A CO2 reading of 7% at the breech and 10% over the fire indicates what issue?

A CO2 reading of 7% at the breech and 10% over the fire suggests a problem with the efficiency of combustion, where the higher CO2 reading over the fire indicates incomplete combustion occurring in the burner area. A difference in CO2 levels between the two locations can point to issues related to how combustion gases are mixing and escaping, which can be affected by infiltration in the heat exchanger.

In a well-functioning system, CO2 levels would be more consistent, and the readings should indicate that combustion is occurring optimally with minimal losses. When infiltration occurs, it disrupts the proper air-to-fuel ratio, leading to inconsistencies in combustion products. This can cause elevated CO2 levels at the fire side while potentially lower readings at the breech. Proper identification of infiltration issues is crucial, as it can influence appliance efficiency and safety.

In this context, a problem with overheating of the burner or excessive combustion efficiency does not directly relate to the CO2 readings, and a leak in the chimney would typically not lead to increased readings at both points in the system. Therefore, identifying infiltration in the heat exchanger as the cause aligns with the readings seen in this scenario.