In the heating system employing water, steam or vapor as a heating medium, the boiler performs two functions - it contains the combustion chamber, wherein the fuel is combusted, and its other function is the transfer of the heat produced by combustion to the heating medium. The piping conveys the heated medium to the radiators and their job is to transfer heat from the medium to the air in the rooms.
In one sense the efficiency of a radiator may be 100 per cent, although, obviously, it does not transfer all of the heat from the water or steam to the air, the thought being that all of the heat lost by the steam or water is transferred to the air in the room. However, the effectiveness of the radiator may be very far from 100 per cent, and this is a matter of major concern to the home-owner. By effectiveness we mean the degree to which the heat transferred by the radiator becomes useful to the dwellers in the home. You can see that if all of the heat taken from the radiator were used to heat the stratum of air within a foot of the ceiling we would have a sad state of affairs, for with no mechanical forces available, we would have an excessively hot ceiling and, perhaps, a floor-line temperature far too low for human comfort.
1 The humidity conditions accompanying these temperatures are not given in their statement.
2 Adapted from "Heating the Small Home," Small Home, April, 1930.
We might, therefore, define the effectiveness of a radiator as its ability, first, to transfer heat to the air of the room at or near the floor line and, second, to offset the effect of infiltrating cold air which enters, in largest quantity, around the window frames.
It is remarkable how poorly and ineffectively the average home is heated, and how large a factor is the improper selection and placement of radiators. In planning radiation for a home, it usually is assumed that each room is an isolated unit, and is considered without reference to other rooms except that they contribute warm walls and other dividing partitions. If we have a radiator in such a room, it heats the air that touches its surfaces, and this air thereby being rendered lighter per unit of volume, will rise if there is an equal volume of colder air that can flow down and replace it. As the coldest and densest air is along the floor line, theoretically there will be set up a circulation around through the room. In practice, however, we usually find a "close" circuit being set up, the tendency being for the air close to the floor to move very slowly to the radiator, the principal and more active circulation being largely above the line of the radiator top. Thus we have a definite and quite strong stream of heated air rising above the radiator, diffusing and spreading to cover the entire area of the room at the ceiling, and much higher temperatures and rate of air movement than at the floor line.
Under these conditions it is difficult to imagine a heated volume much below the mid-line of the radiator, and if this is one of the high type, the inadequately heated lower portion of the room easily may be 3 ft. high. Putting it the other way, it is quite obvious that the most effective radiator is that which delivers its heat at the lowest level; i.e., the lowest radiator of any given capacity. As a general thing, I would recommend a long, low radiator under each window of a room rather than a large single (and usually high) radiator under the largest window. There will be a much better distribution of heat throughout the room, especially in the zone occupied by the occupants - that is, the portion from the floor to about the four-foot line (as it is more common for people to sit than to stand).
Thus far we have been discussing radiator performance based on the air that is heated through transfer of heat from the steam or water, through the iron of the radiator to the air that scrubs along the surfaces of the radiator; in other words, to the heat transfer by convection. But we cannot disregard the fact that a radiator actually radiates heat to no small degree. Many people think of radiant energy as being emitted only from a surface that is so hot that it may be said to be radiant - that is, at least "red hot." If you pick up a white-hot coal from the fire and hold it in a pair of tongs, you readily appreciate the fact that it is giving off energy in a radiant form. This emission is at a tremendous rate, at first, because the temperature of the white-hot surface is so far above the temperature of any surface that can be "seen" from the piece of coal. This energy travels from the surface of the piece of coal just as the light waves travel from its surface. And, as the coal slowly changes color, first to bright, and then to dull red, the rate of radiant emission decreases. Then comes a time when you can see no sign of luminosity - the coal is jet black. But, if you were to touch it with your fingers an ugly burn would result. Yes, radiant energy still is being given off - at a lower rate, to be sure - and it does not cease until the coal has become as cold as the objects surrounding it. If it could be maintained, say by an internal source of heat, at 1800, it would continue what we might call "low-temperature radiation" just as long as heat was supplied. And it would act just as a radiator filled with water at 1800 does. However, the ordinary garden variety of radiator consists of many "sections" with curved surfaces, and as the emission always is at right angles to the surface, you can see that the radiator will "radiate" heat - at a comparatively low rate - into almost every nook and cranny of the room. So we must take account of this radiating ability.
First, we must look at the other end - the reception end - of the radiation phenomenon. If the radiator emits radiant energy, where does this energy go to? It is absorbed, or reflected, or both, by every substance upon which it strikes. If the object is a dull black almost all of the energy that strikes its surface will be absorbed, and this heat absorption raises the temperature of the object. If the object is a glossy white - enamel, for instance - by far the greater part of this radiant energy will be reflected, just as light is reflected by a mirror, and so it will pass into space until some other body gets in the way.
So the radiant energy waves from the (comparatively) low-temperature radiator travel out into the room, striking the furniture and the walls, and the human beings in the room, and warming them all. Now, it is a curious fact that the human being, just like the cat, prefers its heat in the radiant form. Watch a man stand in front of a fireplace and turn himself about, enjoying the sensation of receiving (comparatively) high-temperature radiation from the flame. (Of course, I am not considering radiant energy of a higher order than usually is available in the home.)
Now, as we put radiators in a room mainly for the purpose of making its occupants comfortable it certainly would seem sensible to deliver the requisite amount of heat to the room - and to them - in the form that will give them the greatest comfort. Consequently, research having determined these facts, radiators now are being designed to radiate heat more effectively into the zone of occupation of the room.....
Because we are gaining a more comprehensive idea of these things, radiator design is undergoing a rapid change, and the radiator of the near future will not only heat the air of the room, by convection, at a lower level, but will diffuse its radiant heat emission only through the lower zone. That will mean, in the latter instance, larger surfaces facing the room, occupying, possibly, all of the wall surfaces below all of the windows, instead of being concentrated in a single unit with little surface "facing" the room.
The English have done considerable research in this problem, and have developed both "panel" and "ceiling" heating. The latter is the "panel" system applied to the ceiling instead of the wall. The "panel" system utilizes comparatively large metal panels or containers of water or steam, embedded in the walls, usually occupying spaces that are not useful for other purposes, but effective as areas from which to emit radiant energy.
I may be too visionary to be practical, but I would carry these ideas to the extreme and resort to low-temperature heating of the largest possible area at the lowest possible level - and what would answer this purpose better than the floor of the room? Make the floor of tile, or similar substance, and heat it in any one of a number of possible ways, to a temperature of 8o° to 850 (which English research has shown to be the maximum temperature that will be comfortable to the feet). Then heat will be transferred to the air at the lowest possible level by convection, and the zone between the floor and the four-foot line will be the warmest and most comfortable to the human body. What a contrast in human comfort is evidenced in the person standing on a floor at 85°, with a temperature of 700 at the kneeline, 68° at the breathing line, and the individual with his feet on a 55° floor, his knees in a temperature of 60° and his head bathed in 75° air.
There are several distinct tendencies in American practice today, not all of them of interest to the owner of a small home. First, the trend toward radiator covers or grilles. This is a practice that may lead to troubles for, if the application is not carefully done, the result may be detrimental. It would require too much space to go into details; suffice it to say that covers or shelves set down close to the top of the radiator, or of complete metal enclosures with screen-covered openings at top and bottom, or of enclosures in window recesses may reduce the heat-transmission efficiency as much as 40 per cent. So you will see that this subject should be referred to the heating engineer or to the well-informed heating contractor.
Again, there has come the development of radiators designed to be placed behind the wall line, with grilles top and bottom. Obviously, this treatment of a radiator places it in the category of those just described. To overcome the reduced heat transmission, which is due to the fact that the air-flow over the radiator surfaces is retarded by the screening and reduced areas of the air channels, a very small electric motor and fan can be used, and radiators with this equipment built integral are available. While this class of equipment is expensive, largely on account of the restricted market, there is no reason why the owner of a small home cannot have it, at least in the living and dining rooms, and thus get rid of the usually unsightly radiator.
Then, there is the radiator designed for increased radiant effect, usually installed below the windows and with its outer surface forming the wall line. And we must not forget the new radiators made of metals other than iron or steel - the copper and brass units, with fins, or plates, or, perhaps, tubes, like the automobile radiator. These are coming more and more into use, as they can be used in front of or behind the wall line, and in either case have a given transmission effect with a minimum of volume.
Above all things, make sure that radiators are ample in size to liberate heat at the desired rate, as it is a simple thing to reduce emission by slightly closing the valve. It is impossible, however, to get the desired heating effect from an undersize radiator except by raising the temperature of the water or steam, and often this is difficult, if not impossible.
Here, again, it pays to have a competent heating contractor, as he will intelligently locate and size the radiators while a lower bidder and less competent man may skimp on sizes and locate radiators where they can be piped with the least amount of materials, regardless of the fact that they will not show the same degree of effectiveness.