Demand-controlled ventilation with CO2 sensors

Most ventilation systems are designed for the maximum number of people a space might hold. Yet a conference room for 20 is often empty, a gymnasium is full for a few hours a day, and an auditorium may sit idle for days. Delivering design outdoor air all the time means heating, cooling and dehumidifying air for people who are not there.

Demand-controlled ventilation (DCV) adjusts outdoor air to match actual occupancy. The most common way to estimate occupancy is to measure carbon dioxide (CO2), which people exhale at a fairly predictable rate. In Canadian climates, where outdoor air must be heated for much of the year, DCV can save a significant amount of energy in spaces with variable occupancy.

Why CO2 works as an occupancy indicator

People exhale CO2 in proportion to their metabolic rate. In a space with steady occupancy and steady ventilation, indoor CO2 rises above outdoor levels until the amount removed by outdoor air equals the amount generated. The steady-state difference between indoor and outdoor CO2 is roughly:

Indoor CO2 - Outdoor CO2 = CO2 generation per person / outdoor air per person

For an adult doing office work, CO2 generation is roughly 0.005 L/s per person. With about 7.5 L/s (15 cfm) of outdoor air per person, the steady-state difference works out to around 700 ppm. Lower ventilation per person produces a higher difference; higher ventilation, a lower one.

Two cautions matter:

• CO2 is an indicator, not a measure of air quality. At typical indoor levels, CO2 itself is not the main concern. It indicates how much outdoor air each person is getting, but it says nothing about contaminants from furnishings, cleaning products, outdoor pollution or processes. ASHRAE 62.1 does not set an indoor CO2 limit.
• CO2 responds slowly. It takes time for concentration to build after people arrive. Control sequences must account for this lag.

Where DCV makes sense

DCV saves the most in spaces with high design occupancy that varies a lot over time:

Energy codes require DCV in some cases. ASHRAE 90.1 requires it for larger spaces with high design occupancy served by systems with certain features, and NECB includes related requirements. Thresholds and exceptions differ between editions, so check the code in force.

How DCV fits with ASHRAE 62.1

ASHRAE 62.1 allows outdoor air to be reset based on occupancy, as long as the required ventilation is delivered. Under the ventilation rate procedure, each zone's requirement has a people component and an area component. A compliant DCV sequence typically:

1. Maintains at least the area component whenever the space is scheduled occupied, even with no people present.
2. Increases outdoor air from the area-only rate toward the full design rate as CO2 rises.
3. Delivers full design outdoor air when CO2 reaches the setpoint that corresponds to design occupancy.
4. Provides a pre-occupancy purge where required, to flush contaminants from the building and furnishings before people arrive.

Choosing the CO2 setpoint

Rather than picking a generic number like 1,000 ppm, a better approach is to calculate the expected steady-state CO2 at design occupancy and design ventilation for that space, then set the control to reach full outdoor air at that concentration. Because outdoor CO2 varies, typically around 400 to 450 ppm in many locations and higher near traffic, some systems measure outdoor CO2 and control on the difference rather than an absolute value.

Single-zone and multiple-zone systems

Single-zone systems

DCV is straightforward with a single-zone unit serving one space: a CO2 sensor in the space or return air controls the outdoor air damper position between minimum and design.

Multiple-zone VAV systems

DCV is more complex in multiple-zone systems, because the outdoor air fraction at the air handler is shared by all zones. Approaches include:

• Zone-level CO2 sensors that reset each zone's minimum airflow, combined with a system calculation that resets outdoor air intake, as described in ASHRAE Guideline 36
• CO2 sensors only in high-occupancy zones, with other zones at fixed minimums
• Return air CO2 sensing, which is simple but cannot identify a single crowded room among many empty ones and is generally not considered adequate for multiple-zone systems with varied occupancy

Our guide to ASHRAE Guideline 36 high-performance sequences describes standardized logic for outdoor air reset in VAV systems.

Sensor selection, location and calibration

DCV is only as good as its sensors. Most commercial sensors use non-dispersive infrared (NDIR) technology.

Selection

• Look for stated accuracy in the range of about plus or minus 50 to 75 ppm at typical indoor concentrations, including drift over time
• Prefer sensors with dual-beam or dual-wavelength designs, or a reliable automatic baseline correction that suits the space
• Be cautious with automatic baseline correction in spaces that are occupied around the clock, since it assumes the space returns to outdoor levels regularly

Location

1. Mount wall sensors in the breathing zone, roughly 0.9 to 1.8 m (3 to 6 ft) above the floor.
2. Keep them away from doors, windows, supply diffusers and people's direct breath, such as beside a desk.
3. Use one sensor per zone for spaces with distinct occupancy; large spaces may need more than one.
4. Duct-mounted sensors in return air are acceptable for single-zone systems where the return represents the space.

Calibration and checks

Sensor drift is the most common DCV failure. A drifted sensor that reads low will under-ventilate; one that reads high will waste energy. Check sensors against a calibrated reference at commissioning and periodically after. Our guide on sensor selection, location and calibration covers good practice.

Common problems

• Sensors that have drifted or failed, with no alarm
• Outdoor air dampers that do not move as commanded, or minimum positions that were never set correctly
• DCV logic that drops below the area component during occupied hours
• Sensors in poor locations, such as near a door or on a cold exterior wall
• Economizer control and DCV logic that conflict, causing hunting
• No trend data, so nobody knows whether DCV is working

Commissioning DCV

1. Verify each sensor reading against a calibrated portable instrument.
2. Confirm that outdoor airflow at minimum and maximum matches the design, using airflow measurement where possible.
3. Simulate rising CO2 and confirm that dampers and zone airflows respond as intended.
4. Confirm the area component is maintained during occupied periods.
5. Trend CO2, outdoor air damper position and outdoor airflow over several weeks of normal use.

Frequently asked questions

Is 1,000 ppm a code limit?

No. ASHRAE 62.1 does not set an indoor CO2 limit. The 1,000 ppm figure is a historical guideline loosely associated with about 7.5 L/s (15 cfm) per person of outdoor air at typical outdoor CO2 levels. Setpoints should be based on the design ventilation rate for the space.

Can DCV cause poor air quality?

It can if sensors drift, if the area component is not maintained, or if the space has contaminant sources unrelated to occupancy. Proper design and periodic verification prevent this.

Is DCV worth it in an open office?

Savings are usually smaller because the area component makes up a large share of the requirement and occupancy is fairly steady. It is most valuable in densely and variably occupied spaces.

Key takeaways

• DCV uses CO2 as an indicator of occupancy to reduce outdoor air when spaces are partly occupied.
• It saves most in high-density, variable-occupancy spaces, especially in cold climates.
• Compliant sequences maintain the area component and reach full design ventilation at the calculated CO2 level.
• Sensor accuracy, location and calibration decide whether DCV works.
• Commission and periodically verify sensors and dampers; control system verification catches drift and logic errors before they affect occupants.