One of the main functions of a ventilation system is to replenish indoor air, avoiding the accumulation of harmful compounds. In many cases, the required airflow is established based on two factors: Floor area of the space under consideration, and number of occupants. For buildings in New York City, ventilation rates are established by the NYC Mechanical Code in Table 403.3.
For example, the 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 persons, the following ventilation rate would be required:
Airflow based on area = 0.06 CFM/sq.ff. x 1,000 sq.ft. = 60 CFM
Airflow based on occupancy = 5 CFM/person x 10 persons = 50 CFM
Total airflow = 60 CFM + 50 CFM = 110 CFM
You can size a ventilation system based on area and occupancy, and operate at full CFM all the time, but this is not the most efficient approach. In the office example above, if 8 of the 10 occupants are away in a meeting, 110 CFM results in overventilation - a waste of fan power.
Saving Energy with Reduced Airflow
Running a ventilation system at full output all the time may not represent a significant expense in a small area. However, the energy waste is considerable for large building - if a skyscraper uses its ventilation system at full output for a complete month, a hefty power bill is certain.
A smarter mode of operation is adjusting airflow according to occupancy, a concept called demand-controlled ventilation (DCV). As long as the ventilation system maintains the minimum airflow based on square footage, total airflow can be adjusted as occupancy changes. Note that you can only deploy DCV if you have a variable air volume (VAV) ventilation system, since 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 faster, airflow can be increased to the maximum design value, even when occupancy is low.
Airflow control can be accomplished by reducing the runtime of a fan, lowering the average CFM. However, a more energy efficiency approach is reducing fan speed instead of runtime, using a variable frequency drive (VFD).
For example, reducing the runtime of a fan by 10% yields linear savings - also 10%.
On the other hand, speed reduction provides cubic energy savings with respect to speed. A fan at 90% speed only consumes 73% of the power required at full speed (90% cubed is 73%).
In this simplified example, speed control provides energy savings of 27% while a reduced duty cycle only provides 10% savings. Note that the full savings from speed reduction are only achieved when fan discharges freely onto an open space, and they are slightly reduced when the fan must overcome static pressure. However, speed control is much more efficient than reduced duty cycle even when there is static pressure 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 accomplish this, and the best option changes depending on the application.
Carbon dioxide sensors offer the highest accuracy, but also come with the highest price tag. The 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 accesses are effective in applications where most occupants gather in the same area, such as restaurants and auditoriums. These devices are less effective when a building is split into many indoor areas with variable occupancy, since there is no way to tell how occupants are distributed.
Schedule-based controls are effective in areas with predictable occupancy patterns, such as classrooms. There is no need to sense 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 schedule-based controls cannot provide reliable airflow adjustment. For example, they are suitable for buildings split 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. Since the airflow through space heating and air conditioning equipment is lowered, their required output is also reduced.
Controlling Ventilation Systems Based on Air Pollutant Levels
Ventilation control based on occupancy leads to significant energy savings, but there are certain applications where this mode of operation may compromise indoor air quality. Keep in mind that some activities release large amounts of air pollutants, even when carried out by few persons. In these cases, reducing airflow to a minimal value is counterproductive, since the ventilation system takes more time to remove harmful substances from indoor areas.
Air monitoring devices can be deployed to keep track of key pollutants, such as volatile organic compounds (VOC). If the concentration of an unwanted substance is rising, the ventilation system can be ramped up even during low-occupancy periods. The following are some situations where you would want 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 long-term exposure. Ventilation can be used at full capacity during cleaning, and afterwards for the time required to remove these substances completely.
Recently 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, wood furniture is often impregnated with formaldehyde, a harmful VOC.
Installing a ventilation system that runs at rated airflow all the time may seem the simplest option upfront, but this 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 delivering indoor air quality.
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