Ventilation Strategy for Demand Control Ventilation by ASHRAE Standards Using Occupancy Sensors
Demand controlled ventilation, or DCV, is defined by ASHRAE Standard 62.1 as any method that reduces the outdoor airflow supplied to a zone, based on occupancy sensors or other ventilation requirements. This ASHRAE standard also clarifies that an estimated occupant count is acceptable, when knowing the exact number of persons is technically unfeasible.
The main purpose of DCV is avoiding excessive ventilation when indoor spaces are partially full. Ventilation systems must be designed for full occupancy, but this does not mean they should operate at maximum cubic feet per minute all the time. A reduced airflow not only reduces fan energy consumption, but also the extra heating and cooling costs that come with higher airflow.
DCV is also referenced in ASHRAE standards that deal with building performance, being among the energy efficiency measures that can help meet those standards:
- ASHRAE 90.1 Energy Standard for Buildings Except Low-Rise Residential Buildings
- ASHRAE 189.1 Standards for the Design of High-Performance Green Buildings
Designing DCV Systems According to ASHRAE Standard 62.1
HVAC engineers have three procedures to choose from when specifying a ventilation strategy according to ASHRAE 62.1:
- Ventilation Rate Procedure (VRP)
- Indoor Air Quality Procedure (IAQP)
- Natural Ventilation
Natural ventilation has the advantage of consuming no fan power, but its application is greatly limited by climate and building features. Since DCV applies for mechanical ventilation systems with a variable-air-volume (VAV) design, it involves a choice between VRP and IAQP.
The VRP follows a prescriptive approach with ventilation airflow tables, which means it is based on specific design requirements. ASHRAE standards are updated at regular intervals, but the basic requirement under the VRP is the same: providing a minimum airflow based on floor area and the number of people within.
On the other hand, the IAQP is based on performance metrics that deal with air quality, making the design process more flexible. Unfortunately, the IAQP is not recognized yet under some standards such as California Title 24. In addition, the LEED v4.1 certification framework for green buildings does not accept the IAQP. This could change in future versions of both documents, as more performance data on the IAQP becomes available.
ASHRAE also specifies a combined approach, where the VRP is used to determine minimum airflow rates, while the IAQP is used to determine additional ventilation requirements due to specific air pollutants concentration. Alternatively, pollutants can also be controlled with a combination of air purification and air renewal, instead of relying only on increased ventilation.
Using DCV with the Ventilation Rate Procedure
Being based on prescriptive requirements, the VRP is very straightforward. ASHRAE 62.1 provides outdoor air (OA) flow rate tables for each building type, which have an area component and an occupancy component:
- For example, assume the standard specifies 0.06 cfm per square foot and 5 cfm per person for a specific building zone.
- If the area is 1000 square feet and there are 30 people, the ventilation system must provide 210 cfm.
- The airflow component due to area is 60 cfm (1000 sq.ft. x 0.06 cfm/sq.ft.), while the occupancy component is 150 cfm (30 persons x 5 cfm/persons).
The DCV system cannot reduce the oa airflow below the area-based value of 60 cfm. However, airflow can be adjusted from 60 to 210 cfm based on occupancy. For instance, only 100 cfm would be needed if there are 4 persons – 60 cfm to meet the area requirement, and 40 cfm for the occupancy requirement.
The airflow requirements change according to the application, even if two spaces have the same floor area and number of occupants. For example, the requirement for classrooms is 10 cfm per square foot and 0.12 cfm per occupant, which is higher than the example above.
Since the VRP is based on airflow, DCV systems require occupancy data for control. The traditional approach has been to measure CO2 concentration, but any method that provides an accurate estimate of occupancy is acceptable. Of course, some methods are more accurate than others.
Even though the airflow limits, further gains in terms of energy can be achieved through the installation of an economizer.
Using DCV with the Indoor Air Quality Procedure
The IAQP follows a completely different approach, since it does not provide predetermined outdoor airflow values. Instead, the IAQP consists on identifying key air pollutants and minimizing their concentrations. As long as air pollutants are kept under control, the IAQP allows lower airflow rates than the VRP.
In simple terms, the ventilation design process under the IAQP can be summarized as follows:
- Identifying the contaminants of concern, or COC.
- Indoor and outdoor sources must be identified for each COC, as well as the emission rate from each source.
- Establishing a threshold value and exposure period for each COC, following the guidelines of an authoritative reference (such as the World Health Organization).
- Specify an acceptable level of indoor air acceptability, which is based on occupant perception. This step of the IAQP is the most subjective.
- For each zone, the OA flow rate must achieve COC levels below the threshold, and indoor air acceptability above the established percentage.
Since the IAQP can achieve lower ventilation rates, reduced energy costs are possible. However, identifying the contaminants of concern is critical to ensure air quality. If a key air pollutant is missed during the design process, the equation will be incomplete, and the ventilation system will not respond to it. This can lead to poor air quality, and occupant perception cannot be used as a validation method because it is subjective: many air pollutants are colorless or odorless, and there are also pollutants with no short-term effects. It’s also critical to understand how temperature and humidity affect the release of most indoor pollutants, and so the importance of having at least one temperature sensor and one humidity sensor per ventilation area.
ASHRAE standards give HVAC designers a choice between the ventilation rate procedure and the indoor air quality procedure. The VRP follows prescriptive measures, while the IAQP is based on performance. However, local building codes should be checked as the first step, to verify if the IAQP is accepted. Also, the VRP is the only option for building owners seeking a LEED certification.
Global air quality standards could favor an increased use of the IAQP, by providing a common baseline for all ventilation systems designed under that approach. The guideline values provided by the World Health Organization are a useful reference, but they are not integrated with standards or enforced. It is also important to note that some air pollutants may become more important in specific applications, such as offices right next to production facilities.