Optimisation of [`]radiator' shelf design in order to achieve energy thrift

The traditional hot-water-filled [`]radiator', i.e. a vertical, flat heat emitter, fitted below a window, can often lose > 10% of its heat output through the window (i.e. without making any useful direct contribution to heating the main body of the room in which it is placed). A reduction in this rate of heat loss can be achieved by replacing the single glazing by double glazing. However, a much lower capital-cost, energy-thrift option, which is considered here, merely requires installing a suitably designed horizontal shelf between the radiator and the window. By optimising the geometry of this shelf, placed between the two buoyancy-driven air flows, i.e. downwards from the window and upwards from the radiator, a reduction in the steady-state rate of heat loss through an experimental window of ~14% of the radiator output has been achieved, resulting in a simple pay-back period for the installation of the shelf of ~2 years. The wide variations in the dimensions of windows and radiators as well as their relative placements and surface temperatures preclude a simple optimal thermal design of the horizontal shelf, which is suitable for all occasions, being proposed. However, significant improvements in the system's energy performance (i.e. lower rates of heat loss through the window) will be achieved when the following are employed: either (1) a straight horizontal shelf, protruding from the wall by 84 mm; or (2) a horizontal shelf, with a curved underside of radius 98 mm, when the protrusion from the wall is 98 mm.