What is the function of the RA damper D-3?

What is the function of the RA damper D-3?

• A. To modulate and maintain pressure difference across D-4.
• B. To control outdoor air temperature.
• C. To bypass the cooling coil.
• D. To close the return air path.

Answer: (A)

Pressure-differential control between dampers is a common way to keep an air-handling system balanced. The return-air damper D-3 modulates to hold a specific pressure difference across the downstream damper D-4. By adjusting how much return air flows, D-3 ensures the pressure on either side of D-4 stays within its design range, so the downstream damper operates predictably and the overall air balance and energy performance are maintained. This isn’t about controlling outdoor air temperature, which would be the role of the outdoor-air or mixing dampers; nor is it about bypassing the cooling coil or simply closing the return path, which would disrupt airflow and system operation.

Pressure-differential control between dampers is a common way to keep an air-handling system balanced. The return-air damper D-3 modulates to hold a specific pressure difference across the downstream damper D-4. By adjusting how much return air flows, D-3 ensures the pressure on either side of D-4 stays within its design range, so the downstream damper operates predictably and the overall air balance and energy performance are maintained.

This isn’t about controlling outdoor air temperature, which would be the role of the outdoor-air or mixing dampers; nor is it about bypassing the cooling coil or simply closing the return path, which would disrupt airflow and system operation.