You can’t change physics – but you can change conditions
New facts on the theme of extractor hoods in the kitchen
The underlying study was carried out by the Institut für Holztechnologie (Institute for Wood Technology) and the Institut für Technische Gebäudeausrüstung (Institute for Building Systems Engineering Research and Application), both in Dresden, and the project management of the Passivhaus Institut (Passive House Institute). The kitchen accessories manufacturer Naber GmbH, suppliers of extractor systems and a kitchen furniture manufacturer were involved as technology partners. The study was funded by the Federal Institute for Research on Building, Urban Affairs and Spatial Development, which belongs to the Federal Office for Building and Regional Planning.
The decisive factor is the volume of air conveyed
In order to extract the odours, grease and moisture which are generated during cooking as much as possible, the air volume flow, i.e. the amount of room air conveyed, is decisive for the cleaning performance. This volume of air not only depends on the electric power of the extractor, but also to a large extent on its position and design. Wall-mounted extractor hoods with the right distance to the hob demonstrate the best performance. Head-free hoods and free-hanging island hoods extract the vapours less reliably. Initial tests have shown that hob extractors which suck the vapours down as cooktop or downdraft extractors need a significantly higher air capacity than is required for other types of ventilator. In addition, the measurements showed that even at top power, hob extractors were not able to fully capture the vapours from high pots.
Planning of exhaust air and circulating air systems
The guideline developed from the study contains important foundations as decision aids for an exhaust or circulating air system. Since circulating air systems do not extract moisture loads and the steam generated when cooking stays in the home, the authors recommend combination with a sufficiently dimensioned home ventilation system or additional airing via the windows. The filters of a circulating air system increase the pressure loss. As a result, higher air capacities are required for the same performance. In particular, in order to dry the charcoal filter which is responsible for odour reduction, circulating air systems also require a longer after-run time. The extraction of vapours in the base area of the kitchen cabinet which is often implemented with hob extractors, is expressly not recommended, as this can lead to damage through damp and mould. The alternative is air duct systems which guide the filtered air out of the base area, or an exhaust air system.
For exhaust air systems, the authors recommend wall conducts which are airtight when closed. The guide also provides tips for the dimensioning of the required fresh air supply flow.
Interaction with ventilation systems and single room furnaces
Energy-efficient buildings are regularly equipped with home ventilation systems. The guide provides important tips on this. There are no technical requirements under building law - both exhaust air and circulating air systems can be used. Integration of the extractor system in the home ventilation is not recommended by the authors for reasons of fire protection, hygiene and the ventilation design. The guide also contains detailed instructions for the installation of extractor systems in efficiency houses. Circulating air technology is only recommended for the highest standards (passive houses and efficiency house 40), and especially for small rooms. In existing or new buildings, according to the Energy Saving Ordinance and an Efficiency House 55, planners and buyers are able to choose.
If a hearth or other open fireplace is to be operated together with an extractor hood, the authors generally recommend approved fireplaces which are independent of room air. For exhaust air systems, window contact switches or automatic air supply boxes are required to eliminate the risk of critical negative pressures and flue gases.
The guide, which clearly summarises the wealth of detailed information from the study, can be downloaded for free at compair-flow.com.
New facts on the theme of extractor hoods in the kitchen – 03.04.2019
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