How Does Demand Controlled Ventilation Benefit HVAC Systems and Occupants Beyond Reducing Ventilation Airflow?
Demand control ventilation focuses on optimizing the outdoor airflow according to occupancy. However, the energy savings of DCV extend beyond air handling equipment. The entire HVAC system becomes more efficient, and this includes air conditioning and space heating.
A constant supply of outdoor air (OA) is necessary to conserve indoor air quality in buildings, keeping the concentration of air pollutants low. However, outdoor air is not always at a suitable temperature, and it must be heated or cooled. The outdoor air supplied to a building increases the air conditioning load during summer, especially on the hottest days. The opposite applies for winter, since the cold outdoor air needs heating.
A ventilation system that always provides its maximum airflow is detrimental for energy efficiency. Fans that provide more cfm than necessary are wasting electric power. In addition, the surplus airflow reduces air conditioning efficiency and heating efficiency.
When designing HVAC systems for new buildings, the best recommendation is including DCV from the start. The difficulty of adding DCV to existing installations depends on the configuration used.
- A variable air volume (VAV) system can be upgraded with relative ease, since it can already adjust the ventilation airflow.
- On the other hand, a constant air volume system needs major changes to make DCV possible.
ASHRAE is a global industry authority on HVAC and building efficiency topics. They have covered ventilation design and other related topics in depth, providing a solid reference for design engineers. Building codes from around the world are modeled after Ashrae standards.
Combining DCV with Air Conditioning
Air conditioning is the largest electrical load in most residential and commercial buildings, and its operating cost is very high during summer. However, the cooling load is reduced when the building uses DCV.
During summer, buildings gain heat from both internal and external sources. Some internal heat sources are people and appliances, while external sources include solar radiation and warm air. The ventilation system must constantly draw outdoor air to keep a suitable air quality, but this air reaches high temperatures during summer. DCV reduces the amount of warm air moving into the building, which saves on air conditioning.
Some climate zones are suitable for an airside economizer. Based on the data from a temperature sensor that monitors outdoor air, the economizer increases the air supply to reduce the air conditioning load.
- Since an economizer increases airflow, it cannot be used simultaneously with DCV.
- However, when both systems are present, a control system can choose the one that yields the highest air conditioning savings.
- This results in a more efficient operation that only having one option.
Combining DCV with Space Heating
The benefits of airflow reduction also apply for heating systems. Since the DCV system controls the amount of cold outdoor air, less heating is required. In this case the DCV system can remain active, since an airside economizer has no use in space heating applications.
In general, heating systems use a wider range of energy sources than air conditioners. Most cooling equipment runs with electricity, unless the building has the option of an absorption chiller with waste heat from a process. On the other hand, heating systems can be designed for various fuels like propane, heating oil and natural gas. There are also electrical heating systems, but their operating cost is very high unless they use heat pumps.
Regardless of the energy input, demand control ventilation can reduce heating costs. There is a great opportunity to reduce building emissions if DCV is deployed when switching to a cleaner fuel. For example, if an oil-fired heating system is converted to natural gas and equipped with DCV, the emissions will be cut significantly.
Achieving Energy Efficiency and Indoor Air Quality
Demand controlled ventilation not only saves on air conditioning and space heating. Since it allows precise control over the outdoor air supply, it can also improve air quality. The conventional approach has been to design DCV based on occupancy, but better results are possible if the system also responds directly to air pollutants.
Building codes normally specify CO2 sensors to monitor occupant density, as a control input for DCV. However, the system can use additional sensors to monitor air quality directly. This way it responds not only to occupancy sensors, but also pollutants like volatile organic compounds (VOC) and particulate matter (PM).
While there is a correlation between occupancy and air pollution, it does not apply 100% of the time. Some air pollutants are released independently from occupants. Also, the activities carried out affect ventilation requirements for a given number of people.
A DCV system can optimize airflow better when it tracks both occupancy and air quality. Space heating and air conditioning expenses are also reduced, since the amount of outdoor air supplied to the building is kept under control.
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