Hot End Temperatures
The temperature in the flue pipe of your furnace can run as high as 400 degrees Celsius, which is about 750 degrees Fahrenheit. This temperature is an indication that your furnace is not running at maximum efficiency and you are losing heat up the chimney.
Cool End Temperatures
Your furnace may have a flue pipe temperature as low as 150 degrees Celsius or about 300 degrees Fahrenheit. This indicates that your furnace is not putting out sufficient heat.
For maximum efficiency, your flue pipe temperature should be in the range of 175 to 280 degrees Celsius or 350 to 540 degrees Fahrenheit, according to Natural Resources Canada.