The minimum amount of natural gas needed in the furnace fuel air mixture for proper burning is _____

• A. 2%
• B. 4%
• C. 6%
• D. 8%

Answer: (B)

Flammability limits describe the range of fuel-air mixtures that will burn. For natural gas, which is mainly methane, the mixture must contain a minimum amount of gas to sustain combustion. The lower limit is about 4–5% methane in air by volume, so the smallest amount that will still burn is around 4%. If the mixture has less gas (like 2%), it’s too lean to ignite or sustain flame. Mixtures with more gas (6% or 8%) can burn, but they’re not the minimum required; the question asks for the minimum.