Where should CO testing occur for a 90+ efficiency condensing furnace?

The correct answer is that CO testing for a 90+ efficiency condensing furnace should occur at the vent termination. This is because the vent termination represents the point where combustion gases, including carbon monoxide (CO), exit the furnace system and are released into the atmosphere. Testing at this location allows for the assessment of whether the system is properly venting and whether any harmful gases, such as CO, are being expelled safely.

When testing at the vent termination, it is important to verify the efficiency and performance of the venting system, ensuring that the gases produced during combustion are adequately removed and do not pose a risk to building occupants. The results from this testing can indicate if there are any blockages or other issues in the venting system that could lead to elevated CO levels within the dwelling.

Testing at the other suggested locations, such as the vent above the inducer fan, at the appliance burner, or in the exhaust duct, may provide specific insights into the combustion process or duct integrity, but they do not effectively measure the actual performance of the venting system relative to safe atmospheric conditions. Therefore, conducting CO testing at the vent termination is essential for ensuring that the entire combustion and venting process is functioning safely and efficiently.