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372 CHAPTER 14 1952 Guide : ' ' .i. should not be judged from the value of the CO% in the flue gas without . reference to the ultimate <70j obtainable. Practical values of COt will > usually be from 8 to 14 percent, depending on the gas used. i Ratings for Gas Appliances . Input rating for a gas appliance is established by demonstrating that the appliance can meet the Approval Requirements of the ASA. The tests are conducted at the A.O.A. Laboratories. Output rating is determined from the approved input and an average efficiency stated. in the Approval Requirements, and is the heat available at the outlet. t i !' f 7 Sizing Gas-Fired Heating Plants Although gas-burning equipment usually is completely automatic, | i Table 1. Capacity op Gas Piping Nominal Diameter of Pipe in Inches Length of Pipe m Feet *. 2 Capacity--Cu Ft Per Hr with a 0.6 Sp Gr Gas and Pressure Drop of 03* Water Column 15 172 345 750 30 120 241 535 850 45 99 199 435 700 60 86 173 380 610 75 77 155 345 545 90 70 141 310 490 105 65 131 285 450 920 120 120 270 420 860 150 109 242 380 780 180 100 225 350 720 I r t maintaining the temperature of rooms' at a predetermined figure, there are some manually controlled installations. In order to overcome effec tively the starting load and losses in piping, a manually-controlled gas boiler should have ah output as much as 100 percent greater than the equivalent standard radiation which it is expected to serve. Boilers under thermostatic control, however, are not subject to such severe pick-up loads and consequently, it is possible to use a lower selec tion factor. For a gas-fired boiler or furnace under thermostatic control, a factor of 20 to 25 percent is usually sufficient for pick-up allowance. In those installations, in mild climates where 100 percent outside air is used, furnaces should be of larger size in order to provide adequate capacity and quick pick-up under intermittent heating conditions. The factor to be allowed for loss of heat from piping will vary somewhat, the proportionate amount of piping installed being greater for small installations than for large ones. For selection factors to be. added to installed radiation under thermostatic control, see Chapter 15. Appliances used for heating with gas should bear the approval seal of the A.G.A. Laboratories on the manufacturer's nameplate, together with the official input and output ratings. It is not permissible to operate a gas heating unit above its stated rating. It may be necessary to operate below this rating at elevations above 2000 ft, unless the appliance has been tested and approved for operation at altitudes up to 5200 ft, in which case such approval will be shown on the manufacturer's nameplate. Automatic Fuel Burning Equipment 373 Installations should be made in accordance with recommendations shown in .the publications of the American Gas- Association. Controls Temperature controls for gas burners are described in Chapter 38. Some central heating plants are equipped with push-button or other manual control. The main gas valve may be of either the snap action or throttling type. Automatic electric ignition is available. Sizing of Gas Piping Piping for gas appliances should be of adequate size, and so installed as to provide a supply of gas sufficient to meet the maximum demand without Table 2. Multipliers for Various Specific Gravities For Use With Table 1 Specific Gravity Multiplier Specific Gravity Multiplier .35 1.31 1.00 .775 .40 1.23 1.10 .740 .45 1.16 1.20 .707 .50 1.10 1.30 .680 .55 1.04 1.40 .655 .60 1.00 1.50 .633 .65 .962 1.60 .612 .70 .926 1.70 .594 .75 .895 1.80 .577 .80 .867 1.90 .565 .85 .841 2.00 .547 .90 .817 2.10 535 undue loss of pressure between the point of supply (the meter) and the burner. The size of gas pipe required depends upon the following: 1. Maximum gas consumption to be provided. 2. Length of pipe and number of fittings. 3. Allowable loss in pressure from the outlet of the meter to the burner. 4. Specific gravity of the gas. `J To obtain the cubic feet per hour of gas required by the burner, divide the Btu input at which the burner will be adjusted, by the average Btu heating value per cubic foot of the gas. Capacities of different sizes and lengths of pipe, in cubic feet per hour, with a pressure drop of 0.3 in. of water column for a gas of 0.60 sp gr, are shown in Table 1. In adopting a 0.3 in. pressure drop, due allowance for an ordinary number of fittings was made. To convert the figures given in Table 1 to capacities for another gas of different specific gravity, multiply the tabular values by the multipliers shown in Table 2. FUEL BURNING RATES The burning rate for automatic fuel burning devices is determined by the gross heat output required of the boiler, or furnace, to carry the net heating load, plus allowances for system losses and pick-up. General