A supply air window is a device that by using the space between inner and outer sashes as an air path enables ventilation air in winter to be pre-heated before it enters a room. For optimum results it is necessary to maximise the window's capability as a heat reclaim device (by entraining into the air flow heat that would otherwise escape from the room) and its ability to absorb radiant energy from the sun. The principal requirements for achieving best performance include defining appropriate dimensional characteristics for the window, and the correct location of a low emissivity (low-E) coating within the glazing assembly. To determine these aspects tests were carried out using a PASSYS test cell. The performance of the window was monitored relative to climate conditions measured at an adjacent weather station. A simulation model of the test cell was built and comparison was made of the actual performance with forecast figures. (C) 2003 Elsevier Science B.V. All rights reserved.