Most ventilating systems have shortcomings in the controlof their minimum outdoor air intake. They do not alwaysdeliver the intended ventilation to the occupants, their realperformance is hard to check and they do not adjust minimumoutdoor air intake as the ventilation demand varies. Demandcontrol ventilation (DCV), based on sensing the rise in spaceCO2 concentration, can address these problems8,9 but is diffi-cult to apply to multiple space systems. Supply air CO2 control(SACO2) effectively applies CO2 technology to recirculatingsystems. It is inexpensive, saves energy, and helps ensure goodindoor air quality.