ASHRAE 62.1
Definitions & Purpose

Like all ASHRAE Standards, this one assumes one out of five occupants (20%) may not be satisfied with the results of compliance and may express dissatisfaction with the indoor air quality, even if the Standard is followed perfectly. Many sources have concluded that the majority of HVAC systems designed in the U.S. do not meet the minimum ventilation rates prescribed during operation. In that case, the actual occupant dissatisfaction level is exponentially greater in practice. It is not uncommon for rates to fall below levels that result in occupant dissatisfaction significantly greater than 50%. Many systems cannot meet the minimum airflow requirements at the occupied space during operation because of design choices and equipment limitations, or due to the dynamic nature of mechanical ventilation systems and the constant external forces acting on the building envelope.

The impacts from these continuously changing external conditions are not limited to Variable Air Volume (VAV) systems. Outdoor airflow rates will also vary for systems that provide a Constant Volume of supply air (CAV) to the conditioned space, as a result of:

a.) changes in wind and/or stack conditions on the intake system,

b.) changes in filter loading,

c.) changes in airflow requirements during an economizer cycle.

The lack of specific guidelines to overcome the effect of changing system dynamics on ventilation rates and air distribution for today's HVAC systems is partially to blame for many design deficiencies observed.

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According to the standard, acceptable IAQ is achieved if the occupants' dissatisfaction levels are 20% or better. In practice, dissatisfaction levels are much higher as a result of inadequate instrumentation, poor control strategies and inadequate system maintenance.