T and B hoodThe common practice for commissioning HVAC systems in new commercial buildings consists of verifying the following aspects of the system.

  • The thermodynamic performance characteristics of individual heating and cooling components
  • Airflows in the conditioned air distribution system and in the ventilation air system
  • The operating characteristics of the system management and control components.

Inherent in this process is the assumption that commissioning procedures will detect differences between the design intent and operating characteristics of the building HVAC system. A primary cause of air leakage is depressurization of the building by the HVAC system although most HVAC designs for hot humid climates at the opposite intent that is to pressurize the building.

Unfortunately, current HVAC commissioning procedures are unable to accurately determine if the HVAC design intent has been accomplished. This is because these techniques are based on measurement of airflows at delivery and extraction points such as supply registers and exhaust grills. Airflow measurements at these points alone cannot properly assess the performance of the HVAC system or its impact on the pressure characteristics of the building because they fail to fully consider air distribution. For example, the delivery of makeup air for a hotel Corredor does not necessarily compensate for the extraction of air from guestrooms by bathroom vent fans. Furthermore, relatively large quality quantities of air often either leaked from or drawn into the air distribution ducting.

Since this portion of the HVAC system is usually hidden from view and is difficult to access its contribution to the air flows within the building is not accounted for by the standard commission process. In most cases, pressure gradients developed in building interiors chases wall systems and other cavities within the building if uncontrolled these pressures will introduce infiltration of outside air.

Present commissioning protocols do not provide for measurement of these pressures. More important they also do not provide for direct measurement of building pressures that are induced by operation of the building HVAC components.

This post was taken from an article titled, “Moisture Problems: Why HVAC Commissioning Procedures Don’t Work in Humid Climates”. by George DuBose and J. David Odom, and Philip Fairey, and was previously published by CH2M Hill.