Question 25 CEL01 - Chief Engineer - Limited

The Correct Answer is B **Explanation for Option B being Correct:** Option B accurately describes the standard operation and control mechanism for the dampers in a basic single-zone HVAC system utilizing pneumatic controls for economizer function: 1. **Normally Closed/Normally Open States (Fail-Safe Position):** The system is designed to minimize the introduction of unconditioned outside air when the controls fail or are off. Therefore, the **Exhaust (EA) damper** and the **Outside Air (OA) damper** are typically wired/piped to be **Normally Closed (NC)**, ensuring they seal shut when no pilot air is present. Conversely, the **Recirculation (RA) damper** must remain open to allow the fan to draw return air from the zone, making it **Normally Open (NO)**. 2. **Single Pilot Air Signal (Linked Control):** In a standard economizer setup designed for minimum ventilation and free cooling, the OA, EA, and RA dampers operate in tandem. They are modulated simultaneously by a single controlling signal (often referred to as the pilot air signal) originating from the temperature controller (or an economizer logic panel). As the pilot air pressure increases, the NC dampers (OA and EA) open, and the NO damper (RA) closes, maintaining a consistent air volume into the unit. **Why the Other Options Are Incorrect:** * **A) The exhaust and outside air dampers are normally open and the recirculation damper is normally closed and each damper is controlled by its own pilot air signal.** This is incorrect. Having the OA and EA dampers normally open is inefficient and defeats the fail-safe goal of minimizing outside air intake. Also, these three dampers are typically linked to a single signal, not controlled individually. * **C) The exhaust and outside air dampers are normally open and the recirculation damper is normally closed and all three dampers are controlled by a single pilot air signal.** This is incorrect because the outside air (OA) and exhaust air (EA) dampers are normally closed, and the recirculation air (RA) damper is normally open for fail-safe operation. * **D) The exhaust and outside air dampers are normally closed and the recirculation damper is normally open and each damper is controlled by its own pilot air signal.** This is incorrect because while the normally open/closed positions are correct, the dampers are linked and controlled by a single pilot air signal for synchronized economizer modulation, not individual signals.