Ventilation is important in passive solar design to maintain good quality indoor air (air that is not too humid, too dry or smelly). Often, this is achieved using the "open window" method, but at present far better alternatives are present in the form of natural ventilation and heat recovery ventilation.
Air source heat pumps or heat recovery ventilators can allow for greater ventilation while minimizing heat loss.
Natural ventilation
Natural ventilation is useful for new buildings (e.g. earthships) and (old) appartment buildings. This as the buildings need to lend themselves to this and thus often need to be made specifically for this ventilation method.
Heat recovery ventilation
Heat recovery ventilation is useful for improving most other old buildings. Before implementing a heat recovery ventilation system, it is best to first perform an energy audit and plug any holes that were found troughout your home in the process. See Ultra-low_energy_house#Plugging_holes