Gas/Propane Basics Trane Co. In addition to proper light-off, there is safe combustion after light-off. The proper mixture of gas and primary air at the primary opening ensures a soft blue flame on the burner, not lifting or flashing back. A safe light off time with these burners, up to 400,000 BTUs, is four seconds. The control of secondary air and also excess air on atmospheric design burners is pretty much fixed and not adjustable. It is this air which affects the combustion analysis for safe combustion numbers. The numbers expected with these burners is around 7–7.5% O2 (oxygen) and 7.5% –8% CO2 (carbon dioxide). This will provide about 50% excess air. Those numbers, along with a net stack temperature of 400°F, will give us an efficiency number of about 78%. The level of CO (carbon monoxide) with those numbers needs to be less than 100 PPM (parts per million) air free. What tends to cause problems with those numbers and, in particular, too much excess air which lowers efficiency, are doors not in place on equipment—especially on water heaters. In addition, the flame is cooled and carbon monoxide levels are increased. With boilers that get their excess air from underneath the boiler, the problem occurs when they are mounted on cement blocks instead of a full platform underneath the boiler. ICM Source: Fundamentals of Gas, Volume I. Gas Appliance Service Training and Consulting, 1997. Figure 6: Crossover arrangement in which each burner feeds a portion of the gas-air mixture into the crossover where it is ignited by the pilot flame as it leaves the crossover part. 26 ICM/March/April 2015
ICM September-October 2014
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