How to calculate ventilation

Calculating the Ventilation Rate Procedure is key in any building project.

By Cory Duggin, PE, LEED AP BD+C, BEMP; TLC Engineering for Architecture, Brentwood, Tenn. August 21, 2018

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

The ventilation effectiveness (Ez) is multiplied by Vbz to get Voz. For single-zone systems, the ventilation-rate procedure (VRP) ends with the Voz. For multizone systems that mix return and ventilation air, the required outdoor-air intake flow (Vot) is calculated from the uncorrected outdoor air intake (Vou) and system ventilation efficiency (Ev). 

The most common mistake made when performing the VRP is to not treat the calculation of Ev as iterative. A low Ev 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 Ev is driven by the zone with the highest primary outdoor-air fraction (Zpz). The zone with the highest Zpz is known as the critical zone. To achieve a more optimal Ev, 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 Vot. 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.

Author Bio: Cory Duggin is the energy modeling wizard at TLC Engineering for Architecture, providing building-performance simulation efforts and high-performance design solutions. He is a member of the Consulting-Specifying Engineer editorial advisory board.