into a building, then warm and humid air can also migrate out of a building which can lead to moisture problems inside the construction.
• Without a mechanical ventilation system in new building, one can only try to use the strategy of opening the windows widely at regular intervals. To achieve an air change rate of 0.33 ach, one would have to open all the windows at least once every three hours for some 5 to 10 minutes at a time – even during the night (see the cartoon). This would obviously be both impractical and unacceptable in most dwellings. As a result, the indoor air quality would be poor coupled with increased humidity. It is difficult for humans to easily determine the quality of indoor air and we can not estimate the amount of air that is exchanged by opening a window. Even for an “expert” it is difficult to successfully and properly ventilate a room simply by opening windows. If the ventilation is inadequate, the indoor air quality will be unhealthy and there will be a significant risk of condensation. If the ventilation is too high, on the other hand, the air will be too dry and there will be a high energy loss.
• The function of a ventilation system is to supply fresh air in the right amount into the living space. The simplest solution is an exhaust fan ventilation system. It will extract the stale and humid air from kitchens and bathrooms. Pulled by the small under-pressure caused by the fan, fresh air will flow through inlet air valves into the living spaces. These simple systems are nowadays standard in France and in Sweden where there is experience for more than 50 years using exhaust fan ventilation systems. In Germany that could be a satisfactory solution for new constructions just meeting the requirements of the energy conservation ordinance and for refurbishment of existing buildings. But these simple systems are not suitable for Passive Houses because it is cold air, which is supplied to the rooms and therefore the heat loss will be far too high (see IR-photo at the left hand side). To maintain a high level of comfort, a relatively high peak load will be required with a radiator near the inlet valve. The resulting annual energy consumption for heating will be at least double that of a truly passive house.
• Systematic research in dwellings has shown, that for a appropriate distribution of fresh air to all rooms and an sufficient volumes of extract air from the wet rooms a controlled supply and exhaust air system is most suitable. Fresh air will be supplied to the living, working and sleeping rooms. Each of these rooms will have a supply air valve. Similarly in the exhaust fan ventilation system, the used air will be extracted from the kitchen, bathrooms and other rooms with high indoor air pollutants through extract valves.This will result in cross flow ventilation in the dwelling: The fresh air will at first enter the main living rooms (see section) from where it will flow through the overflow zones into the wet rooms. The wet rooms will have a quite high ventilation rate and therefore it will not take a long time for towels to dry. The principle of cross flow ventilation allows for an optimal utilisation of the fresh air. It will at the beginning maintain an excellent indoor air quality in the living rooms, then taking away stale air in the overflow zones (e.g. from the wardrobe in the hall) and last, but not least, dehumidify the air in the wet rooms.
• Supply- and exhaust ventilation systems open the opportunity for heat recovery from the exhaust air to the incoming fresh air. In dwellings in Central Europe the ventilation heat loss with an appropriate air change rate will be between 20 and 30 kWh/(m²a). Compared to all other heat flows in an energy efficient house that is a very high value, in fact more than the overall heat requirement in a typical passive house. There are currently available heat exchangers that achieve a 75 to 95% recovery rate for these heat losses. These highly efficient recovery units have been developed especially for the use in passive houses. This equipment has a distinct separation between exhaust and fresh air, require only a small amount of electricity to run and are extremely quiet. Using such an efficient heat recovery, the remaining ventilation losses will be negligible; between 2 and 7 kWh/(m²a) (this is a precondition for a passive house). By the way the heat recovery will increase the temperature of the supply air to a value quite near to the room air temperature. The supply air thus will not be "cold" any longer. That opens the opportunity to reduce the peak heating load and to reduce the cost for heat distribution throughout the house.
• In a passive house - and only in a passive house - there will be another advantage: The opportunity, to heat the supply air rooms by heating the supply air. Fresh air has to be supplied to living, study and sleeping rooms anyway - therefore this air can be used for heat distribution, too. But it is only fresh air (not recirculated air!), the mass flow is limited (to avoid dry air conditions) and the temperature is not allowed to exceed 55 °C. Therefore this type of fresh air heating will only work for buildings with a very low heat requirement - precisely the defining condition for a passive house. This gives the opportunity to use quite smart and space saving solutions for the building services, e.g. the ventilation compact units.

Passive houses will always use an integrated mechanical ventilation and often this is the central unit for the whole building services. Only high quality ventilation technology is suitable for passive houses. There are certain requirements for central ventilation units defined by the Passive House Institute: Not only should the efficiency of the heat exchanger be high, but the consumptions of the fans should be low, the operational conditions should clean and the unit should be very quite. At the exhibition during the 10th Conference on passive houses units and components for mechanical ventilation will be demonstrated.

(updated: 2006-09-23 author: Wolfgang Feist - thanks to Tomas O'Leary (mosart Ireland) for proof reading the translation of the 1st edition 

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