The answer is garage. The garage will not have an air return to the HVAC system, and when a central air duct is prolonged into a garage, it can make an irregular pressurization in there. This forces the rest of the home to become somewhat depressurized because the air in the garage can’t get back to the air return. While this change in pressure can't really be sensed. Negative pressure in your home has to be reassured somehow, and what usually happens is that your home will lure air in from outside. Outdoor air bypasses the HVAC filter system, allowing pollutants and allergens to build up inside. And it also makes it harder to keep your home cool, since there is a continuous flow of warm air coming in from outdoors. Also, tying the whole house air conditioner to the garage is the possible for dangerous fumes to enter your home via the ductwork. Whether you’re using the mower or just warming up the car, all kinds of fumes are in your garage at any given time. Some of them just smell odd, but others, like carbon monoxide from exhaust, can be deadly.
