Which of the following is a common method for improving HVAC energy efficiency?

Upgrading to high-efficiency filters is a common method for improving HVAC energy efficiency because these filters are designed to capture a greater amount of airborne particles, allowing for better airflow and reducing the load on the system. When an HVAC system operates with less resistance due to improved airflow, it can work more efficiently, leading to lower energy consumption. High-efficiency filters also help maintain good indoor air quality, prolong the lifespan of the HVAC components, and reduce maintenance costs by preventing dust and debris from accumulating in the system.

Other options, while they may have specific applications, do not typically lead to improved energy efficiency in the same way. Increasing the size of ducts could potentially disrupt airflow patterns and create additional pressure drops, which could force the system to work harder. Temporarily shutting down the system would not lead to energy efficiency improvements but rather lead to discomfort and inefficiency during operation periods. Using less refrigerant could hinder system performance and cooling capacity, resulting in increased energy use as the system struggles to maintain desired temperatures. Thus, upgrading to high-efficiency filters stands out as a proactive method to enhance overall efficiency.