Your furnace is a big box that takes in cold air,cleans it with a an air filter, heats it up using a steel heat exchanger then it blows it through out your house through the duct work.  There is a difference between a conventional and condensing furnace. The difference between the heat exchanger technology used to extract heat from the combustion process and the method used to exhaust the combustion gases.  Have your furnace checked out before the cooler air comes in.  Call furnace repair Denver today!