Indoor air quality is essential for the health of occupants and also contributes to productivity in business environments. Many serious health conditions have been linked to prolonged exposure to air pollutants, and there are also substances that cause immediate irritation of the respiratory system.
There is a common misconception that indoor air quality is at odds with energy efficiency. Although mechanical ventilation requires electricity, there are ways to improve air quality without excessive fan power. Ideally, a building should receive enough outside air to keep concentrations of air pollutants low, but without wasting energy.
Achieve a ventilation design that delivers energy efficiency and air quality.
ASHRAE offers two design procedures for mechanical ventilation. The Ventilation Rate Procedure (VRP) uses research-based prescriptive airflow values, while the Indoor Air Quality Procedure (IAQP) is based on direct control of air pollutants. Unfortunately, many building codes only accept the VRP, as there is no standardized reference for air pollution levels under the IAQP.
- With VRP, typical air pollutants found in each building type are considered in prescriptive ventilation rates.
- On the other hand, the IAQP requires an assessment of air pollutants during the design phase, and the ventilation system is sized accordingly.
If a ventilation system is designed directly for air pollution control, without prescriptive airflow values, there are two very important requirements. All relevant pollutants must be clearly identified and the ventilation system must maintain their concentrations below the limit values of reliable references, such as the World Health Organization.
How Air Pollution Limits Can Improve Ventilation Design
Prescriptive airflow rates are easy to understand and their calculation procedure is straightforward. HVAC engineers must only know the floor area and number of occupants, then multiply the corresponding outdoor air rates in cfm/square feet and cfm/person. However, a ventilation rate lower than the prescriptive value may be appropriate when air pollution levels are very low.
A ventilation system that responds to pollutant concentrations may be more efficient than one that responds to occupancy. Although there is a correlation between the number of people and air pollution, there are exceptions:
- Cleaning products release large amounts of volatile organic compounds, which means air pollution can be high when just a few people clean indoor spaces.
- Some buildings, such as convention centers, are used for multiple activities, and this can change ventilation needs. For example, 2,000 people sitting at a large conference require less ventilation than 2,000 people at a fitness or dance event.
Prescriptive ventilation rates provide an established and tested procedure. However, a ventilation design based on pollutant levels has an additional feature: it responds to changes in air pollution even when occupancy does not change. An air pollution standard with limits for all relevant substances would simplify the ventilation design process with the indoor air quality procedure. Building codes largely limit this approach because there is no benchmark.
How Reduced Ventilation Achieves HVAC Savings
Ventilation systems have much lower energy consumption than heating and air conditioning equipment. However, how the ventilation system determines the volume of outside air that must be heated or cooled has an impact on the total HVAC load. For example, if outdoor airflow is reduced by 50% on a cold winter day, the required heating input to outdoor air is also reduced by half. The same applies to air conditioning during the summer, where reduced airflow equals less cooling.
Demand-controlled ventilation systems respond to occupancy, which is often monitored with carbon dioxide sensors. A ventilation control that responds to other air pollutants could follow the same approach, and the only difference is that more variables are considered. For example, volatile organic compounds and particles can also be monitored for ventilation control.
To maximize the efficiency of a smart ventilation system, it can be equipped with variable frequency drives and NEMA Premium efficiency motors. This combination minimizes electricity consumption for each cfm or air supplied. In the case of fractional power applications, brushless DC motors offer high efficiency with integrated speed control. They are also known as electronically commutated motors.
Final design recommendations
An oversized ventilation system may seem like a quick fix for improving indoor air quality, but this approach is expensive and inefficient. A larger ventilation system has a higher initial cost and more power requires more electricity. A high-capacity ventilation system with speed controls is a much better solution, avoiding unnecessary expenses.
Designing a ventilation system to control air pollution is a promising concept, but note that many building codes still specify minimum airflow rates according to tables. However, air pollution monitoring is useful even with this limitation – the ventilation system can respond to variations in air pollution levels even when occupancy has not changed. Ventilation efficiency could be improved by integrating air pollution standards into building codes. There would be no need to provide a minimum flow of outside air as long as pollutant levels are kept under control.
