Demand controlled ventilation (DCV) adjusts outdoor airflow to the actual number of occupants by measuring CO₂ in the space and raising or lowering ventilation accordingly. Instead of running a fixed 10 L/s per person all day, a DCV system trims airflow when the room is empty and increases it as people arrive, typically cutting ventilation energy 20–40% while keeping indoor CO₂ below 1,000 ppm. The method is most valuable in spaces with variable occupancy such as offices, classrooms, and meeting rooms.
Why CO₂ Is the Signal
People exhale about 0.005 L/s of CO₂ each, so indoor CO₂ tracks occupancy directly. Outdoor air sits near 400–450 ppm; a space at 1,000 ppm implies roughly 600 ppm above ambient, which at steady state means enough ventilation for the present load. DCV uses a setpoint—commonly 700–1,000 ppm—and modulates the outdoor-air damper with a variable-speed fan so the concentration stays in band without overventilating.
DCV vs. Constant Ventilation
| Factor | Constant ventilation | CO₂-based DCV |
|---|---|---|
| Airflow | Fixed at design peak | Tracks real occupancy |
| Energy use | Higher (overventilates) | 20–40% lower |
| CO₂ control | Can swing wide | Held near setpoint |
| Best spaces | Steady occupancy | Variable occupancy |
| Sensors | None | CO₂ (ppm) sensors |
The table shows DCV trades a few sensors for steady energy savings; the saving grows with how variable the occupancy is and how strict the outdoor-air heating or cooling load becomes.
Setting the Numbers
Design to ASHRAE 62.1 minimum of about 10 L/s per person plus a base area rate, but let DCV fall toward a lower bound when empty. A 50-person meeting room at full occupancy needs ~500 L/s; at 08:00 with two people it may need only 50–100 L/s. Keep the CO₂ setpoint at 700–1,000 ppm and place sensors 1.0–1.5 m above the floor, away from supply jets and doors, to read the true breathing zone.
When DCV Pays Off
- Use DCV in spaces with occupancy that swings more than 50% across the day: offices, classrooms, gyms, halls.
- Use it where outdoor air must be heated or cooled, since trimming flow cuts conditioning cost.
- Avoid it in spaces with steady occupancy such as 24/7 labs or cleanrooms where setpoint never drops.
- Add it when energy modeling shows ventilation fan and coil energy above 15% of the bill.
- Pair it with MERV-13 filtration so lower flow still meets air-quality goals.
Decision Checklist
- Confirm occupancy varies enough that trimmed hours exceed sensor and control cost.
- Set the CO₂ setpoint at 700–1,000 ppm and the lower airflow bound at code minimum.
- Locate sensors in the breathing zone, not near doors, returns, or supply diffusers.
- Calibrate sensors every 12–24 months; CO₂ cells drift and cause hunting.
- Verify the sequence in commissioning so the fan never drops below the code minimum.
Control and Reliability
DCV logic reads the highest CO₂ among zoned sensors and commands the outdoor-air damper; a proportional band of 100–200 ppm prevents short cycling. Because the fan may run at 20–100% of design, components balanced to G2.5 and performance-tested before shipment keep the damper and wheel stable across that range. A well-tuned DCV holds air quality while saving roughly a quarter of ventilation energy.
The Control Loop
The DCV controller reads the highest CO₂ among zoned sensors and commands the outdoor-air damper with a proportional band of 100–200 ppm around the setpoint, so the fan slows smoothly instead of stepping. A PI loop on the damper holds 700–1,000 ppm with under 50 ppm offset; too tight a band causes hunting, too wide a band lets CO₂ climb past 1,200 ppm. Tune the loop during commissioning using a packed-room test, not the empty-space default.
Multi-Zone Strategy
In a building with many zones, command the central outdoor-air path from the zone with the worst (highest) CO₂, while local VAV boxes trim per zone. This avoids overventilating the whole building because one gym is full. For a 10-zone floor, the central fan may run at 40% of design while only two zones are occupied, saving far more than single-zone logic and still protecting the code minimum everywhere in the building.
Filtration and Air Quality
Because DCV lowers flow when empty, pair it with MERV-13 or better filtration so the air delivered still meets particulate goals at low volume. A low-flow system with poor filters trades energy for health; the standard expects both. In schools and clinics, keep the filter efficiency fixed and let only the flow vary, and verify contaminant levels, not just CO₂, during occupied testing before claiming the saving.
Calibration and Drift
CO₂ cells drift 50–150 ppm per year, enough to shift the whole loop and either overventilate (wasting energy) or underventilate (stuffiness). Calibrate or replace sensors every 12–24 months against a known gas, and alarm on any sensor reading stuck at 400 ppm (failed) or pegged at 2,000 ppm (blocked). A small calibration program protects the 20–40% saving the DCV was installed to deliver across the building.
Where DCV Pays Least
DCV adds little in steady-occupancy spaces such as 24/7 labs, cleanrooms, or server floors where the setpoint never drops; there the fixed minimum already matches the load. Spend the sensor budget on variable spaces instead. A quick test is to trend occupancy: if it varies more than 50% across the day, DCV likely pays back in 1–3 years; if not, skip it and use constant ventilation.
Reporting and Verification
After tuning, log a week of CO₂ and airflow trends to prove the band holds and the fan modulates as occupancy changes. A good DCV shows CO₂ steady at 700–1,000 ppm with fan speed tracking the room, not a flat 100% all day. Hand that trend to the facilities team so they can spot sensor drift early and protect the 20–40% energy saving the control was installed to deliver. A short annual review of those trends is usually enough to keep the system honest and the saving intact for the life of the building.
Keruite manufactures this equipment at an 80,000 m² facility in Dezhou, China, with an in-house AMCA-210-style test laboratory; every unit is balanced to G2.5 and performance-tested before shipment.
FAQ
What CO₂ setpoint should DCV use?
DCV normally uses a CO₂ setpoint of 700–1,000 ppm, with 1,000 ppm a common upper limit for comfort. The system modulates outdoor air to hold that band, never below the ASHRAE 62.1 minimum per-person rate. Lower setpoints save less energy; higher ones risk stuffiness in dense occupancy.
How much energy does DCV save?
In spaces with variable occupancy, CO₂-based DCV typically cuts ventilation energy 20–40% by trimming airflow when rooms are empty. Savings rise with how much the outdoor air must be heated or cooled and how widely occupancy swings; steady-use spaces see little benefit and may skip DCV.
Where should CO₂ sensors be placed?
Place CO₂ sensors 1.0–1.5 m above the floor in the breathing zone, away from supply diffusers, return grilles, and doors that would give false readings. In multi-zone spaces use one sensor per zone or per representative zone and command the system from the highest reading to protect air quality.
Is DCV required by code?
Some energy codes credit or encourage DCV for variable-occupancy spaces above a certain size, and ASHRAE 90.1 allows it as a compliance strategy, but it is not universally mandatory. Check the adopted edition; where outdoor air is conditioned, DCV is often the most cost-effective measure to meet the fan-power and energy targets.