In the progression of functional performance testing, which stage follows ‘complete systems’?

• A. Interfaces between HVAC systems
• B. Individual items of equipment and sub-systems
• C. Interfaces between HVAC systems and non-HVAC systems
• D. Start-up testing

Answer: (A)

In functional performance testing, after you’ve confirmed that complete systems operate as intended, the next focus is on how those systems interact with each other. This stage involves verifying the interfaces between HVAC systems—how air handling units, boilers, chillers, pumps, and other subsystems communicate, coordinate control signals, and follow the overall sequence of operation. Ensuring these cross-system interfaces work correctly prevents conflicts, confirms proper sequencing, and ensures the building’s air, energy, and comfort goals are met when multiple systems respond together.

Earlier steps target individual equipment and sub-systems, and start-up testing brings systems online to observe basic operation and alarms. Checking HVAC-to-HVAC interfaces first makes sure the integrated control environment functions smoothly before expanding to interfaces with non-HVAC systems.