Supply Air Temperature Reset: How Does It Work?

Introduction

In the quest for energy efficiency and sustainability, optimizing HVAC systems stands out as a significant opportunity. Supply Air Temperature Reset (SATR) is a proven strategy to enhance energy efficiency in heating, ventilation, and air conditioning (HVAC) systems. This article delves into the intricacies of SATR, exploring its principles, benefits, implementation strategies, and the pivotal role of energy audits in identifying and maximizing its potential.

 Understanding SATR

The significance of HVAC systems in energy consumption cannot be overstated. Among various strategies, SATR emerges as a promising method to reduce energy consumption while maintaining comfort levels. This article aims to provide a comprehensive understanding of SATR, its implementation strategies, and the crucial role of energy audits in optimizing HVAC systems. SATR involves adjusting the supply air temperature based on real-time conditions to optimize HVAC system performance. By modulating the supply air temperature, SATR ensures energy efficiency while meeting comfort requirements.

Efficiency Considerations

SATR involves increasing and decreasing the supply air temperature based upon zone damper positions. Low supply air temperatures require increased energy from the cooling system. Increasing the supply air temperature can enhance HVAC system efficiency due to higher evaporator pressure, which reduces compressor lift. Warmer air entering the evaporator coil leads to higher refrigerant vaporization pressure, reducing the workload on the compressor. This reduction in compressor lift results in energy savings and improved system performance.

The benefit of lower supply air temperature is that if the building has a high need for cooling, the building zones will be quickly satisfied. However, when the weather is mild and there is not a high demand for cooling, supplying the same air volume and temperature becomes wasteful. The level of demand for cooling can be determined by the zone damper position. If all or the majority zone dampers are at 100%, then there is a high demand for conditioned air. As the zone dampers begin to close, the demand also begins to fall. Another issue with low supply temperature is that many zones will need to utilize electric reheat in order to bring the air temperature back up to a desirable supply temperature into the zone.

Implementation Strategies

Implementing SATR involves various components and considerations. Zone damper position and VAV reheat systems are critical elements in SATR implementation. Zone damper positions are adjusted based on zone demand to optimize airflow distribution and temperature control. Proportional-Integral-Derivative (PID) loops are commonly used in SATR implementation to control HVAC system components such as dampers, valves, and heating coils. PID controllers continuously monitor supply air temperature and adjust system parameters to maintain setpoint values. By fine-tuning PID loop parameters, SATR ensures precise control and efficient operation of HVAC systems under varying load conditions.

A typical range for SATR setpoints is between 55 °F to 70 °F, balancing energy savings with occupant comfort needs. When cooling demand is higher, a setpoint of 55 °F is more appropriate. During mild weather conditions, when cooling demand is low (no heat demand), the setpoint can rise to 70 °F. See simplified sequence below:

A PID example of SATR could look like the following.:

The SATR sequence will operate according to a PID (proportional, integral, derivative) loop. The minimum setpoint should be set to 55 °F. From this minimum value, increase the setpoint gradually, in 1 °F increments, until two zone dampers are at 100% open. The system should then maintain that value. As more dampers begin to open to 100%, gradually decrease the setpoint in 1 °F increments until only two dampers are at 100% open. The supply air volume and temperature will begin to equalize and maintain the desired zone conditions.

Role of Energy Audits in Identifying SATR Opportunities

Energy audits play a pivotal role in identifying SATR opportunities by assessing HVAC system performance and energy consumption patterns. Through detailed analysis and data collection, energy audits identify areas for improvement, including supply air temperature setpoints and control strategies. Energy modeling and simulation techniques help quantify potential energy savings from SATR implementation, guiding decision-making and prioritizing recommendations.

 Conclusion

Supply Air Temperature Reset (SATR) offers a viable solution for enhancing energy efficiency and sustainability in HVAC systems. By leveraging SATR and energy audits, organizations can achieve significant cost savings, improve comfort, and reduce environmental impact. As we strive for a greener future, SATR remains a valuable tool in our arsenal for building a more sustainable built environment.