In the following applications, the numerical values given above will be used as if they were exact, but it must be understood that they are merely representative and would not apply exactly to any particular case except by chance. The effect of painting on the capacity of a radiator depends upon the size and design of the radiator. The reduction in capacity produced by the application of aluminum paint is less for large radiators than for small ones, especially so in the case of large radiators having many columns or tubes per section. In a large tubular type radiator having 7 tubes per section, more than ¾ of the heat is carried away by the air directly, and painting with aluminum consequently reduces the capacity of the radiator only about 10 per cent. If only the visible portions of a radiator are painted with aluminum paint, the reduction in capacity is also obviously less than if the entire surface is covered.
Suppose a house in which all the radiators are painted with aluminum paint, and that the radiator in one room is found to be too small, so that when the other rooms are warm enough, this one is too cold. If the radiator in this room is repainted with non-metallic paint either white or colored, the heat emitted by it can be increased from 10 to 20 per cent without affecting conditions in the other rooms, although it will be necessary to burn more fuel to supply the additional heat in the one room. If the increase is sufficient the expense of installing a larger radiator may thus be avoided.
Similarly, it is possible, by using bronze or aluminum paint on radiators in rooms which are overheated, and colored or white paints in rooms not sufficiently heated, to improve conditions without going to the expense of installing new radiators of larger or smaller sizes.
In installing radiators in a new house, somewhat smaller radiators may be installed if they are to be painted with colored paints, rather than bronze or aluminum paints.
If the radiators on a hot-water system are painted with metallic paint, and are all too small, so that the water must be kept hotter than is desired in order to heat the house, they may be repainted with non-metallic paint, and it should then be possible to heat the house with the water in the system not quite so hot. There will be no noticeable saving of fuel.
Since basements are usually overheated so that much of the heat supplied there is wasted, some economy can be affected by painting the heater and pipes with metallic paint. This can not, however, serve as anything more than a poor substitute for a covering of good insulating material, about an inch thick, which is capable of making an appreciable saving in the coal bill. The insulating material will remain effective for years, while the paint becomes ineffective if covered with dust.
If a radiator is situated next to an outside wall, as most of them are, it is evident that the heat supplied directly to this wall is more or less wasted. Some slight economy may be obtained, therefore, by using metallic paint on the side facing the wall and non-metallic paint on the visible portions. The gain is not large enough to be important, but on the other hand, in putting non-metallic paint over metallic, it is not worth while to go to the trouble of repainting the side next the wall.
Results of emissive tests of paints for decreasing or increasing heat radiation from surfaces, and a discussion of various applications of the results found, are given in Bureau of Standards Technologic Paper No. 254, which may be purchased from the Superintendent of Documents, Government Printing Office, Washington, D.C.