Demand Control Ventilation Strategy for a Building Beyond Optimizing the Ventilation Airflow (CFM)

The basic principle of demand controlled ventilation (DCV) is adjusting the outdoor airflow (OA) based on occupancy. However, consider that every building is unique, and the optimal DCV strategy changes accordingly. Ventilation systems can also be equipped with air pollution sensors, allowing a response to pollutants that are not related with occupants.

All DCV designs must consider two important factors: the building size and layout, and the requirements of local construction codes. Building features determine the capacity and distribution of ventilation equipment, while codes establish the minimum performance requirements.

Building owners must meet additional requirements if they want to exceed codes, or if they are seeking a specific building certification. Consider the following examples:

To get an ENERGY STAR score above 90 for a US building, it must be among the top 10% most efficient properties of its type.
Building certification programs like LEED and WELL have requirements as well, which are normally more demanding than those in local codes.
A net zero energy building (NZEB) is one that produces enough energy to meet its consumption. The installations in a NZEB must be as efficient as possible, and this includes ventilation.

A DCV system for a high-performance building has more demanding requirements, compared with one that is only intended to meet local codes. Improved performance has a cost, like in any engineering decision, but the rewards are energy efficiency and enhanced IAQ.

Should the DCV System Respond to Occupancy or Air Pollution?

Buildings tend to have a higher concentration of air pollutants when they are full, and ASHRAE provides two design approaches for ventilation.

The Ventilation Rate Procedure (VRP) provides table values to calculate cfm, based on floor area and occupancy.
On the other hand, the Indoor Air Quality Procedure (IAQP) focuses directly on controlling air pollutants.

The Ashrae standard provides a choice, but this is not always the case for building codes. The VRP has become more common because it provides prescriptive values, while the IAQP follows a more open design process. For this reason, many building codes mandate the VRP, and measuring the CO2 concentration through a dedicated sensor is also a common requirement.

Building certifications like LEED and WELL also affect design decisions for ventilation systems. For instance, WELL accepts both design procedures provided by ASHRAE, but LEED only accepts the VRP.

A building owner can request a ventilation system that responds to air pollution, even if local codes only accept the VRP. As long as the DCV design meets the minimum airflow values, adding controls that respond to pollution is acceptable. However, two conditions are necessary to design ventilation only with the IAQP:

Having local building codes that don’t mandate the VRP.
Having no plans for a building certification that specifies the VRP.

Note that occupancy sensors have other uses beyond ventilation control. For example, they can be used to disconnect the lighting when a room is empty. These can be infrared or ultrasonic sensors, which are simpler and cheaper than a dedicated CO2 sensor that measures CO2 concentration.

How the Building Size and Layout Influence Ventilation Design

Regardless of the design process, ventilation requirements are strongly dependent on the physical properties of a building. The total fan power increases with floor area, and the number of building zones determines the distribution of air ducts and equipment.

Single-zone DCV systems are easier to design, since the ventilation airflow must only be controlled for one building area. The design complexity is increased for multi-zone installations, since the DCV system must ensure all areas get the required outdoor airflow (OA).

Combining DCV with other Ventilation Measures

DCV is not the only measure that improves the performance of ventilation systems. If other measures are deployed, the control system must ensure they achieve synergy, instead of interfering with each other.

Energy recovery ventilation (ERV) can be combined with DCV easily. An ERV system exchanges heat and humidity between the air supply and the exhaust, reducing the workload on heating and cooling equipment. DCV and ERV systems interact, but they don’t interfere:

When a DCV system reduces the outdoor airflow, energy recovery is also reduced.
However, the net effect is positive: there are less cfm of outdoor air that require heating or cooling.

Together, DCV and ERV can reduce the energy needed to control the temperature of outdoor air. Even greater efficiency is possible if the building control system also adjusts the thermostat setpoint as needed.

Combining DCV with an airside economizer is more challenging, since the systems may compete with each other. While the DCV system reduces airflow to save energy, the economizer increases it when the outdoor air is cool enough to replace air conditioning. The control system must be smart enough to determine which of the two benefits is greater, and give priority to that ventilation measure.

Final Recommendations

The optimal demand controlled ventilation strategy is determined by local requirements and the plans you have for the building. The following checklist summarizes the points covered in this article:

Do local building codes mandate a specific design procedure?
Do you want a system that responds directly to air pollutants?
Are you seeking a building certification such as LEED or WELL?
What is the size and layout of your building?
Will you combine DCV with other ventilation measures?

DCV design is influenced by mandatory requirements and building owner preferences. A ventilation system that meets the specific needs of each property can achieve a higher energy efficiency, while conserving indoor air quality.

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