In the humid climate we deal with here in Louisiana, which I wrote about in August, every CFM of unnecessary outdoor air is costing you real money. This is not a subtle cost buried in efficiency percentages. On a typical Louisiana summer afternoon, outdoor air arrives at your air handler carrying both heat and moisture that your cooling system has to remove before that air reaches your occupants. The more outdoor air you bring in beyond what you actually need, the harder your cooling system works and the more you spend.
Demand-controlled ventilation is the strategy that addresses this directly. The concept is straightforward: instead of providing ventilation based on a design assumption about how many people will be in a space, you measure actual occupancy in real time using CO2 sensors and adjust ventilation accordingly. Empty rooms get minimal ventilation. Full rooms get full ventilation. Rooms in between get something in between.
The energy savings from DCV are among the most consistent we see across different building types and climates, and in humid climates like ours, they tend to be at the higher end of what published research predicts.
What ASHRAE 62.1 Actually Requires
To understand why DCV saves energy, you first need to understand what we are reducing from.
ASHRAE 62.1, the ventilation standard that most building codes adopt by reference, specifies minimum outdoor air rates based on two components: a people component (measured in CFM per person) and an area component (measured in CFM per square foot). The total outdoor air requirement for a space is the sum of these two components adjusted for the zone air distribution effectiveness.
The people component is what varies with occupancy. A 2,000-square-foot conference room designed for 50 occupants might require 250 CFM of outdoor air for the people component at full occupancy. With DCV, when that room has 8 people in it, the people component drops to approximately 40 CFM. The area component (say, 40 CFM for this room) remains constant because it is not occupancy-dependent.
Without DCV, your system delivers outdoor air based on the design occupancy of 50 people, 365 days a year, regardless of whether 50 people are actually there. In a typical commercial building, design occupancy in conference rooms, training rooms, classrooms, and assembly spaces is rarely achieved more than a few times per year.
How the CO2 Measurement Works
CO2 is used as a proxy for occupancy because humans exhale CO2 at a predictable rate (approximately 0.3 liters per minute per person at light office activity). As more people occupy a space, CO2 levels rise. As people leave, CO2 levels fall as the ventilation system dilutes and exhausts the accumulated CO2.
Outdoor air typically contains approximately 400-420 ppm of CO2. A well-ventilated space with normal occupancy might run 600-800 ppm. A crowded, under-ventilated space might reach 1,500 ppm or higher. ASHRAE 62.1 provides guidance on the relationship between CO2 concentration and occupancy-based ventilation effectiveness.
In a DCV implementation, the CO2 sensor reports back to the BAS, which modulates the outdoor air damper to maintain a target CO2 setpoint in the space. Common setpoints range from 800 to 1,000 ppm above outdoor ambient (approximately 1,200-1,400 ppm total), which corresponds to ventilation rates that meet or slightly exceed the ASHRAE 62.1 people-plus-area requirement for the actual number of people present.
Sensor Placement Matters More Than You Might Expect
Where you put a CO2 sensor significantly affects the accuracy of the occupancy signal it provides. Common mistakes:
Placement near doors or operable windows. CO2 levels near a frequently opened door will be diluted by air exchange from the corridor or outdoors, giving a falsely low reading that will under-ventilate the space.
Placement near a supply air diffuser. Fresh supply air from the diffuser will locally dilute CO2 around the sensor, giving a falsely low reading.
Return air duct placement only. Return air duct CO2 sensors are appropriate for monitoring average building CO2 and for controlling outdoor air at a central air handler, but they average the CO2 from all zones served by that air handler. For zone-level DCV control (at individual VAV boxes), space-mounted sensors are needed.
Placement too high. CO2 is slightly denser than air, but in a well-mixed space, it distributes fairly evenly. The more important consideration is that the sensor should be at breathing height (approximately 5 feet above finished floor for seated or standing occupants) to measure the CO2 concentration in the occupied zone, not in the stratified ceiling layer.
For most commercial applications, we mount CO2 sensors on the wall at approximately 4.5 to 5 feet above finished floor, away from doors, windows, and supply air diffusers, in a location representative of the occupied area.
The Integration with Your Air Handler Control Sequence
DCV does not operate in isolation. The CO2 sensor data feeds into the air handler's outdoor air control sequence, and that integration needs to be designed carefully.
For a single-zone air handler serving one space with one CO2 sensor, the integration is straightforward: the BAS modulates the outdoor air damper to maintain the CO2 setpoint, subject to a minimum damper position that ensures the area-based ventilation requirement is always met.
For a VAV system serving multiple zones with multiple CO2 sensors, the integration is more complex. The outdoor air quantity for the system must be calculated to meet the most demanding zone's ventilation requirement, accounting for the fact that the system air is a mixture of return air and outdoor air that gets distributed to all zones. ASHRAE 62.1 Section 6.2.5.2 provides the multi-zone recirculating system procedure for calculating the required system-level outdoor air fraction.
Most modern BAS platforms have library functions or reference implementations for the ASHRAE 62.1 multi-zone calculation. The key is ensuring that the implementation actually uses the real-time CO2 data from each zone, not a fixed design occupancy assumption.
The ROI in a Humid Climate
The payback for DCV varies by building type, occupancy patterns, and climate. Published research shows typical paybacks of 2-4 years in most commercial buildings. In humid climates like Louisiana, paybacks tend to be shorter, often 1-3 years, for several reasons.
First, the latent load associated with outdoor air is significantly higher in humid climates. Every pound of moisture your cooling system removes from outdoor air requires roughly 1,060 BTU of cooling energy. In July in Covington, the outdoor dew point might be 75 degrees, meaning outdoor air carries approximately 130 grains of moisture per pound of dry air. Reducing unnecessary outdoor air reduces this latent load directly.
Second, Louisiana cooling seasons are long. The months when outdoor air carries significant heat and moisture energy penalties run from April through October in most years. That is seven months where every unnecessary CFM of outdoor air is a meaningful cost.
Third, electricity rates in Louisiana, while historically lower than the national average, have been rising. The energy cost per CFM of unnecessary outdoor air is not static.
CO2 sensors for DCV typically cost $150-$400 each installed, depending on the mounting location and wiring requirements. For a building with 10 qualifying spaces, the hardware cost might be $2,000-$4,000, plus programming and commissioning. The annual energy savings from eliminating unnecessary ventilation in those spaces, in a Louisiana climate, typically cover that cost within one or two cooling seasons.
If your building has conference rooms, training spaces, classrooms, or other variable-occupancy areas served by your BAS, and those spaces are not already on DCV, this is one of the first controls upgrades we recommend.