I recently came across this circular sent out by the US Department of Commerce's National Bureau of Standards on July 19, 1935. It's fascinating stuff and I thought you might want to keep it on file. Here 'tis.

Painting of Steam and Hot Water Radiators

For a number of years this subject has received considerable attention from the public, and it is apparent that the essential facts have not always been understood. The object of this note is to supply the more important facts in the case.

It will appear that as far as their effect on the performance of radiators is concerned, paints fall into two classes. First, those in which the pigment consists of small flakes of metal, such as the aluminum and bronze paints, most commonly used for painting radiators, which produce a metallic appearance and will be called metallic paints. Second, the white and colored paints, in which the pigment consists not of the metals but of oxides or other compounds of the metals. Thus white lead paints, or those containing compounds of zinc or other metals, will be called non-metallic paints. These non-metallic paints are obtainable in practically all colors, including white and black, while the metallic paints have the color of the metal or alloy of which the flakes are composed.

We will state at the outset the principal conclusion, which will be explained in more detail later, that the last coat of paint on a radiator is the only one that has an appreciable effect. And that a radiator coated with metallic paint will emit less heat, under otherwise identical conditions, than a similar radiator coated with non-metallic paint. In order to obtain the same amount of heat from the two radiators just considered the temperature of the one painted with metallic paint must be somewhat higher. Under these conditions, exactly the same amount of heat is being supplied to the two radiators. And since neither the boiler efficiency nor the heat wasted in the pipe lines is appreciably affected by small changes in radiator temperatures, practically the same amount of fuel is required to supply the heat in each case. In other words, while it may be desirable for various reasons to avoid the use of metallic paints on radiators, no appreciable saving in fuel will result from the use of non-metallic rather than metallic paints.

The purpose of a heating system is to maintain the rooms in a house at some temperature higher than that prevailing out of doors. The heat that is developed by burning fuel is transferred to the rooms by means of the radiators. A radiator neither creates nor destroys heat and a large radiator, while it can put more heat into a room than a small one, must be supplied with all of the heat it puts in. In the sense that they ultimately transfer all the heat supplied into the room, all radiators are 100% efficient. The word "efficiency" is, however, used in other ways, and it is now customary to use it on all possible occasions, but it is hardly correct to say that putting metallic paint on a radiator reduces its efficiency when the effect is merely to reduce its capacity. The size of the radiators in a house can only affect the fuel required for heating by increasing or decreasing the heat wasted in transmission from boiler to radiator and that lost up the chimney. Only when the radiators are so small as to render the whole heating plant ineffective is an appreciable saving of fuel to be expected by installing larger radiators.

After these preliminary explanations, we may proceed to consider the kind of effects that may be obtained by the use of various kinds of paint. The heat emitted from a radiator is removed in two ways. First, the air streaming past the radiator and rising from it is heated and carries the heat to other parts of the room. Second, the hot surface of the radiator emits heat by radiation just as the glowing electric and gas heaters do. Most types of steam and hot water radiators emit less than half their heat by radiation and evidently the name "radiator" although universally used is not a particularly appropriate one.

To take concrete case, a particular sectional cast iron radiator, if painted with any non-metallic paint, might transfer into the room 180 Btu per hour for each square foot of its surface, if supplied with the necessary amount of heat from a boiler. The burning of one pound of good coal produces about 12,000 Btu, and if the coal is used in a domestic heating plant, perhaps half of this, or 6,000 Btu, might finally be transferred from the radiators into, the rooms. Most of the other half of the heat produced is inevitably lost via the chimney.