Demand controlled ventilation (DCV) is often discussed in building designs and energy efficiency projects. The concept may sound technical, but the underlying principle is very simple. Providing just the right airflow needed is more efficient than using fans at full power all the time.
There are several basic requirements to make demand controlled ventilation possible. The specific design details change depending on the building, but all DCV systems are based on a common principle:
- The occupancy of buildings is measured directly, with a sensor, or programmed into a control system if it follows a predictable schedule.
- The ventilation system establishes the outdoor airflow (OA) according to occupancy sensors.
The simplest way to modulate the airflow provided is with a damper at the air handler outlet. However, the pressure drop across the damper represents a waste of fan power. A more efficient design is achieved if the fan speed is controlled with a variable frequency drive (VFD). Assuming the same airflow, a fan at reduced speed uses less power than a full-speed fan with a damper.
Designing a Demand Controlled Ventilation System
To deploy DCV, the ventilation control system must be supplied with occupancy data. Occupancy can be monitored directly, through a dedicated sensor as previously mentioned, or programmed into the system when known in advance.
- The number of occupants can be determined at building accesses, by counting the number of people entering or leaving the building. This can be achieved with devices such as turnstiles or cameras.
- In the case of paid events, the number of tickets also provides an accurate estimate of building occupancy.
- CO2 sensors are a more expensive method, but they are very accurate. In fact, building codes make them mandatory in some applications. These sensors are based on the correlation between CO2 concentration and occupancy.
- Occupancy is more predictable in some applications, such as classrooms and data centers. The number of people present is known beforehand, as well as their schedule. This data can be programmed in a DCV system to reduce the need for monitoring.
ASHRAE standards provide two design approaches for ventilation systems, the Ventilation Rate Procedure (VRP) and the Indoor Air Quality Procedure (IAQP). The VRP uses predetermined airflow values based on occupancy, while the IAQP is based on controlling air pollutants directly. Many building codes only accept the VRP, and it has become the most widespread method as a result. However, this does not mean the IAQP should be disregarded. Hybrid designs that use both the VRP and IAQP are acceptable according to ASHRAE. The VRP requirements ensure code compliance, while the IAQP elements improve control over air pollutants.
Adjusting the Outdoor Airflow in Demand Controlled Ventilation
The cfm from an air handler can be controlled with a damper or with fan speed adjustment. Both methods provide the same OA for a given application, but there are important differences:
- An air damper restricts the fan outlet to reduce cfm. However, this causes a static pressure drop across the damper, which represents wasted fan power.
- Speed adjustment with a VFD achieves just the right airflow, with no need to restrict the fan outlet. As a result, the fan requires less power.
Another common option to reduce the average airflow is using a fan intermittently. However, this approach results in alternating periods of high airflow and no airflow, which affects IAQ and occupant comfort. Also consider that most building codes only allow intermittent ventilation in certain cases.
The potential of variable frequency drives extends beyond the ventilation system. They have also been used to modulate the speed of air conditioning compressors. If an HVAC design uses water piping to provide heating or cooling, VFDs can also modulate pump speed.
An airside economizer may sometimes override the DCV system. It increases outdoor airflow when its temperature is suitable to provide “free cooling”, saving on air conditioning.
Exceeding the Minimum Requirements in DCV Design
Energy codes normally establish minimum requirements for DCV. However, both energy efficiency and indoor air quality can be enhanced by exceeding code requirements. A good example is designing a DCV system that responds to both occupancy and air pollutants. Code requirements are normally based on occupancy, while air pollution monitoring allows a performance level that exceeds the code.
Of course, a simple and effective way to save energy is using a moderate thermostat setpoint instead of extreme values. Even the most efficient system wastes energy if the thermostat is set too high or too low.
When building owners seek a certification like LEED or WELL, the recommendation is aiming for the highest performance level possible. DCV can help meet the basic requirements of both certifications, while earning points that count towards a higher certification level.