Fresh air is fed directly from outside into the ventilation system initially through a filter, then the heat taken from the extracted air is used to warm the fresh filtered air in the heat exchanger and then enters the ducting system. By continuously supplying preheated air into living areas and extracting contaminated air from wet rooms, you create a whole house ventilation system. Air is allowed to circulate from supply air rooms to rooms with extract via undercuts on all internal doors and / or by the use of transfer grilles.