1950s: Mechanical supply and extract air ventilation becomes common in European buildings
After the Second World War, there was a great demand for new housing and non-residential buildings. In order to reduce building costs, ceiling heights were lowered and, subsequently, the need for well-functioning ventilation systems increased. American ventilation technology, with mechanical supply and extract air solutions, was introduced in a number of countries in Europe. To save heating energy, the return air principle was used, whereby a proportion of the extract air was mixed with the fresh supply air. In wintertime, more than half of the extract air was often reused in this way. This was a cheap and very efficient solution and was used almost exclusively in non-residential buildings. However, in the early 1980s, in Sweden as well as in other countries, critical voices were raised challenging the suitability of having to breathe extract air, even if diluted with fresh air. In the USA and countries with humid climates, the return air principle is still used and is the completely dominant solution.
1960s: Mechanical supply and extract air ventilation introduced into apartment blocks
Even if mechanical extract air systems were the most common forms of ventilation solutions in housing in the 1960s, apartment blocks were sometimes built with supply and extract air solutions. However, in contrast to non-residential buildings, the return air principle was only used to a small extent.