Ducts are the primary means of moving air through the rooms in a house ventilation system. Typically, they are hidden in the floor, interior walls, and ceiling, but they may be exposed. In some cases, a house ventilation system is designed for air to flow through walls and doors as well, and there are specialty products to ensure air movement through these “transfer zones” between rooms where fresh air is introduced and rooms where stale air is removed (see Miscellaneous Products).
Dampers may optimize flow through ducts during certain periods (for example, greater exhaust air flow from bathrooms or kitchens on demand and greater supply air flow when there are many people in a living or dining room).
Ducts are only part of the fresh air distribution system. Registers and valves (adjustable registers) connect the ducts to the rooms they serve. These registers may disperse fresh air at low velocity throughout a room or direct it to a precise location within the room.
Sometimes these registers supplying fresh air are called “diffusers”, and registers removing stale air are called “returns.” However, the “returns” do not function like the return registers of old forced air heating systems. In a Passive House, the stale air does not return to the furnace where it is reheated and then directed back to rooms in the home. Rather a Passive House ventilation system extracts stale air from bathrooms, kitchens, and other rooms where moisture and contaminants are introduced and then ejects it from the home. Passive House buildings do not recycle contaminated air. The stale air ejected from the house is replaced with an equivalent volume of fresh air drawn from outside the home by the heat/energy recovery ventilator.
Some diffusers supply fresh air at high velocity in order to ensure the fresh air mixes with air already within the room. Or in the case of “throwing diffusers” that fresh air is thrown across a room to reach distant locations.