1. System Components

• Supply fan
• Return/exhaust fan (optional)
• Filter section
• Heater battery (electric or hot water coil)
• Temperature sensors
  • Return air temperature sensor (RAT)
  • Supply air temperature sensor (SAT)
  • Room/zone temperature sensor (ZT)

---

2. Control Objectives

• Maintain supply air temperature to meet room temperature setpoint.
• Protect heater from overheating or low airflow.
• Sequence operation safely.

---

3. Control Sequence

a. System Start/Stop

• AHU starts when enable signal is ON (manual switch, time schedule, or BMS command).
• On start:
  1. Supply fan starts.
  2. After airflow is proven (from differential pressure switch or airflow sensor),
     the heater is enabled.
• On stop:
  • Heater is turned OFF immediately.
  • Fan stops after a short post-run delay (e.g., 30 seconds) for cooling.

---

b. Temperature Control

• Control Mode:
  Maintain supply air temperature (SAT) at setpoint (e.g., 20 °C).
• Logic:
  • If SAT < setpoint → modulate heater output UP.
  • If SAT > setpoint → modulate heater output DOWN.
• For electric heaters:
  • Use step control (e.g., 3 stages) or phase-angle modulation (SCR).
• For hot water coils:
  • Modulate control valve (0–100%) to maintain SAT.

---

c. Safety Interlocks

• Heater operates only when:
  1. Supply fan is proven running.
  2. Airflow switch = ON.
  3. No high-temperature trip.
  4. Enable command = ON.
• Alarms / trips:
  • High temperature cut-out (manual reset).
  • Airflow failure (heater lockout).
  • Sensor fault (fail-safe → heater OFF).

---

d. Optional Room Temperature Control (Cascade Loop)

If tighter space control is needed:

1. Outer loop: Room temperature controller adjusts supply air setpoint.
2. Inner loop: Supply air temperature controller modulates heater.

Example:

• Room setpoint = 22 °C
• If room temp < 22 °C → increase supply air setpoint from 20 °C to 24 °C

4. Summary of Typical Setpoints