One of the main functions of a ventilation system is to replace indoor air, preventing the accumulation of harmful compounds. In many cases, the required airflow is established based on two factors: usable area of the space under consideration and number of occupants. For buildings in New York City, ventilation rates are established by the New York Mechanical Code in Table 403.3.
For example, airflow requirements for an office are 5 CFM per person and 0.06 CFM per square foot. If a 1,000 sq. ft. office is used by 10 people, the following ventilation rate would be required:
- Area-based airflow = 0.06 CFM/sq.ff. x 1,000 square feet. = 60 CFM
- Airflow based on occupancy = 5 CFM/person x 10 people = 50 CFM
- Total airflow = 60 CFM + 50 CFM = 110 CFM
You can size a ventilation system based on square footage and occupancy and operate at full CFM at all times, but this is not the most efficient approach. In the office example above, if 8 of the 10 occupants are absent from a meeting, 110 CFM will result in excessive ventilation – a waste of fan energy.
Saving energy with reduced airflow
Operating a ventilation system at full power all the time may not represent a significant expense in a small area. However, energy waste is considerable in large buildings – if a skyscraper uses its ventilation system at full capacity for a full month, it is certain to have a high energy bill.
A smarter mode of operation is to adjust airflow according to occupancy, a concept called demand-controlled ventilation (DCV) . As long as the ventilation system maintains minimum airflow based on square footage, total airflow can be adjusted as occupancy changes. Please note that you will only be able to deploy DCV if you have a variable air volume (VAV) ventilation system, as airflow adjustment is not possible with a constant air volume (CAV) system.
Design your ventilation system for maximum efficiency.
Some modern buildings have ventilation systems that also respond to air pollution. For example, some cleaning and maintenance activities release harmful compounds during low occupancy periods. To remove these substances more quickly, the airflow can be increased to the maximum design value, even when occupancy is low.
Airflow control can be accomplished by reducing the running time of a fan, decreasing the average CFM. However, a more energy-efficient approach is to reduce fan speed rather than run time by using a variable frequency drive (VFD).
- For example, reducing the operating time of a fan by 10% generates linear savings – also 10%.
- On the other hand, reducing speed provides cubic energy savings in relation to speed . A fan at 90% speed consumes only 73% of the energy required at full speed (90% cubed is 73%).
In this simplified example, speed control provides 27% energy savings, while a reduced duty cycle provides only 10% savings. Note that the full savings from speed reduction are only achieved when the fan discharges freely in an open space and are slightly reduced when the fan must overcome static pressure. However, speed control is much more efficient than reduced duty cycle, even when static pressure is involved.
Controlling ventilation systems based on occupancy
For demand-controlled ventilation, the system must be equipped with a way to measure occupancy. There are many ways to do this and the best option changes depending on the application.
- Carbon dioxide sensors offer the highest accuracy, but they also come with the highest price. Human metabolism produces carbon dioxide and its concentration in the air can be correlated with the number of occupants in a space.
- People counting devices at building entrances are effective in applications where the majority of occupants congregate in the same area, such as restaurants and auditoriums. These devices are less effective when a building is divided into many interior areas with varying occupancy, as there is no way of knowing how occupants are distributed.
- Schedule-based controls are effective in areas with predictable occupancy patterns, such as classrooms. There is no need to detect occupancy directly – it is known in advance and the ventilation system is programmed accordingly.
Carbon dioxide sensors should be used in applications where people counters and programming-based controls cannot provide reliable airflow adjustment. For example, they are suitable for buildings divided into many areas with random occupancy.
In addition to reducing the energy consumption of ventilation systems, occupancy-based control leads to heating and cooling savings. As airflow through heating and air conditioning equipment is reduced, the required output is also reduced.
Control of ventilation systems based on atmospheric pollutant levels
Occupancy-based ventilation control leads to significant energy savings, but there are certain applications where this mode of operation can compromise indoor air quality. Keep in mind that some activities release large amounts of air pollutants, even when carried out by few people . In these cases, reducing the airflow to a minimum value is counterproductive, as the ventilation system takes longer to remove harmful substances from indoor areas.
Air monitoring devices can be deployed to track key pollutants such as volatile organic compounds (VOCs). If the concentration of an unwanted substance is increasing, the ventilation system can be increased even during periods of low occupancy. The following are some situations in which you would like to increase ventilation even if the room is empty:
- Cleaning products often release VOCs , which irritate the human respiratory system and lead to more dangerous health conditions with prolonged exposure. Ventilation can be used at full capacity during cleaning and thereafter for as long as necessary to completely remove these substances.
- Freshly painted walls and new furniture also release harmful compounds . A ventilation system that responds to pollutants may have to work harder in new buildings or after renovations, but indoor air quality is improved. For example, wooden furniture is often impregnated with formaldehyde, a harmful VOC.
Conclusion
Installing a ventilation system that operates at rated airflow at all times may seem like the simplest option at first, but it leads to significant energy waste in the long run. Ventilation systems can become smarter if they are equipped with controls that respond to occupancy and air pollution, achieving energy efficiency while providing indoor air quality.
