What is the life expectancy of a flue fan in a furnace?

The actual name of that item is called a power ventor. They usually last anywhere from 7 to 12 years, but can break wear out after only a year or two. Sometimes, if your furnace is not set up properly, (according to the heat rise check that should be done after installing any new furnace), your power ventor can burn out prematurely. The heat rise calculation of a forced air furnace is calculated by measuring the difference in temperature between the Return Air and the Supply Air (incoming air and outgoing air to and from the blower). Dont get this confused with exhaust temperature from the Power Ventor. This heat rise reading that you get will ensure that the exhaust temperature is ok, as long as the heat rise value is within the acceptable parameter. The range of temperature where the heat rise parameter should fall is listed on the manufacturers rating plate on the Furnace...Usually inside the cabinet somewhere. Example: It will say... Heat Rise 35 F to 70 F. So as long as the difference in the temperature readings taken from the return air (Cold Air) and the supply air (Hot Air) is a reading anywhere between 35 and 70 degrees Farenheit, your system should be venting properly....unless you have had a wrong type of venting system installed for that furnace, or a blocked chimney, the unit should vent (exhaust ) properly. Having a properly vented furnace is key to longevity of its parts. Good Luck...ps. I forgot to tell you one thing... When doing the heat rise check, you should crank up your thermostat in heat mode. Make the heat run steady for a good 10 minutes. This allows your furnace to reach its peak operating temperature and start running at a steady state. This steady state is where you want to take your readings. If you go to a Plumbing or Refrigeration supplier, you should be able to get a probe type of thermometer for about 20 to 30 dollars. It kinda looks like a thermometer that you stick in a nice Beef roast. If you have a duct system you can drill a 3/16 hole in each duct plenum to insert your probe. Make sure you dont drill into your AC Evaporator coil that is usually mounted in the supply (hot air plenum) of a forced air furnace that works as the air handler for the central AC system.