Demand-Controlled Ventilation

Ventilation that scales with how many people are actually in the room.

Quick Answer

Demand-controlled ventilation (DCV) is a control strategy that adjusts the amount of outdoor air delivered to a space based on actual occupancy instead of a fixed maximum. It commonly uses carbon dioxide sensors or occupancy sensors. DCV reduces the energy used to heat, cool, and move outdoor air while still meeting ventilation requirements.

The Full Picture

Buildings must bring in outdoor air to dilute pollutants people and materials produce. Conventional systems supply the design ventilation rate all the time, as if the space were fully occupied. In conference rooms, auditoriums, classrooms, and gyms, occupancy swings widely, so much of that air is conditioned and then wasted.

DCV closes that gap. Sensors measure a proxy for occupancy, most often carbon dioxide concentration, which rises as people breathe. The controls modulate outdoor air dampers or fans so ventilation rises when the space fills and falls when it empties. Some systems use occupancy counters or schedules, and many combine methods.

Ventilation rates themselves come from codes and standards, most notably ASHRAE 62.1 and the mechanical code adopted by the jurisdiction, and energy codes such as ASHRAE 90.1 and the IECC require DCV in certain high-density spaces, with thresholds and exceptions that vary by edition. A DCV design must still ensure that minimum ventilation for the building's own sources, not just the people, is maintained.

On projects, DCV is a design and controls question as much as an equipment one. Sensor placement, calibration, controls programming, and commissioning decide whether it works as intended. Sensors that drift or are poorly located can quietly cause under-ventilation or erase the expected savings.

Real Examples

→Lecture hall: A university lecture hall is fully occupied a few hours a day. CO2 sensors let the air handler reduce outdoor air when the room is empty and ramp it up during class.

→Energy code compliance: A design team adds DCV to a high-density assembly space because the adopted energy code requires it above an occupant-density threshold.

→Commissioning catch: During commissioning, a CO2 sensor is found mounted near a supply diffuser, so it reads low and the system under-ventilates. Relocating it fixes the control response.

Common Misconceptions

People assume: DCV means turning ventilation off when a room is empty.

Actually: Standards still require minimum ventilation for building-related sources, so DCV reduces outdoor air toward a floor rather than to zero.

People assume: CO2 level measures overall air quality.

Actually: CO2 is a proxy for people-generated pollutants, not a measure of everything in the air. It does not detect pollutants from materials, equipment, or processes.

Frequently Asked Questions

What sensors does DCV use?

Most systems use CO2 sensors. Others use occupancy sensors, people counters, or schedules, sometimes in combination.

Is DCV required by code?

In many jurisdictions the energy code requires DCV for certain high-occupant-density spaces above a size threshold, with exceptions. The exact rule depends on the adopted code edition and local amendments.

How does DCV save energy?

Less outdoor air means less energy spent heating, cooling, and dehumidifying that air, and less fan energy, particularly in climates with extreme weather.

Does DCV reduce indoor air quality?

When designed and maintained correctly it keeps ventilation at or above code minimums. Problems usually come from poor sensor placement, drift, or bad control logic.