What Is the Cost of Demand Control Ventilation? Using Occupancy Sensors and More to Optimize Ventilation Airflow
Demand controlled ventilation achieves energy savings by optimizing the outdoor ventilation airflow provided to a building. This not only saves fan power, but also the energy cost of heating and cooling the outdoor air. However, consider that DCV is an investment, and having an accurate budget of the installation is very important.
Providing a general rule to estimate DCV costs is not possible, since the design changes drastically depending on building conditions.
- Two similar buildings with the same area can have very different DCV costs, depending on factors like their internal layout and occupancy patterns.
- Weather conditions such as temperature and humidity also influence the overall HVAC design, which includes the ventilation system.
Consider that ASHRAE provides two design approaches for ventilation systems, and this also determines the most suitable DCV strategy. The Ventilation Rate Procedure establishes outdoor airflow rates based on area and the number of occupants, while the Indoor Air Quality Procedure focuses on achieving direct control over air pollutant concentrations. Both approaches improve IAQ, but the IAQP controls pollutants more directly while preventing overventilation.
Typical DCV Costs in a High-Rise Building
As previously mentioned, an accurate DCV cost can only be determined from a detailed design by HVAC engineers. DCV costs of $300 to $1000 per room are typical, where the variation is due to the unique design of each building.
- In a 10-floor apartment building with 100,000 square feet and 100 dwelling units, a cost estimate for a DCV project would be $233,000, according to a case study by the firm New York Engineers.
- This price estimate considers CO2 concentration sensors and control devices.
- The typical savings would be in the range of $45,000 to $50,000 annually, achieving a payback period of around 5 years.
If a DCV project uses existing extraction equipment such as kitchen hoods, the cost is around $600 per unit, with a typical payback period of two years. The installation cost would be $60,000 in a building with 100 dwellings, with savings of around $30,000 per year.
CO2 sensors are expensive components in a DCV system. According to the New Jersey Green Building Manual, the installation cost per sensing zone reaches up to $700. Some DCV systems use occupancy sensors instead of CO2 sensors, but this is not as efficient to guarantee quality indoor air. A good approach is to combine these sensors: CO2 sensors and motion detectors.
In the case of retrofits, the cost of implementing a DCV also depends on whether the existing installation uses CAV (Constant Air Volume) or VAV (Variable Air Volume) boxes as parts of the ventilation system. The cost will be much lower in a building using VAV boxes.
The Minnesota Division of Energy Resources estimated DCV installation costs of $1 to $3 for every cfm or outdoor air, along with annual maintenance costs of $0.12 per cfm. With the climate of Minnesota, DCV achieves median savings of $0.50 per cfm per year. Considering the upfront cost and maintenance savings, the payback period for a conventional DCV system ranges from 2.5 to 8 years.
The savings from DCV can vary just like installation costs, depending on building conditions and operating schedules. A study by the Mikkeli University of Applied Sciences found that annual savings can range from €0.39 to €7.75 per square foot when deploying DCV.
Why Such an Amplitude in Estimated Costs for DCV?
In addition to the sensors and equipment involved when installing a DCV system, there is a great deal of “integration” to be done. Indeed, sensors must be connected to the building management system (BMS), which will then send pre-determined commands for the HVAC system. BMS specialists are in charge of doing this software integration, and this adds to the cost.
When considering the widely different costs per square foot mentioned above, we need to compare apples with apples. For example, a basic DCV system with a direct wired connection between a sensor and a fan is not comparable to a building equipped with a fleet of sensors, connected to the entire HVAC system. The quality of the rules programmed makes a huge difference in air quality, thermal comfort, and energy saved.
What also affects the cost is the density of sensors. For instance, buildings may use less sensors and still consider their system as DCV. The WELL building certification suggests one sensor every 3,500 sq ft. After our measures in office, we Foobot go beyond that and recommend one sensor every 1,000 sq ft. Of course, it depends on the distribution of walls in the space, as one large open space will require less sensors than many smaller rooms.
Lastly, a DCV upgrade project provides an excellent chance to deploy other measures that also increase savings. For example, an airside economizer can reduce air conditioning costs. Using a temperature sensor, the device detects when outdoor air can provide “free cooling” for indoor spaces.
Only a professional assessment of your building can provide an accurate estimate of DCV costs and energy savings. However, previous research and case studies can give you an idea of what to expect. Sensor calibration, quality of the rules programmed, and overall maintenance are important to ensure a DCV system continues to save energy in the long run.