Oklahoma Heating, Ventilation, and Air Conditioning (HVAC) Practice Exam

The correct answer is based on standard safety and performance practices in HVAC systems regarding duct insulation and temperature regulation. When a duct has an opening temperature of 150 degrees Fahrenheit, it is important to insulate it to ensure that the surface temperature doesn't exceed a certain limit for safety and efficiency.

In many HVAC guidelines, including those related to metal or duct insulation, the maximum allowed exposed surface temperature is often set to a level that reduces the risk of burns or heat-related issues. Limiting the surface temperature to 120 degrees Fahrenheit is consistent with these safety measures.

Maintaining a maximum surface temperature of 120 degrees F helps in minimizing heat loss, improves energy efficiency, and ensures safety for anyone near the duct. This prevents overheating of surrounding areas and protects against possible hazards related to high surface temperatures, such as burns during maintenance work or unintended thermal impact on nearby materials.

Insulating to this level also aids in enhancing the system's overall performance by ensuring that the heated air remains at an effective temperature without unnecessary heat loss, contributing to a balanced and efficient HVAC operation.