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TABLE 304.1 <br /> COMBUSTION AIR REQUIREMENTS FOR GAS-FIRED <br /> APPLIANCES WHEN THE COMBINED INPUT IS UP TO <br /> AND INCLUDING 400,000 Btu/hr <br /> TOTAL INPUT OF APPLIANCES, REQUIRED FREE AREA OF AIR-SUPPLY ACCEPTABLE APPROXIMATE ROUND DUCT <br /> THOUSANDS OF Btu/hr OPENING OR DUCT,SQUARE INCHES EQUIVALENT DIAMETERZ, <br /> (kW) (sq mm) INCH(mm) <br /> 25(8) 7(4,500) 3(75) <br /> 50(15) 7(4,560) 3(75) <br /> 75(23) 11(7,000) 4(100) <br /> 100(30) 14(9,000) 4(100) <br /> 125(37) 18(12,000) 5(125) <br /> 150(45) 22(14,000) 5(125) <br /> 175 (53) 25(16,000) 6(150) <br /> 200(60) 29(19,000) 6(150) <br /> 225(68) 32(21,000) 6(150) <br /> 250(75) 36(23,000) 7(175) <br /> 275(83) 40(26,000) 7(175) <br /> 300(90) 43 (28,000) 7(175) <br /> 325(98) 47(30,000) 8(200) <br /> 350(105) 50(32,000) 8(200) <br /> 375(113) 54(35,000) 8(200) <br /> 400(120) 58(37,000) . 9(225) <br /> 1. For total Inputs failing between listed capacities, use next largest listed Input. <br /> 2. If flexible duct is used, Increase the duct diameter by one inch. <br /> *Flexible duct shall be stretched with minimal sags: <br /> BTU Amount for Non-direct vent appliances <br /> i <br />