Why a mechanical ventilation system is recommended - at least in Passive Houses

Ventilation using supply for living romms and exhaust from wet rooms

The scheme of a comfortable ventilation system. Stale air (brown) is removed permanently from the rooms with the highest air pollution. Fresh air (green) is supplied to the living rooms. (Section from the Passive House estate at Hannover Kronsberg, design by Rasch & Grenz. These row houses have been opened during the field trip of the 10th Conference on Passive Houses, Sunday, May 21st 2006)


Heat recovery by a counterflow heat exchanger

This is how a counterflow heat exchanger works: The warm air (red, extract air) flows through a channel and delivers heat to the plates. This air will leave the exchanger cooled (orange, then called exhaust air). On the opposite side of the exchanger plates the fresh air (blue) flows in separate channels. This air will absorb the heat and it will leave the exchanger with a higher temperature (but still unpolluted), then called supply air (green). The counterflow principle allows for almost 100% recovery of the temperature difference, if the exchanger is long enough. In practise, systems with 75% to 95% are available.

The health and comfort of the inhabitants are the most important objectives of a Passive House design. Excellent indoor air quality is indispensable. But this can only be achieved if stale air is exchanged with fresh outdoor air at regular intervals. This can definitely not be done by just opening windows twice a day.

Ventilation will work accurately only if polluted air is removed constantly out of kitchen, bathrooms, and all other room with significant air pollution. Fresh air has to be supplied to the living room, children’s room, sleeping rooms, and workrooms to substitute the removed air.
The system will supply exactly as much fresh air as is needed for comfort and for good indoor air quality; only outdoor air will be supplied – no recirculated air. This will lead to a high level of indoor air quality.

What has been discussed so far could be satisfied by using a simple exhaust fan ventilation system, where the air is supplied through direct vents in external walls. These vents allow fresh (cold) air to enter the room at the required rate. However, for a Passive House, the heat losses caused by such a system are much to high.

In Central Europe Passive Houses will only work with highly efficient heat recovery. Heat from the exhaust air is recovered and applied to the supply air by a heat exchanger. The air flows are not mixed in the process. State of the art ventilation systems may have heat recovery rates of 75% to more than 95%. Of course this only works with counterflow heat exchangers and very energy efficient ventilators (using so called EC-motors with extraordinarily high efficiency). With this technology the recovered heat is 8- to 15-times higher than the electricity needed.

Find more information about ventilation in a Passive House.

An additional opportunity to increase the efficiency of ventilation systems is the use of earth buried ducts. The ground during winter has a higher temperature than outdoor air, and during the summer a lower temperature than outdoor air. Therefore it is possible to preheat fresh air in an earth buried duct in winter, or to cool it in summer. This can be done directly with air ducts in the ground, or indirectly with brine circulating in earth buried pipes and heating or cooling the air with a water-to-air heat exchanger.

In the exhibition, Passive House ventilation systems can be seen: Heat recovery systems of highest efficiency, quiet and suitable to be used in dwellings.

Compact systems - the integration of the complete building services in the ventilation system. This is possible in Passive Houses.

(updated: 2006-09-23 thanks to Dylan Lamar for proof reading of the 1st edition
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