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CHAPTER 24 1953 Guide Fig. 6. Warm Air Floor Panel Construction cases, the heated air is recirculated in a closed system. In others, all or a part of the air is passed through the room on its way back to the furnace to provide supplementary heating and ventilation.* Figs. 5 and 6 indicate two common types of construction. Care must be exercised to assure com pliance with any building codes that might apply. (See also section on Warm Air Ceiling Panels in Chapter 20). '' Electrically Heated Ceilings, Walls, or Floors Several different forms of electric resistance units are available for heating the interior room surfaces. These include: (1) resistance cables that may be imbedded in a manner similar to hot water piping in concrete or plaster; (2) prefabricated electric heating panels to be attached'to room surfaces; and (3) electrically heated fabrics or other materials for applica tion to, or incorporation into, finished room surfaces. Again, the problem of compliance with applicable codes must be considered.' Figs. 7 and 8 indicate two methods that have been used. OUTPUT FROM PANEL SURFACES / The heat transfer from a panel is accomplished by two basic heat trans fer processes:'radiation and convection, which are considered in following paragraphs. Radiation Transfer The radiation transfer can be evaluated by means of the relationship set up by Stefan and Botzmann: g, = 0.173 |7i-Y_(.YT \100/ \100/ (1) sruo space /--INSULATION 1 S--10 LATH H^fATIHO CABLES fcT (stapled to lati$ riHlSHCO PLASTER Fig. 7. Electric Heating Cables in Plaster FILLER PANEL INSULATION HEATING PANEL INSULATION IN PANEL ATTACHED TO JOISTS ?? fUCATED PANEL ATTACHED to joists ___ ;ctRic RESISTANCE Clement in PANEL Fig. 8. Prefabricated Electric Panel Panel -Heating 555 where ,, .qT-. = heat transfer by radiation, Btuper (squarefoot)(hour).. . .. . T, = absolute temperature of panel heated surface, Fahrenheit. .. , Tw = absolute mean radiant temperature of all unheated surfaces, Fahrenheit. . F = the configuration factor (dimensionless). Ft = the emissivity factor (dimensionless). For large parallel planes or large enclosed surfaces as ordinarily' en countered in panel heating practice: F. Cl Gt (2) ! = pane! surface temperature Fig. 9. Heat Output by Radiation where ei and ei = emissivities of the respective surfaces. In heating practice ei and et are usually equal to 0.9 and Fe to 0.82. Also, the configuration factor F,, is equal to 1 for large parallel planes, long concentric cylinders, or smaller bodies in large enclosures. There fore, for ordinary rooms with parallel walls, regular floors and ceilings, with an emissivity factor of 0.82, Equation 1 can be simplified to: Irregularities in room surfaces and materials may introduce some error m the application of this radiation relationship. However, most author ities are in agreement that the heat emission by radiation, as calculated in this manner, can be considered accurate within 10 percent. Radiation values are shown in Fig. 9 for various surface temperatures. These values , are applicable for either floor, wall, or ceiling panel radiation outputs.