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Plenty of steam around extractors

In a new study, the Passive House Institute focuses on systems for vapour extractors, with the research report also resulting in a handbook for extractor hoods in Passive House buildings. This contains the key principles for a compatible system and its dimensioning. In view of greater energy efficiency of buildings and increased demand for thermal comfort by occupants, the scientific focus has shifted to extractor hoods particularly in airtight buildings. Oliver Kah and Kristin Bräunlich of the Passive House Institute evaluated the various extractor hood systems together with the ITG Institute for Building Systems Engineering in Dresden, the IHD Dresden Institute for Wood Technology, and the company Naber GmbH. The interaction of the extractor systems with the building was also studied.

Wall mounted hoods take precedence

"The design of the extractor hood and its position in the room have considerable influence on the amount of vapours collected and the volume flow rate that is necessary for this purpose", explains Oliver Kah of the Passive House Institute. The results indicate for example that already at lower operating stages the amount of fumes captured by extractor hoods mounted on the wall (recirculation air and extract air) is comparable to that of cooktop extractors.

Circulation air is preferable

In buildings with a very small heating demand, such as Passive Houses, the Passive House Institute recommends the installation of recirculation air hood systems. Exhaust air hood systems may increase the heating demand of energy efficient buildings significantly; in addition, it is usually easier to integrate recirculation air systems. The tests also showed that in combination with a home ventilation system, the results achieved by good recirculation air hood systems are almost similar to those obtained with exhaust air systems with reference to the reduction of odours.

Removal of moisture

Unlike exhaust air extractor hoods however, recirculation air hoods do not remove the moisture arising during cooking. Basic ventilation is therefore recommended for the adequate removal of moisture in kitchens. "In Passive House buildings and frequently also in other energy efficient buildings, controlled ventilation constitutes a part of the building concept in any case. Thus adequate basic air exchange is already ensured in these buildings, due to which the moisture loads are removed", explains Kristin Bräunlich of the Passive House Institute.

Hardly any cost differences

There are hardly any differences when considering the overall costs for the exhaust air and recirculation air systems: while annual costs for replacing the active carbon filter are incurred with the recirculation air systems, exhaust air systems cost slightly more due to the installation costs and incur higher heating energy costs caused by the additional ventilation heat losses.

Essential for exhaust air hood systems

For buildings with extremely low heating demands such as Passive House buildings, the Passive House Institute recommends the installation of recirculation air hood systems. However, if an exhaust air system is installed, then attention should be given to the following points:

o Solutions should be provided for incoming air flow.
o The exhaust air outlet and air inlet must be equipped with airtight seals. Non-return valves are usually insufficient.
o Preference should be given to systems which ensure adequate air capture using moderate exhaust air flows.
o In smaller homes the additional ventilation heat loss may increase the specific area- related heating demand and also the heating load significantly. Exhaust air extractor hood systems are therefore not advisable in buildings with mainly smaller dwelling units.

This research on extractor hood systems in energy efficient buildings was funded by the research initiative "Future Building" of the German Federal Institute for Research on Building, Urban Affairs and Spatial Development (reference number : SWD-10.08.18-7-17.27).

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