- Abstract A hybrid space-cooling system for hot arid zones previously investigated by the authors was based on the nocturnal radiative cooling of water circulated through flat plate radiators. Preliminary investigations also indicated that the same system, with no modifications to the physical set-up, could provide a significant proportion of the winter heating requirements of buildings exposed to these climatic conditions, where summers are hot yet winters are frequently cold enough to justify the installation of heating systems. The heat output of the system averaged 370 W/m 2 of collector under the sunny but cool conditions typical of Sde-Boker winters. However, on windy and overcast days the thermostat control prevented water circulation and the system was inoperative. The primary factors determining the heat output were the intensity of global solar radiation incident on the collectors, wind speed and the temperature difference between the water in the roof pond and the ambient air. An expression was derived linking these parameters, which may be used to predict the heat output of the specific system with a high degree of accuracy, and thus define the climatic conditions where such a system may be of value.