Question 63 3AE02 - Third Assistant Engineer (Alt)

The Correct Answer is C **Explanation of why Option C is correct:** In a single-zone HVAC system that incorporates both temperature (thermostat) and humidity (humidistat) control, a diverting relay (or changeover relay) is often used to establish a priority sequence for the control loops. When cooling or dehumidification is required, both functions typically rely on activating the cooling coil and manipulating the air dampers (outside air, return/recirculation, exhaust). The standard control strategy prioritizes humidity control (dehumidification) over temperature control (cooling) because high humidity is often damaging or uncomfortable, and cooling inherently provides some dehumidification. 1. **If the space humidity is below the humidistat setpoint:** Humidity is acceptable. The system operates primarily to maintain the desired temperature. Therefore, the diverting relay directs the **room thermostat control signal** to modulate the cooling coil valve and the dampers (O.A., R.A., E.A.). 2. **If the space humidity exceeds the humidistat setpoint:** Dehumidification is required. This becomes the overriding control priority. The diverting relay switches the control signal source and directs the **room humidistat control signal** to modulate the cooling coil valve and the dampers, running the coil primarily for dehumidification until the setpoint is met, regardless of minor temperature deviations. Option C perfectly describes this standard humidity-priority control logic managed by a diverting relay. **Why the other options are incorrect:** * **A) The diverting relay processes the humidistat control signal as long as the space humidity is below the humidistat setpoint...** This is incorrect. If humidity is below the setpoint, the system focuses on temperature control (thermostat signal), not humidity control (humidistat signal). * **B) The diverting relay processes the humidistat control signal as long as the space temperature is below the thermostat setpoint...** This is incorrect because the diverting relay in this application switches based on the **humidity level** (the priority variable), not the temperature level. Temperature is typically the secondary control loop when switching priority. * **D) The diverting relay processes the room thermostat control signal as long as the space temperature is below the thermostat setpoint...** This is incorrect for the same reason as Option B. The switch between primary control signals (Thermostat vs. Humidistat) is determined by whether the humidity setpoint is satisfied, not whether the temperature setpoint is satisfied.