So once we’ve located the furnace, the first thing we need to do is survey the furnace. We need to identify what type of furnace we will be cleaning, where our supply and return air ducts are, and also if the system has air conditioning or not. The furnace we are cleaning today is an 80% efficient natural gas unit. This particular unit is a Carrier. There’s a multitude of different furnaces out there that you are more than likely to encounter. There’s wood-burning, oil-burning, coal-burning, electric, mid-efficient and high-efficient natural gas, and the list goes on. Natural gas is the most common, but the important thing to remember is that they all operate on the same principles.