States that carbon dioxide-controlled mechanical ventilation systems can be an effective and energy conscious method of providing adequate ventilation air only while a space is occupied. Provides the results of measurements of CO2 concentrations in buildings and illustrates the pitfalls in using a CO2 controlled ventilation system in three buildings. Presents the required parameters and a procedure for calculating the evolution of CO2 in buildings. Sets out a logical sequence of simple calculations to make an initial assessment of the efficacy of providing a CO2 control system in any particular building and highlights those buildings types which are likely to be most suitable. If this proves promising, the algorithms and validation of a model are provided to make a fuller assessment. Illustrates energy savings in the form of an example of the reduction in ventilation air for a common building type appropriate for the application. Comments on reliability and maintenance factors. Presents the results of an evaluation of various indoor air quality sensors in the laboratory.