Assessment of CO<inf>2</inf>-based demand controlled ventilation requirement for a flexible work environment with ductless split air conditioners

Use of ductless split air conditioners in small workspaces leads to accumulation of carbon dioxide due to the absence of fresh air entry. Additional mechanical ventilation is required to meet the ASHRAE standard. Providing constant-rate ventilation would reduce the CO2 concentration but would lead to high energy consumption when there is variation in occupant density. Instead, demand-controlled ventilation, that is, ventilation rate linked with the occupant density, can reduce energy consumption and maintain adequate indoor air quality. The purposes of this study were to quantify the temporal variation of CO2 in a workspace, to determine the factors influencing the demand-controlled ventilation strategy, and to assess its performance in workspaces with flexible work environment (varying occupant density with time). A parametric study on demand-controlled ventilation design indicates that the ventilation cycle time depends significantly on the occupant density and the range of threshold levels of CO2. The occupants can decide the lower CO2 threshold level based on the requirements. Use of a lower CO2 threshold limit of 700 ppm in a typical workspace with varying occupant density resulted in an energy saving of around 90% when compared with the ventilation rate required by ASHRAE 62.1. The assessment would be helpful in identifying the workspaces that do not require external ventilation. The ventilation design strategy thus developed would help the building managers in improving indoor air quality and lowering energy costs when compared with the constant rate ventilation system.