What is demand-controlled ventilation?

Demand-controlled ventilation means the outdoor-air rate follows who is actually in the space, not the design occupancy that sized the air handling unit. When zones empty out, DCV cuts outdoor air toward the code minimum instead of conditioning peak outdoor air all day. The acronym is DCV; operators also say demand control ventilation. Same plant, different outdoor-air reset.

How DCV works: CO₂ vs occupancy

CO₂-based DCV treats zone or return CO₂ as a proxy for people density. As CO₂ rises toward a setpoint, the controller raises outdoor-air (or zone primary-air) rate; when CO₂ falls, outdoor air backs off. Placement and calibration matter: a sensor in a dead corner or a drifted probe writes a lie into the loop. Occupancy-sensor DCV uses PIR, ultrasonic, badge, or workplace occupancy feeds to switch between occupied, occupied-standby and unoccupied outdoor-air rates. Occupied-standby keeps ventilation above the empty-room floor when sensors say someone is present but density is unknown. Either approach can run at zone level (per VAV box or zone damper) or as a system outdoor-air reset on the AHU mixing dampers. Many commercial jobs combine both: VAV boxes meter zone air while the AHU resets total outdoor air from a ventilation load calculation.

Where it sits on VAV AHUs

On a VAV system the AHU still holds duct static and supply temperature. DCV changes how much outdoor air that AHU pulls and how low zone boxes may go for ventilation. Pressure-independent VAV boxes already expose airflow setpoints and measured flow; DCV logic writes higher ventilation minima when zones are full and lower ones when they are empty. Constant-volume AHUs reset outdoor-air damper position (or OA CFM) from CO₂ or occupancy without a zone flow loop. Either way the BMS owns the sequence; analytics only reads whether the dampers and sensors match the claim.

What to check before trusting the savings

Published DCV savings are real in the right buildings and fake when the plant is lying. Before you bank a percentage, check: - Sensor placement and calibration drift. Wall CO₂ that never sees the breathing zone, or a probe that has drifted 200 ppm, will over- or under-ventilate while the trend looks fine. - Assumed vs measured outdoor CO₂. Many sequences hard-code outdoor CO₂ near 400 ppm. If the real outdoor baseline is higher, or the OA sensor is wrong, the differential that drives the loop is wrong. - Code minima still apply. ASHRAE 62.1 still requires minimum outdoor-air rates; DCV modulates above that floor, it does not erase it. - Damper authority and stuck OA dampers. A damper that never closes, or has no authority at low OA, leaves you conditioning peak outdoor air while the graphic says "DCV enabled." - Concurrent heating and cooling. Over-ventilation in shoulder seasons loads the cooling coil while reheat or perimeter heat fights back. The meter rise is not a DCV win. - Baseline quality. Savings claims need a weather- and schedule-aware baseline. A mild week after enablement is not proof. Field commissioning reports (for example Minnesota CARD work on implementing DCV) keep returning to the same theme: the control idea is sound; the sensors and dampers decide whether you get the energy.

Related control ideas (not separate pages)

Optimum start (also called optimal start) brings the plant on early enough that zones hit setpoint at occupancy, then backs off. It is a schedule/thermal problem, not a ventilation-rate reset, though bad start logic and DCV often share the same BMS schedule block. Night setback lowers (or raises) temperature setpoints when the building is empty. It cuts thermal load; DCV cuts outdoor-air conditioning load. Both need a trustworthy occupancy or schedule signal. Occupancy-based HVAC control is the broader pattern: mode changes from people presence (setpoints, minima, lighting, and ventilation). DCV is the outdoor-air slice of that pattern. For how those sequences sit inside a commercial HVAC BMS, see HVAC building management system.

What Explore already reads

FrostLogic Explore is Sensor Intelligence. It reads CO₂ and occupancy points already on the BMS, including Nimway workplace occupancy where that feed is connected, and flags CO₂-sensor anomalies in the ranked queue. Smart Schedules includes a demand-controlled ventilation AHU template: available in pilot, default-off, never assumed on for every building. Once a schedule is approved, the LLM is not in the control loop. Explore is not a CMMS and not a dashboard replacement. The commercial path for the ranked BMS queue is BMS analytics in Explore.