How to calculate ventilation

The procedure calculations begin by determining the required volume of ventilation in each zone (V_oz). V_oz is based on the breathing-zone outdoor airflow (V_bz).  

The ventilation effectiveness (E_z) is multiplied by Vbz to get V_oz. For single-zone systems, the ventilation-rate procedure (VRP) ends with the V_oz. For multizone systems that mix return and ventilation air, the required outdoor-air intake flow (V_ot) is calculated from the uncorrected outdoor air intake (V_ou) and system ventilation efficiency (E_v). 

The most common mistake made when performing the VRP is to not treat the calculation of E_v as iterative. A low E_v can cause the system to have much more outdoor air than is actually required. This significantly increases the first cost and lifecycle cost of the HVAC equipment. The E_v is driven by the zone with the highest primary outdoor-air fraction (Z_pz). The zone with the highest Z_pz is known as the critical zone. To achieve a more optimal E_v, the minimum flow of the critical zone must be incrementally increased. Another zone will eventually become the critical zone, and then the process begins again. Care must be taken to balance the increase in minimum flow with the decrease in V_ot. There is a limiting point where increasing the minimum airflow will cause more reheat than the energy being saved by reducing the required amount of ventilation air.

Do you have experience and expertise with the topics mentioned in this content? You should consider contributing to our WTWH Media editorial team and getting the recognition you and your company deserve. Click here to start this process.