Air can be used in a gas cycle to produce refrigeration; the so-called air cycle systems are the basis of most aircraft air conditioning systems. Using air as refrigerant has enormous advantages over conventional refrigerants, many of which have harmful environmental effects, and are flammable or toxic. However, using air for mainstream refrigeration in buildings has been held back by the perception of poor energy efficiency of air cycle systems. This paper reports the latest collaborative research by BRE and the University of Bristol to build an air cycle system for simultaneous heating and cooling in buildings, with lower overall energy consumption and environmental impact than conventional heating and refrigeration plant. 6 pages.