How is the effectiveness of a gas furnace's combustion typically assessed?

The effectiveness of a gas furnace's combustion is typically assessed by measuring carbon monoxide levels. This method is crucial because carbon monoxide is a byproduct of incomplete combustion. When a furnace operates efficiently, it should produce minimal carbon monoxide, indicating that fuel is being burned completely. If elevated levels of carbon monoxide are detected, it suggests that there may be an issue with the furnace's combustion process, such as insufficient air supply, improper fuel mixture, or a malfunctioning burner.

In contrast, checking the temperature rise can provide information about the heating performance of the system but does not directly assess combustion effectiveness. Adjusting the thermostat is related to controlling the heating output rather than determining how effectively the gas is burned. Using a gas pressure gauge can reveal if the correct pressure is supplied to the furnace, but it doesn’t specifically measure combustion efficiency or its byproducts. Therefore, measuring carbon monoxide levels stands out as the most direct method to evaluate the combustion effectiveness of a gas furnace.