If there are no return vents, the air conditioning system would be forcing hot air in all directions. This would cause a rapid increase of the temperature in any room or area that is being cooled by the AC unit. Without any way to recycle this heat away from users, this increased temperature can cause excessive fatigue and discomfort. Additionally, since the air conditioner is only working to cool one location, this could also lead to a loss of efficiency as well as damage to materials such as furniture if not Properly Presented.