Where must a hole be made to perform a combustion efficiency test?

To conduct a combustion efficiency test, it is critical to measure the combustion gases produced by the burner as they exit the system. Making a hole in the flue on the furnace side of the draft regulator allows for this measurement because it captures the emissions directly as they leave the combustion chamber. This location is optimal for accurately assessing the performance of the burner and the efficiency of the combustion process.

By obtaining data from this point, technicians can evaluate key parameters such as carbon monoxide levels, oxygen content, and flue gas temperature, all of which are essential for determining combustion efficiency. This information helps in optimizing the burner performance and ensuring compliance with safety standards.

The other options would not provide the same level of insight into the combustion process. For instance, testing at the burner unit may not accurately reflect the conditions in the flue, while checking the air intake vent does not focus on gas emissions. Similarly, examining the fuel line is unrelated to combustion gases and would not yield relevant data for efficiency testing.