Heat is lost from the human body through radiation, convection, and evaporation. Radiation heat loss represents the transfer of energy by means of electromagnetic waves. The convection loss is the heat carried away by the passage of air over the skin and cloth- ing. The evaporation loss is the heat used up in converting moisture on the surface of the skin into vapor.
Heat transfer, whether by convection or radiation, follows the same physical laws in the radiant heating system as in any other; that is, heat flows from the warmer to the cooler exposure at a rate directly proportional to the existing temperature difference.
The natural tendency of warmed air to rise makes it apparent that this induced air current movement is greater at the cooler floor and exterior walls of the average heated enclosure than at its ceil- ing. It is through absorption by these air currents that the radiant panel releases the convection component of its heat transfer into the room air.
The average body heat loss is approximately 400 Btu per hour; total radiation and convection account for approximately 300 to 320 Btu of it. Because this is obviously the major portion, the prob- lem of providing comfort is principally concerned with establishing the proper balance between radiation and convection losses.
It is important to understand that bodily comfort is obtained in radiant heating by maintaining a proper balance between radiation and convection. Thus, if the air becomes cooler and accordingly the amount of heat given off from the body by convection increases, then the body can still adjust itself to a sense of comfort if the heat given off from the body by radiation is decreased. The amount given off from the body by radiation can be decreased by raising the temperature of the surrounding surfaces, such as the walls, floor, and ceiling. For comfort, the body demands that if the amount of heat given off by convection increases, the heat given off by radiation must decrease, and vice versa.
The principles involved in radiant heating exist in such common- place sources of heat as the open fireplace, outdoor campfires, electric spot heaters, and similar devices. In each of these examples, no attempt is made to heat the air or enclosing surfaces surrounding the individual. In fact, the temperature of the air and surrounding surfaces may be very low, but the radiant heat from the fireplace or campfire will still produce a sensation of comfort (or even discomfort from excess heat) to those persons within range. This situation can occur even though a conventional thermometer may indicate a temperature well below freezing. Radiant heat rays do not perceptibly heat the atmosphere through which they pass. They move from warm to colder surfaces where a portion of their heat is absorbed.
This chapter is primarily concerned with a description of radiant panel heating, which can be defined as a form of radiant heating in which large surfaces are used to radiate heat at relatively low tem- peratures. The principal emphasis will be on hydronic and electric radiant floor heating.
Types of Radiant Panel Heating Systems
Radiant panel heating systems use water-filled tubing or electric heat- ing mats or rolls installed in the floors, walls, and ceilings to dis- tribute the heat. Radiant floor heating is by far the most popular installation method in residential and light-commercial construction.
Note
The word panel is used to indicate a complete system of tubing loops in a single room or space in a structure. It may also be used to indicate a premanufactured radiant floor heating panel.
Floor Panel Systems
Floor panels are usually easier to install than either ceiling or wall panels. Using floor panels is the most effective method of eliminating cold floors in slab construction. Another advantage of heating with floor panels is that much of the radiated heat is delivered to the lower portions of the walls. The principal disadvantage of using floor panels is that furniture and other objects block portions of the heat emission.
Floor panels are recommended for living or working areas con- structed directly on the ground, particularly one-story structures. Partial ceiling or wall treatment may be used as a supplement wher- ever large glass or door exposures are encountered. A typical floor installation is shown in Figure 1-1.