Modern UK office buildings have a reputation of being energy profligate, largely due to the fan power requirements of commercial air conditioning. Most architects and HVAC designers only associate low-energy consumption with natural ventilation. However, the UK electricity utilities have peak maximum demands in winter, and buildings need to be designed for year-round lowenergy usage. Relatively few monitored studies of the total annual energy implications of natural and mechanical ventilation strategies operating in conjunction with fabric thermal storage have been published.