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384 CHAPTER IS 1952 Guide 2. Liquid and Gaseous Fuels. The combined efficiency of boiler, furnace and burner is the ratio of the heat absorbed by the water and steam in the boiler per pound or cubic foot of fuel, to the calorific value of 1 lb or cubic foot of fuel, respectively. The following efficiencies apply to current designs of boilers operated tinder favorable conditions at their gross output ratings. Some older boilers, designed primarily for hand firing, may have lower efficiencies when automatically fired. Anthracite, hand-fired .......................................................... 60 to 75 percent Bituminous coal, hand-fired........................................... 50 to 65 percent Stoker-fired................................................................................... 60 to 75 percent Oil and gas-fired.......................................................................... 70 to 80 percent Higher, efficiencies for hand-fired bituminous coal may be obtained by careful firing of either a regular or a smokeless boiler. RATING OF BOILERS In referring to boiler rating, it is necessary to know the basis on which .t he - rating has been established in order to understand the exact meaning of the term. The following example will illustrate the meaning of three ratings which might be established for the same boiler; Assume that an installation has the following loads determined in accordance with the section Selection of Boilers: ` Net Load...................... '............................ ......... 1000 sq ft of steam radiation Piping Tax.............................................................. 200 sq ft of steam radiation Design Load.................................. 1200 sq ft of steam radiation Pickup Allowance.................................................. 240 sq ft of steam radiation Maximum or Gross Load...................... 1440 sq ft of steam radiation :. A boiler that is just large enough to carry this system might be said to have a net load rating of 1000 sq ft, a design load rating of 1200 sq ft, or a gross load rating of 1440 sq ft, depending on the basis on which the boiler is rated. On a net load basis the boiler would be rated 1000 sq ft of steam radiation and .would have sufficient excess capacity to supply the normal piping and pickup load. Net I = B -- R Ratings, SBI Net Ratings, and Net Load Ratings of the Heating, Piping and Air Conditioning Contractors National Association are established on this basis. On a design load basis the boiler would be rated 1200 sq ft of steam radia tion and would have sufficient excess capacity to supply the pickup load. It would be of adequate size for a system in which the sum of the net load and the piping heat loss did not exceed 1200 sq ft of steam radiation. The SBI Ratings shown in columns 1, 2, 3, 10, 11 and 12 of Table 2 (not to be confused with SBI Net Rating) are established on a design load basis. On & gross output basis of rating, the boiler would be rated 1440 sq ft of steam radiation and would be of adequate size for a system in which the sum of the net load, piping load, and pickup load did not exceed 1440 sq ft of steam radiation. Gross 1= B -- R Output and A.G.A. Ratings are established on a gross output basis. In the determination of boiler ratings, the Gross Output is the quantity of heat available at the boiler nozzle, with the boiler normally insulated and when operating under limitations stipulated in the code or method by. which the boiler is rated. The boiler may be capable of producing a greater nozzle output, but in doing so would exceed some of these limitations. Heating Boilers, Furnaces, Space Heaters 385 SELECTION OF BOILERS General Factors ' The Maximum Load or Gross Load on the boiler is the sum of the four following items. The Design Load is the sum of items 1, 2, and 3. The Net Load is the sum of items 1 and 2. 1. Radiation Load. The estimated heat emission in Btu per hour of the connected radiation (direct, indirect, or forced convection coils) to be installed. The connected radiation is determined by calculating the heat losses for each room in accordance with data given in Chapters 9,10 and 11. The sum of the calcu lated heat losses for all the rooms represents the total required heat emission of the connected radiation, expressed in Btu per hour.. As practically all boilers are now rated on a Btu basis, it is unnecessary to convert the radiation load to square feet of equivalent direct radiation. 2. Hot Water Supply Load. The estimated maximum heat in Btu per hour re quired to heat water for domestic use. I = B = R recommends that allowance for hot water supply load be made only for bathrooms in excess of two, as follows: Instantaneous' Coil 12,000 Btu per hour, and for Storage Tank installation 120 Btu per (hour) (gallon of tank capacity). For instantaneous coil installations the boiler capacity should not be less than required to heat 2 to 3 gal of water, 100 deg per min. See also Chapter 48. 3. Piping Tax. The estimated heat emission in Btu per hour of the piping con necting the radiation and other apparatus to the boiler. As the heating industry as a whole is not entirely agreed upon piping tax allowances for different sizes of installations, it is better to compute the heat emission from both bare and covered pipe surface in. accordance with data in Chapter 27. In average house heating systems, it is common practice to consider the piping tax to be equal to 25 percent of the Net Load. In determining Net I = B = R Ratings from Gross I = B = R Output, the piping factor allowed varies from 30 percent for small boilers to 12 percent for larger boilers. 4. Warming-Up or Pick-Up Allowance. The estimated increase in the normal load in Btu per hour caused by the heating up of the cold system. The warming-up allowance represents the load due to heating the boiler and con tents to operating temperature, and heating up cold radiation and piping. The factors to be used for determining the allowance to be made should be selected from Table 4. Table 4. Warming-up Allowances for Hand-Fired Low-Pressure Steam and Hot Water Heating Boilers" 0 ,DESIGN Load (Rbpbsssntino Summation or Iran 1 2, and 3) Btu per Hour Equivalent Square Feet of Radiation* PSBCKNTAOS CAPACITY TO ADD roB Warmino-Upo Up to 100,000 100,000 to 200,000 200,000 to 600,000 600,000 to 1,200,000 1,200,000 to 1,800,000 Above 1,800,000 Up to 420 420 to 840 840 to 2500 2500 to 5000 5000 to 7500 Above 7500 65 60 55 50 45 40 * This table is taken from the A.S.H.V.E. Code of Minimum Requirements for the Heating and Ventila tion of Buildings, except that the second column hn been added for convenience in interpreting the design load in terms of equivalent square feet of radiation. b See also Time'Analysis in Starting Heating Apparatus, by Ralph C. Taggart (A.S.H.V.E. Transac tions, Vol. 19, 1913, p. 292); Report of A.S.H.V.E. Continuing Committee on Codes for Testing and Rating Steam Heating Solid Fuel Boileis (A.S.H.VJ3 Transactions, Vol. 36, 1930, p. 35); Selecting the Right Sixe Heating Boiler, by Sabin Crocker (Heating, Piping and Air Conditioning, March, 1932). c This table refers to hand-fired, solid fuel boilers. A factor of 20 percent over design load is adequate when automatically-fired fuels are used. d 240 Btu per square foot.