building diagnosticsThe startup and shutdown sequence of a complex HVAC system is closely related to the management and control components of the HVAC system. Improper start-up sequencing of HVAC forced air systems can result in severe moisture problems.

Generally, start up a forced air HVAC systems and humid climates should consist of the following sequence of activities operation of conditioned

  • The operation of the conditioned make-up air systems, if applicable
  • The operation of all main conditioning units
  • The operation of the exhaust air systems.

Experience has shown that the start-up sequence often is reversed and exhaust systems become operational first.  This occurs because the exhaust system is the easiest system to make functional and because of the belief the drawing air through the building will help dry out the structure. This assumption is untrue in humid climates because outside moisture levels typically exceed inside moisture levels. In humid climates, this scenario often results in severe moisture accumulation problems from depressurization of the building. This is especially destructive if the main conditioning units become operational before the make-up air units.

Two techniques, pressure differential measurement and flow visualization, are valuable additions to the commissioning process. These methods are helpful because the measurement of airflows at the termination point forced air systems is insufficient for determining the overall pressure relationship between the buildings interior and exterior. The pressure differential measurement is a direct measure of the driving force of airflows while flow visualization confirms and verifies the pressure differential measurement.

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.