Fans are the cornerstone of ventilation systems, establishing the airflow needed in the building. The fans in ventilation systems can be classified into two types, based on how they move air:
- Supply fans move air into the building, causing a positive pressurization.
- Exhaust fans remove air from the building, causing a negative pressurization.
The two fan types are used individually or in combination, depending on how the ventilation system is designed. Building codes require exhaust fans in rooms that produce plenty of air pollution, such as kitchens and restrooms.
The capacity of a ventilation system is specified in cubic feet per minute (CFM) or air changes per hour (ACH). When airflow is specified in CFM, the number does not depend on the space being ventilated, since cubic feet and minutes are both fixed units. However, the definition of ACH changes depending on indoor space dimensions. For example, 5 ACH in a large conference hall is a much higher airflow than 5 ACH in a small office.
ASHRAE provides two methods for calculating the mechanical ventilation load, the Ventilation Rate Procedure (VRP) and the Indoor Air Quality Procedure (IAQP).
The VRP uses airflow tables that have been determined experimentally by ASHRAE, finding the optimal ventilation rate for each type of indoor space. The VRP rates consider three main factors:
- Purpose of the ventilated space: Classroom, office, restaurant, warehouse, etc.
- Floor area: Assuming the same occupancy classification, a larger space needs more ventilation to preserve air quality.
- Number of occupants: A full office needs more ventilation than an equally-sized office that is only 25% full, for example.
The VRP does not require air pollutants to be monitored directly. ASHRAE has already taken care of that task, when finding the optimal airflow rates for each type of indoor space. However, ventilation systems designed with the VRP have a key limitation - they cannot respond to a sudden increase in air pollution.
The IAQP is based on controlling air pollutants directly, without using the prescriptive airflow values in the VRP. This provides more flexibility when designing the ventilation system, and the process can be summarized in three main steps:
- Identify all the air pollutants that can be expected in the building.
- Determine the maximum concentration of each pollutant, based on reliable sources like the WHO or US EPA.
- Specify a ventilation system that provides enough airflow to keep all pollutants below their respective thresholds.
Many building codes make the VRP mandatory, since it uses prescriptive airflow rates from ASHRAE, instead of the open design approach of the IAQP. However, both methods can be combined to design a ventilation system that meets building codes, while enhancing indoor air quality.