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How to determine the gas flow to the burners
The gas orifice
Below are charts that give the amount of gas (the first 2 charts are for natural gas) that will flow through different size orifices at different pressures. The size of the orifice is usually marked on the orifice. If there is no marking, the sizes correspond to the number drills. The orifice to the right is a number 42. Using the chart at 3.5 in wc, this orifice will flow 24,950 BTU.