Balanced ventilation (system D)
With balanced ventilation (Figure 1) ventilation air is continuously blown inward and sucked outward through the ventilation ducts (mechanically). In general, air is blown into the living room and bedrooms. Areas with restrictor valves are specifically kitchens, toilets and bathrooms. By means of air transfer facilities (usually a slit under the door), the supplied air can flow to the rooms from where the air is removed. Most modern systems are equipped with a multi-position switch, with which residents are able to adjust the air discharge to a higher or lower level and thus determines the amount of air supply and discharge (rate of flow). With this ventilation system, the ingested air passes through a filter in the ventilation unit before the air is distributed any further. This filter protects the ventilation unit against impurities and cleans the air supply (the degree to which the air is cleaned depends on the quality of the filter). This type of ventilation is usually combined with heat recovery (HR). With HR, the heat from the discharged air is used to preheat the cold fresh air (the air flows will not directly have contact with each other). As a result, less energy is required for heating in the wintertime. Obviously, in the summertime, heat recovery is not necessary. Therefore, modern ventilation systems are equipped with a bypass with which the supplied air in hot weather is diverted around the HR unit to prevent unwanted heating.
Basic diagram of a ventilation system with mechanical supply and discharge (balanced ventilation) (blue is supplied air, red is discharged air). The ventilation system described is usually implemented as a central system, with which the ventilation unit for the entire house is connected to a one-channel system.