The process of processing or replacing air in any space to provide high indoor air quality (i.e. to control temperature, replenish oxygen, or remove moisture, odors, smoke, heat, dust, airborne bacteria and carbon dioxide).
A ventilation system is a mechanical structure of connected devices that controls airflow within confined spaces, commonly homes and offices. Its main function is to introduce a constant supply of fresh air, usually from the outside, while channeling stale air back out. @georgesplane-A heat recovery ventilation system is a good idea for a colder climate, but it requires an overhaul of the entire system. That would probably be something better for a new build, or if the building is in need of new HVAC units. Air infiltration and exfiltration: In addition to intentional ventilation, air inevitably enters a building by the process of ‘air infiltration’. This is the uncontrolled flow of air into a space through adventitious or unintentional gaps and cracks in the building envelope. Purpose provided (intentional) ventilation: Ventilation is the process by which ‘clean’ air (normally outdoor air) is intentionally provided to a space and stale air is removed. Definitions covering ventilation and the flow of air into and out of a space include: Purpose provided (intentional) ventilation: Ventilation is the process by which ‘clean’ air (normally outdoor air) is intentionally provided to a space and stale air is removed. Ventilation. Movements of the ribs, rib muscles and diaphragm allow air into and out of the lungs. Take care-this is called breathing or ventilation, not respiration. When we breathe in, we inhale. When we breathe out, we exhale. Air passes between the lungs and the outside of the body through the windpipe, called the trachea Ventilating (the V in HVAC) is the process of processing or replacing air in any space to provide high indoor air quality (i.e. to control temperature, replenish oxygen, or remove moisture, odors, smoke, heat, dust, airborne bacteria and carbon dioxide). Ventilation includes both the exchange of an air to the outside as well as circulation of air within the building. It is one of the most important factors for maintaining acceptable indoor air quality in buildings. Ventilation is the mechanical system in a building that brings in fresh outdoor air and removes the contaminated indoor air. In a workplace, ventilation is used to control exposure to airborne contaminants.  Natural ventilation is the ventilation of a building with outside air without the use of a fan or other mechanical system. It can be achieved with openable windows or trickle vents when the spaces to ventilate are small and the architecture permits. Ventilation includes both the exchange of an air to the outside as well as circulation of air within the building. It is one of the most important factors for maintaining acceptable indoor air quality in buildings. Guidelines: Roof Ventilation. Ventilation provides the conditions that allow air to flowout with the hot air and in with the cool air. When stale, overheated air in your home or attic is vented out and fresh air is pulled in you have an air exchange.. Effective ventilation requires a balancing act. 
http://www.wisegeek.com/what-is-a-ventilation-system.htm http://www.aivc.org/resources/faqs/what-ventilation http://www.aivc.org/resources/faqs/what-ventilation http://www.bbc.co.uk/bitesize/ks3/science/organisms_behaviour_health/life_processes/revision/4/ https://en.wikipedia.org/wiki/Ventilation_(architecture) http://www.ccohs.ca/oshanswers/prevention/ventilation/introduction.html https://en.wikipedia.org/wiki/Ventilation_(architecture) http://www.aussieroofing.com/ventilation.htm
what is a ventilation system