There several key differences between residential and large commercial HVAC systems. One the central differences is in how the two handle outside or “fresh” air as it interacts with the internal system. Many small residential central HVAC systems work in a closed loop where a fan forces air in a circle through one end of the ventilation system, through the rooms in the house, and then back into the other side of the ventilation ducts where it’s sent to the air handler to be chilled again. Although a minimal amount of fresh outdoor air is needed to keep the pressure equalized as the air circulates, in most homes enough air leaks in through door and window cracks to fulfill this need. In larger buildings with a sizable number of occupants, a percentage of the air circulating through the heating and cooling system must be drawn from an outside source and is determined based on the average number of occupants in the building daily. If left alone, oxygen would not replenish in the indoor air fast enough as it is being consumed by the occupants and then being transformed into carbon dioxide. Pulling in fresh oxygen from outside air prevents these effects. However, there exist home heating and cooling systems that utilize a similar effect if the homeowner wishes to ventilate the house without using unfiltered, dusty air from open windows or doors. I have a similar intake valve on my HVAC system, but I have a serious problem with tree frogs crawling inside and blocking the airflow. My HVAC technician had to install a special metal netting over the opening to prevent critters from getting inside. The last thing you want to smell is fried tree frog caked all throughout the insides of you ductwork and air handler as you have your AC blowing at full speed.