The transient velocity field of a plane ‘wall jet’ formed on the ceiling of a test room has been studied. This ‘simple’, isothermal flow provided an insight into the mechanisms underlying the more complex jet/room geometries which are employed in intermittent warm-air heating systems for the domestic environment. Data are presented which may be used to predict the development of these intermittent wall jets, in either a starting or stopping mode, within typical room geometries. The time taken for the starting and stopping jets to attain their steady-state values, the so-called ‘growth’ and ‘decrement’ times respectively, were relatively short (less than 60 s). The duration of the supply fan off period required to maintain thermal comfort will therefore largely depend on the strength of local, natural draughts and not on the jet decrement time.
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