When measuring flue gases, the temperature must be recorded at the __________ to the chimney.

The correct choice is to record the temperature at the entrance to the chimney. This is crucial for accurately assessing the performance of the flue system. Measuring the temperature at this point allows for the evaluation of the combustion efficiency and the effectiveness of the venting system. It provides insight into how well the exhaust gases are being carried away, which directly impacts the safety and efficiency of the heating system.

The entrance to the chimney is where the flue gases first enter, and capturing the temperature here helps determine if there are any issues with the exhaust before it exits into the atmosphere. This measurement can indicate whether the system is operating within the proper temperature ranges and help identify problems such as insufficient draft or blockages.

Other locations, such as the exit point or midpoint, do not provide the same level of insight into the initial conditions of the flue gases as they are affected by various factors such as cooling or potential condensation that may occur after the gases have traveled through part of the chimney. Therefore, measuring at the entrance ensures a more accurate representation of the flue gas conditions.