Why ventilation should respond to demand rather than run constantly at maximum output

A meeting room is designed for a large group, but for most of the day only a few people use it. A conference hall is used occasionally, yet its ventilation runs at maximum output. Capacity is needed for peak loads, but that does not mean it should be used every hour.

What demand-controlled ventilation means

Demand-controlled ventilation adjusts the outdoor air supply to suit the needs of the rooms. One input may be CO₂, which is associated with occupancy. Savings potential depends on occupancy, climate, the system and the control algorithm; there is no single percentage for all buildings.

Fewer occupants do not mean the outdoor air damper can simply be closed. Requirements relating to materials, moisture and other sources of pollution remain. An engineering calculation determines the minimum airflow, while the sensor selects the operating mode within permitted limits.

Where the savings come from

An illustrative system supplies 3000 m³/h at full load. For six hours, operation at 1800 m³/h is permissible, and the temperature difference between outdoor and supply air is 20 °C. Without heat recovery, the reduction in heating demand is 8,16 kW, or 48,96 kWh of heat over six hours.

ΔQ = 0,34 × (3000 − 1800) × 20 / 1000 = 8,16 kW

Assuming the same heat recovery efficiency of 70%, the reduction in supplementary heating demand would be 14,69 kWh of heat. This is not yet electricity or seasonal savings: the heat source, duration of operating modes and fan consumption must be taken into account.

Why one sensor is not enough

A sensor in the common extract duct averages conditions across rooms, while a sensor near a supply air grille may sample unrepresentative air. Installation location, verification of readings, actual occupancy and the zone’s response time all matter.

There is also an aerodynamic aspect: at lower airflow rates, diffusers may serve the room less effectively. Part-load operation must be checked for air distribution as well as concentration levels.

What conventional controls can already do

Scheduling, CO₂-based control, minimum outdoor airflow and coordination between fans and dampers can all be implemented without AI. Predictive algorithms are appropriate when expected occupancy, the response of a particular zone, weather and overall system constraints must be considered together. In either case, the starting point must be reliable measurements and functioning basic controls.

Energy-efficient ventilation does not supply the minimum amount of air at any cost. It avoids supplying excess air where the required demand has already been met.