Application Example: Swimming Pool AHU

 

This control philosophy outlines the operation of an Air Handling Unit (AHU) for a swimming pool environment. The AHU ensures optimal control of temperature, humidity, and ventilation, focusing on energy efficiency and operational stability. It uses inlet, recirculation, and outlet dampers to manage airflow dynamically, prioritizing recirculation and Free Cooling to reduce energy demands.

 

System Overview

 

The AHU operates to maintain poolside air temperature at 1°C above water temperature (typically 28–30°C during occupied hours) and relative humidity (RH) between 50–60% when occupied and 55–65% during unoccupied hours. The system achieves this through staged heating, staged dehumidification, and dynamic damper modulation. The inlet and recirculation dampers work together to balance fresh and recirculated air, while the outlet damper remains fully open to ensure consistent exhaust airflow.

 

Occupied Mode

 

During occupied hours, the AHU ensures comfort and proper air quality by maintaining precise control of temperature, humidity, and ventilation.

 

For temperature control, recirculation heating is prioritized to retain internal heat. The recirculation damper remains fully open while the inlet damper stays at a minimum position of 10% to provide fresh air. If recirculation alone cannot maintain the temperature, the heating coil modulates to provide additional heating.

 

For humidity control, the system employs staged dehumidification. When external conditions are favorable, the inlet damper opens incrementally to bring in drier air for Free Cooling, while the recirculation damper closes proportionally to prioritize fresh air intake. If Free Cooling is insufficient, the cooling coil activates to remove moisture. After dehumidification, reheating is achieved by prioritizing recirculated air blending first. If the return air is not warm enough, the heating coil modulates to reheat the supply air to the setpoint.

 

Ventilation is maintained dynamically. The inlet damper provides a baseline fresh air supply of at least 10%, while the recirculation damper modulates to balance airflow. Fan speeds are adjusted using Demand-Controlled Ventilation (DCV) principles, ensuring airflow aligns with real-time demand. The outlet damper remains fully open to exhaust air.

 

Unoccupied Mode

 

When the pool is unoccupied and typically covered, the AHU transitions to a setback mode to conserve energy. The poolside air temperature is relaxed to 26–27°C, and the RH target is widened to 55–65%.

 

The recirculation damper fully opens to maximize internal heat retention while the inlet damper closes to eliminate unnecessary fresh air intake unless required for humidity control or Free Cooling. If the setback temperature cannot be maintained through recirculation, the heating coil modulates to prevent condensation or maintain the target temperature.

 

For humidity control, the cooling coil activates if RH exceeds 70%, ensuring the space remains within acceptable levels. Free Cooling is used when outdoor conditions allow, with the inlet damper opening incrementally to admit fresh air and the recirculation damper closing proportionally.

 

Fans operate at reduced speeds to minimize energy use while maintaining sufficient airflow to prevent stagnation. The outlet damper remains fully open to ensure proper exhaust airflow.

 

Free Cooling

 

Free Cooling is utilized whenever outdoor conditions are favorable to reduce temperature or humidity. The inlet damper opens incrementally, bringing in external air when the outdoor temperature is within ±2°C of the setpoint and the RH is below 60%. To optimize energy efficiency, the recirculation damper closes proportionally but is never fully shut to allow blending of warmer return air if required. This minimizes the need for reheating. Fans increase speed as needed to support the additional airflow.

 

If the supply air temperature drops below the setpoint after Free Cooling, reheating is managed dynamically. The system prioritizes recirculation to reheat the air. If return air blending is insufficient, the heating coil activates to restore the temperature.

 

Energy Saving Features

 

The AHU incorporates multiple strategies to minimize energy consumption. Staged heating and cooling prioritize recirculation and Free Cooling before activating the heating or cooling coils. Supply air temperature is adjusted dynamically to prevent overconditioning, and a night purge feature introduces cooler outdoor air during mild seasons to precondition the space. Fan speeds are modulated dynamically using DCV principles, ensuring airflow matches real-time demand without excess energy use. During unoccupied hours, setback targets for temperature and humidity further reduce energy consumption.

 

Monitoring and Alarms

 

The AHU continuously monitors all critical components to ensure efficient operation and early fault detection. Differential pressure sensors track filter performance and notify operators when airflow resistance increases, allowing for timely maintenance. Damper positions are monitored to ensure they respond to commands and do not cause airflow imbalances. High and low temperature and humidity alarms are triggered if conditions deviate from set ranges. Fan energy consumption is tracked against benchmarks to detect inefficiencies, with alarms raised if motors exceed expected thresholds. Hand/Off/Auto (HOA) switch positions are also monitored, triggering alarms if devices are left in manual mode, which may override automated control and waste energy.

Programming Tips

Below is a breakdown of each condition described in the control philosophy, presented in plain English for easier integration into software logic. Each condition is described individually, and the corresponding device actions are outlined.

 

Condition 1: Maintain Poolside Temperature During Occupied Hours

Trigger: Poolside temperature is below the setpoint (1°C above pool water temperature, typically 28–30°C).

 

Actions:

• Open the recirculation damper fully to retain internal heat.
• Keep the inlet damper at a minimum of 10% open to provide fresh air.
• If the recirculation damper cannot maintain the setpoint, modulate the heating coil to provide additional heating.
• Ensure the outlet damper remains fully open for constant exhaust airflow.

 

Condition 2: Staged Dehumidification During Occupied Hours

Trigger: Poolside relative humidity (RH) exceeds 60%.

Actions:

• Stage 1 (Free Cooling): Incrementally open the inlet damper to admit drier outside air (external RH <60% and temperature within ±2°C of the setpoint).
• Close the recirculation damper proportionally as the inlet damper opens to prioritize fresh air intake.
• Stage 2 (Active Cooling): If Free Cooling is insufficient to control RH, modulate the cooling coil to remove moisture.
• Increase fan speeds as required to enhance airflow and moisture removal.
• After dehumidification, reheat the supply air by prioritizing recirculation. If recirculation is insufficient, activate the heating coil to maintain the temperature setpoint.

 

Condition 3: Ventilation During Occupied Hours

Trigger: Occupied mode is active.

 

Actions:

• Keep the inlet damper at a minimum of 10% open to ensure baseline fresh air supply.
• Modulate the recirculation damper dynamically to balance fresh and recirculated air based on temperature and humidity demand.
• Keep the outlet damper fully open for consistent exhaust airflow.
• Adjust fan speeds using Demand-Controlled Ventilation (DCV) to match real-time demand.

 

Condition 4: Maintain Poolside Temperature During Unoccupied Hours

Trigger: Poolside temperature is below the setback range (26–27°C).

Actions:

• Fully open the recirculation damper to maximize internal heat retention.
• Close the inlet damper unless required for Free Cooling or humidity control.
• Modulate the heating coil to prevent condensation or maintain the setback temperature.
• Ensure the outlet damper remains fully open for exhaust airflow.

 

Condition 5: Staged Dehumidification During Unoccupied Hours

Trigger: Poolside RH exceeds 70%.

Actions:

• Stage 1 (Active Cooling): Modulate the cooling coil to remove moisture.
• Stage 2 (Free Cooling): If external conditions are favorable (external RH <60% and temperature within ±2°C of the setback temperature), incrementally open the inlet damper to admit fresh air.
• Close the recirculation damper proportionally as the inlet damper opens.
• Operate fans at reduced speeds to conserve energy while ensuring sufficient airflow.

 

Condition 6: Free Cooling

Trigger: External conditions are favorable (external RH <60% and temperature within ±2°C of the setpoint).

Actions:

• Incrementally open the inlet damper to admit fresh air.
• Close the recirculation damper proportionally but do not fully shut it to allow blending with warmer return air.
• Increase fan speeds as needed to support airflow.
• If the supply air temperature drops below the setpoint after Free Cooling, prioritize reheating with recirculated air. If return air is insufficient, activate the heating coil to maintain the temperature setpoint.

 

Condition 7: Filter Monitoring

Trigger: Differential pressure across filters exceeds a pre-defined threshold.

 

Actions:

• Alert operators to increasing resistance before reaching the alarm threshold for proactive maintenance.
• Raise an alarm if resistance exceeds the maximum allowable limit, indicating the need for immediate filter replacement.

 

Condition 8: Damper Position Monitoring

Trigger: Dampers fail to achieve the commanded position or misalignment occurs.

Actions:

• Raise an alarm to notify operators of damper position errors.
• Log the fault for maintenance.

 

Condition 9: Fan Energy Monitoring

Trigger: Fan motor energy consumption exceeds benchmarked thresholds.

Actions:

• Raise an alarm to indicate potential inefficiencies due to increased load, mechanical failure, or clogged filters.
• Notify operators to investigate and address the issue.

 

Condition 10: Hand/Off/Auto (HOA) Monitoring

Trigger: HOA switch is left in “Hand” mode.

Actions:

• Raise an alarm to alert operators that a device is in manual mode.
• Notify operators that automated control is overridden, potentially leading to energy waste or system imbalance.

 

Condition 11: Night Purge

Trigger: Night purge mode is activated during mild outdoor conditions.

Actions:

• Open the inlet damper incrementally to introduce cooler outdoor air.
• Maintain the recirculation damper partially open to blend internal and external air if needed.
• Keep the outlet damper fully open to exhaust air.
• Operate fans at reduced speeds to precondition the space while minimizing energy use.