​Passive stack ventilation is a means of removing unwanted moisture from dwellings. The proper design and installation of these systems is vital to their performance. All properties require a supply of fresh air for the health and comfort of the occupants, to control condensation and to ensure the safe and efficient operation of combustion appliances. One of the most important of these is the removal, preferably at source, of moisture generated by normal household activities. One solution to the problem of unwanted moisture is to remove it from kitchens, utility rooms, bathrooms and WCs via ducts from the ceiling of each of these rooms to an outlet terminal on the roof. Such ventilation systems are commonly known as passive stack ventilation. They are suitable for houses and blocks of flats of up to four storeys.