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432 CHAPTER 16 1957 Guide Radiant heaters are required to have a radiant efficiency of not less than 28 percent. Design Considerations Some factors important in the design of gas heaters are: - 1. Proper design of the burner head, port sizes and locations'so that the flame will not lift, float, or flash back, and be excessively noisy in operation. 2. Proper venting of combustion chamber for relief of forces resulting from igni tion of an explosive mixture of gas and air. 3. Protection of valve handles to prevent excessive temperature rise during opera tion. 4. Insulation and baffling of heater to prevent over-heating of walla and floor. INSTALLATION OF SPACE HEATERS The two most important considerations involved in the installation of a space heater are safety and chimney draft. The items of chimney details and flue connections which should have special attention, are outlined in Chapter 17, Chimneys and Draft Calculations. In all cases, it is recom mended that installation be made in accordance with the current National Building Code. REFERENCES 1 See A.S.H.V.E. Transactions, Vol. 35, 1929, p. 322. * See A.S.H.V.E. Transactions, Vol: 35, 1929, p. 332. * See A.S.H.V.E. Transactions, Vol..37, 1931, p. 23.. 4 See A.S.H.V.E. Transactions, Vol. 44, 1938,,p. 366.: ' 6 f=B=R Testing and Rating Code for Low Pressure Heating Boilers, July 195? Institute of Boiler and Radiator Manufacturers). e Testing and Rating Code for Boiler-Burner Units, July, 1954 (Heating, Piping and Air Conditioning Contractors National Association). I l=B = R Ratings for Cast-Iron Boilers May 10, 1955 (Institute of Boiler and Ra8dEiantogrinMeearninugfaScttuarnedrsa)r.ds, Part II, Net Square Feet Radiation Loads in 70 Deg Fahr, Recommended for Low Pressure Heating Boilers, July, 1954 (Heating, Piping and Air Conditioning Contractors National Association) 9 Comfort Heating, 1938, pp. 35 to 39 (American Gas Association). ' " Gravity Code and Manual for the Design and Installation of Gravity Warm Air Heating Systems, Manual No. 5, Fifth Ed., (National Warm Air Heating and .An CoInIdRiteiocnoimngmeAnsdseodciafotiromns)..f.or municipal installation and fire codes are, include.. d.m Manual 7--Code and Manual for design and Installation of Warm Air Winter`Air Conditioning Systems, Fourth Edition, (National Warm Air Heating and Air. Con diti1o1nTinhge DAsesvoecloiaptmioenn).t of a Design of. Smokeless. St. ove for Bituminous Coal, by B. A Landry and R. A. Sherman, presented at the 1948 Annual Meeting of the ASMS.. 19 American Standard Approval Requirements for Gas Fired Room Heaters, ASA Z2114.All,m1e9r5icia(nAmStearnicdaanrdStLainsdtianrgdsRAesqsuoirceiamtieonnt)s? for: Automatic Pilots, Z21:20, 1951; Gas Appliance Thermostats Z21.23,1941, Domestic Gas Appliance Pressure Regula tors, Z21.18, 1936, with addenda effective June 15,1935, July 8,1938, Automatic Valves for Gas Appliances, Z21.21, 1952 (American Standards Association). CHAPTER 17 CHIMNEYS AND DRAFT CALCULATIONS Draft Definitions, Theoretical Natural Draft, Factors Affecting Natural Draft, Available Draft, Required Draft; Industrial Chimneys, Available Draft for the Industrial Chimney, Determining Industrial Chimney Sizes; Residential Chimneys, Available Draft for Residential Chimneys, Short; Chimneys, Determining Residential Chimney SizeB, Draft Requirements of Appli ances, Chimneys for Gas Appliances; Recommendations of the National Board of Fire Underwriters; General Considerations DRAFT is the pressure difference associated with the movement of flue gases through a flue or chimney. Natural draft is caused and meas ured by the difference in weight of a column of flue gas within the flue, and a corresponding column of air of equal dimension outside the flue. Natural draft is always negative and is expressed in inches of water gage. When the movement of air is supplied by a fan, the draft is said to be forced or induced. When the fan is located so as to push the flue gases through the flue, the draft is forced. When the fan is located so as to draw the flue gases through the flue, the draft is induced. THEORETICAL NATURAL DRAFT Theoretical natural draft is produced solely by the difference in weight of the column of flue gases within the flue and that of a corresponding column of. air of equal dimensions toutside the flue. In the determination of theoretical natural draft, no allowance is made for friction or other posi tive or negative effects. Theoretical natural draft is expressed by the following formula: where Dt = theoretical draft, inches of water. H = height of flue or chimney, feet. = barometric pressure, inches of mercury. po = density of air at 0 F and 1 atmosphere pressure, pounds per cubic foot. s = density of flue gas at 0 F and 1 atmosphere pressure, pounds per cubic foot. = temperature of air surrounding the chimney, Fahrenheit absolute. 7\ = temperature of the gases, average or effective, in the chimney, Fahrenheit absolute. ' -Theoretical natural draft may also be expressed by the following simpli fied formula in which it is assumed that the densities of the flue gas and of the air are equal at the same temperature and pressure : . pH(T, -- To) D`~ '5.2T. where: p mean density of atmospheric air, pounds per cubic foot. (2) 433