DCV has the goal of adjusting the air supply as needed, according to the number of occupants. Modulating the available cfm is much more efficient than keeping the fans at full power all the time. There are many methods to monitor the number of present people, such as using motion detection sensors. However, measuring the Co2 concentration is a widely accepted method, which is also specified in many building codes globally.
When designing a DCV system with CO2 detectors, one of the first design requirements is setting the optimal CO2 setpoint. Standards from OSHA and other organizations normally set a limit of 5,000 ppm, while Canadian standards establish a lower limit of 3,500 ppm. These are legally enforced limits, but ASHRAE recommends a much lower value for human comfort: keeping the indoor CO2 level within 700ppm of the outdoor concentration.
According to the latest data published by NASA for May 2019, the global average concentration of CO2 is 411 ppm and considered constant around the globe. Assuming this value, the CO2 setpoint for a DCV system would be 1,111 ppm. However, conditions change depending on the location: CO2 levels below the average can be found away from population centers, and levels above the average can be expected in highly populated areas.
Establishing the CO2 Setpoint for a DCV System
Assuming a DCV design follows the recommendation from ASHRAE, there are two ways to establish the CO2 setpoint:
- Measuring indoor and outdoor CO2 levels, and calculating the difference between both values.
- Measuring only indoor CO2 levels, and assuming the outdoor level.
The first option is more accurate, but the second is more affordable because there is no outdoor sensor. In both cases, CO2 levels are reduced well below the guideline values published by regulating bodies. When the CO2 accumulation difference exceeds 700 ppm, it means the ventilation system must increase the outdoor airflow (OA).
There is an extra control requirement if the HVAC design includes an airside economizer. In simple terms, this is a device that increases the OA when its temperature is suitable to provide “free air conditioning”. This is independent from CO2 levels, and it may use the full cfm available. This machine is programmed to override DCV when the potential cooling savings are higher than those from varying the flow rate.
Enhanced DCV with Air Pollution Monitoring
Carbon dioxide must reach concentrations close to 100,000 ppm to threaten humans. This is an extremely high value, and even buildings with poor airing are unlikely to exceed 10,000 ppm (one-tenth of the lethal concentration). However, other substances become dangerous at much lower levels than CO2.
Particulate matter (PM) and volatile organic compounds (VOC) are two examples of pollutants that have negative health effects at the levels commonly found in building interiors. These substances are released by many sources that don’t depend on the number of present people, which means that CO2-based controls will not remove them effectively.
ASHRAE provides design guidelines for demand controlled ventilation systems that respond to air pollutants and not occupancy, following the Indoor Air Quality Procedure (IAQP). However, many building codes mandate the Ventilation Rate Procedure (VRP), which is based on floor areas and the number of present people. DCV systems with CO2 monitoring follow the VRP, but they can be enhanced with air pollution sensors, such as Foobot ones.
A conventional DCV system will not increase air supply if the measured CO2 level is low, regardless of other air pollutants present. However, when other substances are monitored, the DCV system can increase the ari supply accordingly. Cleaning activities are a good example of this: they are normally carried out by a few persons (low CO2 levels), but cleaning products release chemicals into the air (high VOC levels).
Regardless of the method used to determine the CO2 setpoint, monitoring air pollutants directly enhances performance. Consider that pollutants are not controlled directly when only CO2 is measured. Such systems assume that air pollution is high when the building is full, but there are exceptions.
- Is There a Demand Control Ventilation (DCV) Energy Savings Calculator?
- 2010 Codes and Standards Enhancement Through Demand Control Ventilation in Buildings