What is the condition for return air from a room or space related to the supply air flow rate?

In HVAC systems, the condition for return air from a room or space in relation to the supply air flow rate is critical for maintaining balanced air pressure and efficient system performance. The correct condition is that the return air must not be greater than the supply air flow rate.

This requirement helps to ensure that the HVAC system maintains proper airflow and pressure relationships within the space. If the return air were to exceed the supply air, it could create negative pressure within the room, leading to several potential issues, such as drafts, discomfort, and even ineffective ventilation. Positive pressure can assist in preventing the infiltration of unconditioned outdoor air and helps to maintain the air quality by ensuring that the system can adequately recycle and treat the air being returned.

Additionally, maintaining a situation where return air is not greater than supply air helps in energy efficiency. A well-balanced system allows for proper conditioning of the air, reducing the workload on heating and cooling equipment, thus contributing to lower energy costs.

In summary, having a balance where return air does not exceed supply air flow rate is essential to the overall efficacy, comfort, and efficiency of the HVAC system.