Final answer:
Lower efficiency furnishes require a higher amount of excess air for complete combustion compared to higher efficiency furnaces due to their less effective fuel combustion.
Step-by-step explanation:
The amount of excess air needed for complete combustion in lower efficiency furnaces is
B. Higher than in higher efficiency furnaces
. This is because lower efficiency furnaces are less able to completely combust the fuel, which necessitates a greater amount of excess air to ensure complete combustion. In contrast, higher efficiency furnaces are more effective at fuel combustion, thus requiring less excess air. For example, considering a furnace using natural gas which is largely methane, the combustion reaction requires a specific ratio of methane to oxygen. Inefficiencies can mean not all the methane is combusted, requiring additional oxygen (and therefore air) to fully combust the methane. Hence, consideration of the furnace's efficiency is crucial in estimating the excess air needed.
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