A return duct in a home's attic becomes disconnected and pulls 100% of its air from outside the home. What is the impact on the home?

When a return duct in a home's attic becomes disconnected and begins to pull 100% of its air from outside, it alters the air pressure dynamics within the home. This disconnection can lead to a scenario where the home is pressurized due to the influx of outside air entering through the return duct.

In typical HVAC systems, return ducts are designed to circulate the air within the home, creating a balanced airflow by recirculating conditioned air back to the system. However, when the return duct pulls air entirely from the outside environment, it introduces air that has not been conditioned (either heated or cooled) into the home. This can create an imbalance where the incoming outside air exerts pressure on the interior of the home, leading to pressurization.

In such a situation, it's important to note that the home can experience issues such as increased energy inefficiency, drafts, and discomfort due to the unconditioned nature of the incoming air. Properly sealed and balanced duct systems are essential in maintaining the desired pressure and comfort levels within a building.

Thus, the overall impact of a return duct pulling air entirely from outside is an increase in indoor pressure, affirming that the home will become pressurized.