Uses the example of a single-duct variable air volume (VAV) system to illustrate the difficulties involved in achieving proper ventilation with them. Notes the three-apart challenge of designing for peak ventilation capacity, operating with required ventilation at all loads and controlling to minimise the ventilation energy impact and describes in some detail the correct design procedures to meet these challenges. Stresses that single-duct VAV systems must be designed to meet Standard 62-1989 ventilation requirements. Considers the further challenges of freeze protection and mixing, airflow sensing, building pressure and subcooling at part load.