Displacement ventilation uses air supplied at a temperature, typically around 19 deg C for offices, which is only slightly cooler than the design temperature of the occupied zone. This is a much higher temperature than used for conventional mixed flow ventilation systems and creates opportunities for using fresh-air based ’free cooling’ for a large proportion of the year. Conventional displacement ventilation design practice has suggested that, for normal office buildings, the cooling capacity of many displacement ventilation systems is limited to around 25 W/m2. However, recent BRE research has shown that displacement ventilation on its own using appropriate diffusers can deal with heat loads of around 50-60 W/m2 in typical office environments without causing thermal discomfort outside the diffuser outflow zone. This opens up the possibility of using free cooling to reduce the energy used for cooling. 8 pages.