GLOSSARY

Demand-Controlled Ventilation

Demand-controlled ventilation (DCV) adjusts how much air a ventilation system moves based on measured conditions, rather than running at a fixed rate whenever the building is occupied.

FAN SPEED OVER A WEEKDAY: FIXED VS DEMAND-CONTROLLED12AM6AMNOON6PM12AMFIXED SCHEDULE: 100% WHILE OPENDCV: FANS FOLLOW CO / NO2Fan power falls roughly with the cube of speed, so half speed uses about one-eighth the power.
Gas detectors speed fans up at the morning and evening peaks and slow them the rest of the day, while holding the code minimum airflow. Illustrative profile.

In parking garages, DCV uses gas detectors, most often for carbon monoxide and nitrogen dioxide, to speed fans up when contaminant levels rise and slow them down when the air is clean. Fans are usually driven by variable frequency drives, and because fan power falls sharply as speed drops, running slower most of the day can cut ventilation energy dramatically.

Try it
Run the fans by the air

An office garage fills in the morning and empties in the evening. Run the exhaust fans at full speed whenever the building is open, or let gas sensors set the speed, then compare the energy.

Running 30 kW of exhaust fans at full speed from 6 a.m. to 10 p.m. uses about 490 kWh a day. Letting carbon monoxide sensors set fan speed keeps the fans near idle except at the morning and evening rush, and because fan power falls with the cube of speed, the same garage uses more than 80 percent less fan energy while holding CO near the control setpoint.

Illustrative office garage: 30 kW of exhaust fans, 25 ppm control setpoint, minimum airflow about 7% of full (0.05 of 0.75 cfm per sq ft), fan power proportional to speed cubed. Actual setpoints and minimum rates come from the adopted mechanical code and the design engineer.

How it works

  • Sensors placed across each level report gas concentrations to a controller.
  • Controls stage or modulate fans to hold concentrations below set thresholds. International Mechanical Code Section 404 allows the exhaust rate to drop from 0.75 to a minimum of 0.05 cfm per square foot when detection controls the system.
  • Maintenance matters: gas sensors drift and need periodic calibration or replacement.

The Parkonomics view

Electric vehicles produce no tailpipe CO or NO2, so as the fleet changes, the case for running garage fans by the clock gets weaker every year. DCV is one of the simplest retrofits with a clear payback, as long as the sensor maintenance is budgeted.

Related terms: Variable Frequency Drive, Air Outlet, Enclosed Parking Garage, Plug-in Electric Vehicle

Further reading: The Evolution of U.S. Parking Structures in 2026