Building Air Change Rate Estimates for CBR Analysis
Andrew K. Persily
Building air change rates impact energy consumption for space conditioning and indoor contaminant levels in relation to general indoor air quality issues as well as occupant exposure to airborne chemical, biological and radiological (CBR) agents. With respect to contaminant levels, air change rates impact both the entry and subsequent dilution of agents with outdoor sources and the dilution of those released internally. The air change rate of a particular building at a given time is a function of building layout, HVAC system design and operation, the operation of other building systems such as exhaust fans and vented combustion equipment, building envelope airtightness, exterior weather conditions, and occupant actions, e.g. window and door opening. These dependencies can lead to 10-to-1 or even larger variations in the air change rate of a given building, and even larger variations between buildings. The most reliable means of determining a buildings air change rate under specific conditions is to measure it with a tracer gas technique. Other measurement methods exist but they do not necessarily characterize all of the relevant airflows (e.g. outdoor air intake traverses that dont account for envelope infiltration) or are often associated with higher levels of measurement uncertainty. However, such measurements need to be repeated under a range of conditions to generate a complete understanding of a buildings air change rates for all situations of interest. Alternatively, modeling approaches exist that can predict these rates with an acceptable level of accuracy, assuming the required input data has been determined for the building of interest (ASHRAE 2005). However, measurement and modeling is not always an option in a given building, and generic values may be more useful than building-specific rates to support certain types of analyses. Since such estimates are not building specific, they are associated with a significant level of uncertainty, but they are still useful as long as their approximate nature is understood. This note presents air change rate estimates for a number of different building types: offices, homes, schools (including child development centers), barracks, multi-family residential buildings, retail buildings, restaurants, theaters, hospitals, gymnasiums, warehouses, and commissaries. A table is presented for each building type based on design requirements in ASHRAE Standard 62.1 or 62.2 as appropriate (ASHRAE 2007a and 2007b), the limited measurements available (which do not exist for some building types), and engineering judgment. In each table, the air change rate values are color coded to indicate the associated level of confidence. Air change rates based on design values or field measurements are shown in black and are associated with an uncertainty of about +/- 25 %, though in some cases the uncertainty could be larger. Those shown in blue are based on very limited measurements or design considerations, and are noted to have an uncertainty of roughly +/- 50 %. The values in red are noted as being very speculative and are associated with an uncertainty of +/- 100 %. These uncertainty values are all approximate and in some cases are quite large due to the limited number of field studies that have taken place and due to building, weather and system operation impacts on air change rates. As already noted, without building and condition specific measurements, air change rates estimates are inherently approximate but can still be useful for certain analyses.