HVAC

Zone and system calculations

Learn the basic zone and system calculations using ASHRAE 62.1 tables and equations
By Carol Qing Li, PE, LEED AP, Interface Engineering, Los Angeles August 23, 2019
Figure 2: This example shows an office building’s ventilation calculations. Courtesy: Integral Group

Zone calculations: Determine breathing zone outdoor airflow for each zone, based on prescribed rates in ASHRAE 62.1 Table 6.2.2.1 

  • Calculate outdoor airflow to the breathing zone, using Table 6.1 rates. 
  • Vbz = Rp x Pz + Ra x Az (outdoor flowrate per person by number of people plus outdoor flowrate per square foot by area of the zone). 
  • Determine zone air-distribution effectiveness Eby looking up Table 6.2.2.2. 
  • Calculate outdoor airflow to the zone. 
  • Voz = Vbz / Ez (outdoor air in the space breathing zone divided by zone air distribution effectiveness). 

System calculations: Calculate outdoor air intake flow for the entire system, based on prescribed calculation procedure. 

  • Singlezone system: One air handler serving one zone system outside air requirement: Vot = Voz. 
  • 100% outdoor air system: One air handler serving many zones without recirculation system outside air requirement: Vot = ΣVoz 
  • Multiplezone system: Ventilation is satisfied by providing, at minimum, the outdoor air that represents the sum of the ventilation requirements at each space that it serves. One air handler serving many zones with recirculation, all zones receive the same percentage of outside air, resulting in some zones being over ventilated. This is acceptable considering that the concentration of pollutants in the overventilated spaces return air is low enough that it can be used, along with the ventilation to the other spaces, to dilute pollutants in those spaces. For variable air volume systems, the design outdoor ventilation rate at the system level must always be maintained even when the fan has modulated to its minimum capacity.

Carol Qing Li, PE, LEED AP, Interface Engineering, Los Angeles
Author Bio: Carol Qing Li an associate/senior mechanical engineer with Interface Engineering, and was previously with DLR Group. She has practiced engineering in Australia, China, New Zealand and the United States.