Book - School of Science and Technology

Combined radiant and convective heating 193Increased heat lossDowndraughtFigure 7.16 Mismatch of radiator to windowIt is important to remember that the length of a radiator should be matched to that ofthe window beneath which it is fitted. A narrow high output radiator beneath a widewindow will produce an equally narrow `fountain' of warm air moving upwards, with acascade of cooler air falling at either end of the window (Figure 7.16).The construction of most types of radiator is such as to offer the facility for threedifferent patterns of pipe connection. Two of these are shown in Figure 6.7, that at theleft-hand side with flow and return top and bottom at the same end (TBSE); and that atthe right-hand side with flow and return top and bottom at opposite ends (TBOE). Thethird alternative, probably now the most familiar, has flow and return both at the bottomat opposite ends (BBOE). Catalogue ratings are normally based upon the test routinesrequired by BS EN442-2: European laboratory investigations quoted by Peach suggestthat variation in heat emission with the three connection patterns is:TBSE ˆ 1:00TBOE ˆ 0:97 to 0:95BBOE ˆ 0:78 to 0:84Painting of radiators was a sore subject at one time as a result of the industrial andcommercial practice of using metallic paints, aluminium or bronze. These reduced theemissivity of the surfaces, and in consequence the total output, by a significant extent. Itwas later found that a coat of clear varnish over the metallic paint resolved the problem.Attempts are made from time to time to market radiators which have been stoveenamelled at works and are delivered to the site in a protective package. It is, however,also common practice to provide no more than a primer coat in preparation for a sitefinish.The concept of energy saving by provision of a reflective surface behind a radiator hasbeen widely publicised and extravagent claims of economy made. Two methods have beenadvocated, first by the provision of a metallic-foil covering to the hidden wall surface and,second by attachment of polished metal strips to the rear of the radiator itself. The efficacyof either approach depends to a large extent upon how well the wall construction behindthe radiator has been insulated (i.e. the U value). In the case of a solid 220 mm brick wall,the energy saving for a typical domestic living room might be of the order of 3% but for a260 mm cavity wall with an effective insulant between the leaves, it would be less than 1%.There would of course be no saving at all if the radiator were fitted on a partition walldividing two heated rooms.