When bodies of unequal temperatures are placed near each other, heat leaves the hotter body and is absorbed by the colder one until the temperature of each is equal. The rate by which the heat is absorbed by the colder body is proportional to the difference of temperature between the two bodies—the greater the difference of temperature the greater the rate of flow of the heat.

Heat is transferred from one body to another at lower temperature by any one of the following means:

2. Conduction
3. Convection

Radiation, insofar as heat loss is concerned, refers to the throwing out of heat in rays. The heat rays proceed in straight lines and the intensity of the heat radiated from any one source becomes less as the distance from the source increases. The amount of heat loss through radiation will depend upon the temperature of the floor, ceiling, and walls in the room or building. The colder these surfaces are, the faster and greater will be the heat loss from a human body standing within the enclosure. On the other hand, if the wall, ceiling, and floor surfaces are warmer than the human body within the enclosure they form, heat will be radiated from these surfaces to the body. In these situations a person may complain that the room is too hot.

Knowledge of the (mean radiant) temperature of the surfaces of an enclosure is important when dealing with heat loss by radiation. The mean radiant temperature (MRT) is the weighted average temperature of these surfaces (i.e. the floor, ceiling, and walls). The significance of the mean radiant temperature is determined when compared with the clothed body of an adult (80°F. or 26.7°C.). If the MRT is below 80°F, the human body will lose heat by radiation to the surfaces of the enclosure. If the MRT is higher than 80°F, the opposite effect will occur.

Conduction is the transfer of heat through substances, as for instance, boiler plate to another substance in contact with it. Conductivity may be defined as the relative value of a material as compared with a standard, in affording a passage through itself or over its surface for heat. A poor conductor is usually referred to as a nonconductor or insulator. Copper is an example of a good conductor. The various materials used to insulate buildings are poor conductors. It should be pointed out that any substance, which is a good conductor of electricity, is also a good conductor of heat.

Convection is the transfer of heat by the motion of the heat matter itself. Because motion is a required aspect of the definition of convection, it can therefore take place only in liquids and gases.

Heat from the burning fuel passes to the metal of the heating surface by radiation, through the metal by conduction, and is transferred to the water by convection (i.e. circulation). Circulation is caused by a variation in the weight of the water due to temperature differences. That is, the water next to the heating surface receives heat and expands (becomes lighter) and immediately rises due to displacement by the colder and heavier water above.

Proper circulation is very important because its absence will cause a liquid, such as water, to reach the spheroidal state. This in turn causes the metal of the boiler to become dangerously overheated. A liquid that has reached the spheroidal state is easy to recognize by its appearance. When liquid is dropped upon the surface of a highly heated metal, it rolls about in spheroidal drops or masses, and without actual contact with the heated metal. This phenomenon is due to the repulsive force of heat and the intervention of a cushion of steam.