2.4.2 Mechanical ventilation
Mechanical ventilation systems use electric fans to direct the airflow in the building. Mechanical ventilation can provide a constant air change rate independently of external weather conditions, but it uses electricity and usually cannot change the ventilation rate as the need changes over the day and year.
Several variations exist, as illustrated in Figure 2.4.1. Systems with both supply and extract can be combined with a heat recovery unit, which recovers (reuses) the heat of the extract air that would otherwise be lost. Up to 90% of the energy can be ‘reused’.
It is becoming a standard solution in many North European countries for newbuilt houses to be provided with mechanical heat recovery ventilation in order to meet current energy requirements. This is a very energy efficient solution for the heating (winter) season. However, in the summer season, electricity for running of fans can be saved by using natural ventilation. Systems shifting between natural ventilation and mechanical ventilation are called hybrid ventilation systems.
Mechanical ventilation requires filters to be changed regularly. Dirty filters are a source of pollution of the indoor air and reduce indoor air quality, which, in turn, reduces the performance of the occupants of the building and increases the prevalence of SBS symptoms (Wargocki et al., 2002; Bekö, 2009).
It has been found that SBS symptoms occur more frequently in buildings with air conditioning than in naturally ventilated buildings (Wargocki et al., 2002). If a mechanical ventilation system with heat recovery is to perform energy efficiently, the building must be perfectly airtight. If it is not, a substantial part of the ventilation will come from infiltration, which bypasses the heat exchanger. So mechanical ventilation with heat recovery is often not an energy-correct solution for existing buildings – unless they are made more airtight.
Mechanical ventilation systems can be central or decentral. Central systems have one central unit, with supply and extract fans; if the system has heat recovery, the heat recovery unit is included in the central unit. Ventilation ducts are installed from the unit to most rooms of the house. Decentral ventilation does not use ducts; instead, small units, which can include heat recovery, are installed in individual rooms of a house. Such a system has the advantage of not requiring space for ducts.