Fresh Air Handling Unit (FAHU) BAS Monitoring Points and Control Logic

1. Typical FAHU BAS Architecture

A Fresh Air Handling Unit normally contains or interfaces with the following equipment and field devices:

• Supply air fan
• Fresh air / outdoor air damper
• Cooling coil
• Heating coil, where required
• Motorized chilled-water or hot-water valve
• Supply air temperature sensor
• Heating/cooling coil return water temperature sensor
• Filter differential pressure switch
• Freeze protection thermostat
• Programmable DDC controller
• BAS/BMS supervisory platform

The DDC controller receives digital and analog signals from field devices, executes the programmed HVAC control logic, and sends control commands to the fan, water valve, and fresh air damper.

Depending on the project, the controller can communicate with a supervisory BAS/BMS using protocols such as BACnet/IP, BACnet MS/TP, Modbus TCP, or Modbus RTU.

2. FAHU BAS Monitoring Points

The Building Automation System should continuously monitor the main operating conditions of each Fresh Air Handling Unit.

2.1 Digital Input Monitoring — DI

2.2 Analog Input Monitoring — AI

3. FAHU BAS Control Points

3.1 Digital Outputs — DO

3.2 Analog Outputs — AO

4. FAHU Fault and Alarm Functions

4.1 Fan Fault Alarm

The BAS should continuously monitor the operating status of each FAHU fan.

When the unit receives a start command but the expected fan running status is not confirmed, or when a fan fault signal is detected, the BAS should generate a FAHU Fan Fault Alarm.

This allows facility operators and maintenance personnel to quickly identify abnormal equipment operation.

4.2 Freeze Protection Alarm

For FAHUs equipped with heating coils, freeze protection is particularly important because outdoor air is directly introduced into the unit.

When either condition occurs, the BAS should generate a freeze protection alarm and immediately execute the required safety interlock sequence.

5. Historical Data Recording

The BAS should record important FAHU operating data for maintenance, fault analysis, performance evaluation, and energy management.

Typical historical trend data includes:

• FAHU operating status
• Supply air temperature
• Coil return water temperature
• Supply air temperature setpoint

The BAS should also record fault alarm information, including:

• Alarm description
• Alarm type
• Time of occurrence

6. FAHU Operating Mode Selection

The FAHU supervisory interface should allow centralized management of all Fresh Air Handling Units.

Typical centralized functions include:

• One-click start of multiple FAHUs
• One-click stop of multiple FAHUs
• Unified supply air temperature setpoint adjustment
• Centralized operating mode switching

The system may provide three main operating modes:

1. Subsystem Manual Mode
2. Subsystem Schedule Mode
3. Cloud / Supervisory Control Mode

6.1 Subsystem Manual Mode

When the system operates in Subsystem Manual Mode, subsystem scheduling and cloud supervisory control are disabled.

The user should be able to manually operate each FAHU from the subsystem control interface, including:

• Start or stop individual FAHU fans
• Adjust supply air temperature setpoints
• Adjust indoor pedestrian-street CO₂ concentration setpoints, where applicable

During system commissioning, authorized engineers should also be able to manually override the motorized water valve opening.

6.2 Subsystem Schedule Mode

When operating in Subsystem Schedule Mode, the BAS can automatically start and stop Fresh Air Handling Units according to configured time schedules.

Typical schedules may include:

• Pre-occupancy startup
• After-hours shutdown
• Weekday schedules
• Weekend schedules
• Holiday schedules

For FAHUs serving indoor pedestrian streets or similar public areas, operators may select either:

• Time Schedule Control, or
• CO₂-Based Automatic Start / Stop Control.

The BAS should transmit the relevant operating status points and logic selection points to the cloud or supervisory platform.

6.3 Cloud / Supervisory Control Mode

When the system operates in Cloud / Supervisory Control Mode, the FAHU can operate according to authorized commands issued by a higher-level supervisory or cloud platform.

Typical supervisory commands may include:

• FAHU start / stop
• Operating mode selection
• Supply air temperature setpoint adjustment
• CO₂ setpoint adjustment

The local DDC controller remains responsible for executing the actual HVAC control logic and safety interlocks.

7. FAHU Supply Air Temperature Control Logic

The FAHU automatically adjusts the motorized chilled-water or hot-water valve opening according to the difference between the measured supply air temperature and the configured supply air temperature setpoint.

In practical DDC applications, this function can be implemented using proportional or PID control.

7.1 Summer Cooling Operation

When supply air temperature rises above the setpoint, the controller increases chilled-water flow through the cooling coil. When supply air temperature falls below the setpoint, the valve opening is reduced.

7.2 Winter Heating Operation

During winter operation, the control direction is reversed.

7.3 Transitional-Season Operation

During suitable transitional-season ventilation conditions, mechanical heating or cooling may not be required.

The FAHU can continue supplying outdoor air while avoiding unnecessary chilled-water or hot-water consumption.

8. FAHU Shutdown Logic

When the Fresh Air Handling Unit stops, the system should first close the fresh air damper.

This prevents uncontrolled outdoor air from entering the building when the supply fan is not operating.

The water valve position should then be controlled according to the operating season:

9. CO₂-Based Automatic FAHU Start / Stop Control

For indoor pedestrian streets, shopping mall corridors, and other areas with variable occupancy, indoor CO₂ concentration can be used as an indicator of ventilation demand.

The BAS compares the measured indoor CO₂ concentration with the configured CO₂ setpoint.

9.1 High CO₂ Condition

This increases the amount of outdoor air supplied to the corresponding indoor area.

9.2 Low CO₂ Condition

The ±100 ppm difference creates a control deadband and helps prevent frequent FAHU start/stop cycling around the CO₂ setpoint.

9.3 CO₂ Control Example

For example, if the indoor CO₂ setpoint is:

The control logic would be:

10. Fresh Air Damper and Fan Interlock

The FAHU fan and fresh air damper should operate with an interlock sequence.

A typical startup sequence is:

Where damper status feedback is provided, the controller can verify that the fresh air damper is open before allowing the fan to start.

When the FAHU stops:

This interlock helps prevent uncontrolled outdoor air infiltration when the FAHU is not operating.

11. FAHU Freeze Protection Interlock

Freeze protection should have a high priority in the FAHU control sequence.

Because a Fresh Air Handling Unit directly handles outdoor air, a heating coil may be exposed to very low air temperatures during cold weather.

When a freeze protection condition is detected, the BAS should immediately execute the following sequence:

1. Stop the FAHU fan.
2. Close the fresh air damper.
3. Generate a freeze protection alarm.
4. Fully open the heating-water control valve.

This sequence prevents additional cold outdoor air from entering the FAHU and maximizes hot-water circulation through the heating coil.

12. Typical FAHU BAS I/O Point Summary

13. Why Proper FAHU BAS Control Logic Matters

A Fresh Air Handling Unit is responsible for introducing and conditioning outdoor air, so its control strategy is directly affected by outdoor temperature and climate conditions.

A properly designed FAHU Building Automation System can provide:

• Stable supply air temperature
• Reliable outdoor air ventilation
• Improved indoor air quality
• Automatic CO₂-based ventilation control
• Reduced unnecessary fan operation
• Improved HVAC energy efficiency
• Automatic filter alarm monitoring
• Fan fault detection
• Centralized BAS/BMS operation
• Historical operating data
• Automatic freeze protection
• Easier maintenance and troubleshooting

Freeze protection is particularly important for FAHUs operating in cold climates because the heating coil may be directly exposed to extremely cold outdoor air.

14. FAHU vs. AHU BAS Control

Fresh Air Handling Units and conventional Air Handling Units use many of the same BAS components, but their operating strategies are different.

Because a FAHU directly handles outdoor air, fresh air damper interlocking, supply air temperature control, and freeze protection are particularly important parts of its BAS sequence of operation.