What happens to a room that has a supply duct but no return duct?

When a room has a supply duct but no return duct, the airflow dynamics are altered significantly. The supply duct introduces fresh air into the room, which increases the air pressure within that space. This influx of air without a way for the same amount of air to exit creates a condition of pressurization. As the air pressure builds up, it can lead to the door becoming hard to close or open if there are no other leaks or vents to balance the pressure.

In contrast, an absence of a return duct means that there is no mechanism for the air to exit the room, resulting in a continual increase in pressure as more air is pushed in. Even if some air does leak out through cracks or openings, the overall effect remains that of pressurization. This condition can lead to issues with air circulation and can exacerbate humidity control, but the key takeaway is that the predominant effect is the pressurization of the room.

Other options don't accurately describe the situation: the room does not remain neutral because the introduction of air changes the pressure, and air quality may not necessarily improve if there is no effective circulation of that air. The lack of a return duct means stale air could accumulate over time, negating any potential benefits to air quality.