Gas Fitter Practice Test 2025 – The All-in-One Guide to Mastering Your Certification!

In a well-functioning gas furnace, the approximate oxygen reading typically ranges from 3% to 5%. This range indicates efficient combustion, where sufficient fuel is being burned with minimal excess oxygen present in the exhaust gases.

When the oxygen level is too high, it can signal that the burner is not using the fuel efficiently, leading to higher emissions and decreased efficiency. Conversely, if the oxygen level is too low, it may indicate incomplete combustion, which can be dangerous and produce harmful byproducts such as carbon monoxide.

Reading around 3% suggests that the combustion process is close to being optimal in terms of both efficiency and safety. Thus, a measurement of 3% indicates effective fuel usage, where the furnace is burning the gas properly while keeping oxygen levels low without leading to incomplete combustion. This ensures both a clean burn and reduced pollution levels, which is essential in proper furnace operation.