What is considered the most efficient method of capacity control in HVAC systems?

The most efficient method of capacity control in HVAC systems is speed control, particularly with variable speed drives. This method allows for precise adjustments to the compressor's operational speed, facilitating better matching of the cooling or heating output to the building's actual load requirements. By varying the speed of the compressor (or fan motors), the system can operate at optimal efficiency across a wide range of conditions, significantly reducing energy consumption and minimizing wear and tear on components.

In contrast, the other options provide less flexibility and efficiency in capacity control. Dampers primarily modulate airflow, which can help balance the system but does not adjust the actual capacity of the equipment. Two-stage compressors offer improved efficiency over single-stage compressors by allowing for a low and high capacity but are still limited compared to the continuous adjustment provided by speed control. Thermostatic expansion valves regulate refrigerant flow and can improve performance but do not have the capacity modulation capabilities of variable speed technology. Therefore, speed control stands out as the most effective means of achieving energy efficiency and optimal performance in HVAC applications.