VAV units are used in air conditioning and ventilation systems to improve comfort and reduce energy consumption. The VAV unit is controlled by a thermostat or DDC controller and varies the air quantity based on cooling and/or ventilation needs. This results in considerable savings in fan energy because, with little or no cooling demand, the valve can be partially or even completely closed. The VAV unit can also be used for air quality control. Depending on the CO2 concentration, more or less fresh air is then supplied. The units are available in single and double-walled versions.