Outdoor Air Supply

Adequate amounts of outdoor air should be supplied to the indoor occupied spaces to promote acceptable indoor air qual­ity. ASHRAE Standard 170-2013: Ventilation of Health Care Facilities and ASHRAE Standard 62.1-2013: Ventilation for Acceptable Indoor Air Quality (ASHRAE 62.1-2013) provide common standards for outdoor air ventilation rates in hospitals. Table 7-1 of ASHRAE Standard 170 lists minimum outside and total air change rates for various hospital spaces. For hos­pital spaces not listed in Standard 170, Standard 62.1 is used to determine the required outside air for the space. According to ASHRAE Standard 170, 7.1,6, for air-handling systems serving multiple spaces, the system minimum outside air shall be calcu­lated using one of the following methods:

  1. System minimum outdoor air quantity for an air-handling system shall be calculated as the sum of the individual space requirements as defined by the standard.
  2. System minimum outdoor air quantity shall be calculated by the Ventilation Rate Procedure (multiple zone formula) of ASHRAE Standard 62.1. The minimum outdoor air change rate listed in this standard shall be interpreted as Voz (zone outdoor airflow) for purposes of this calculation.

This is a significant new addition to the ASHRAE standard. Previous editions and iterations implied that the ratio of outside air to total was required. The newer procedure will result in lower, more reasonable outside air flow rates for health care facilities.

Without adequate outdoor air, indoor airborne contaminants may rise to uncomfortable or unhealthy concentrations. ASHRAE 62.1-2013 specifies two means of achieving acceptable indoor air quality from a ventilation standpoint: ventilation rate procedure and indoor air quality procedure. The ventilation rate procedure specifies outdoor air requirements for certain applications. The ASHRAE outdoor air requirements are designed to keep human body odors, carbon dioxide concentrations, and other human bioef­fluent concentrations to levels acceptable to a substantial major­ity (approximately 80 percent) of the building population. Carbon dioxide concentrations are often measured within a building as an indicator of concentrations of other human bioeffluents. Ambient outdoor air contains approximately 370 ppm (parts per million by volume) of carbon dioxide. ASHRAE 62.1-2013 states that if indoor carbon dioxide concentrations are less than 700 ppm above ambient concentrations, human body odors and other bioeffluents are likely to be at levels acceptable to a majority of the building population.

The second method of achieving acceptable indoor air quality in ASHRAE 62.1-2013 is the indoor air quality procedure. For hos­pitals, the IAQ method is not acceptable to Standard 170. This procedure restricts the airborne concentration of all known con­taminants to acceptable levels. These acceptable levels are estab­lished by the Occupational Safety and Health Administration (OSHA), the Environmental Protection Agency (EPA), and other agencies and organizations.

Excerpt from: Mechanical Systems Handbook for Health Care Facilities
J. Robbin Barrick, PE, and Ronald G. Holdaway, PE
ASHE copyright 2014. Available at the ASHE Store.

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