Energy saving in buildings by control of ventilation as a function of indoor carbon dioxide concentration

Many mechanically ventilated and air conditioned buildings are over-ventilated since ventilation rates are usually based on a fixed number of people which is often considerably in excess of the average occupancy. Over-ventilation also occurs because no allowance can usually be made for infiltration. If ventilation rates could be modulated so as never to exceed the requirements of the actual number of occupants considerable savings in energy would be made. The CO2 concentration in the ventilated space can be related to the ventilation rate per person, and by modulating the fresh air flow to maintain a constant CO2 concentration a constant ventilation rate per person would be obtained. This action would reduce total ventilation rates whenever the occupation level is less than the design figure. Reductions in the total air supplied may also be made by ‘holding off’ the mechanical ventilation at certain times until the CO2 concentration reaches a predetermined value. The potential savings in energy due to these actions are discussed and two case studies relating to CO2 concentration measurements carried out in a theatre and clothing store are analysed. In the case of the clothing store the estimated cost saving results in an equipment cost pay-back period of about 2 years.