There are two main reasons you should ensure the area around the furnace is clear before switching it on. First, you need good air circulation around the furnace for it to run officially, and having obstructions around it will interfere with airflow. Secondly, some of the items around the furnace may be flammable, increasing the risk of fire when you start it up.
The furnace also needs to be clean; dust accumulation, combustion residue, and other debris interfere with the operation of the furnace in many ways. For example, some of the debris may be combustible and the debris may also block airflow and interfere with heat exchange. Fortunately, cleaning the unit is as easy as using a vacuum and a brush attachment to get rid of the debris.
The blower belt connects the furnace's motor to the air blower. It transfers power from the motor to the air blower so that it can force heated air through the ducts and vents and circulate it throughout the house. This means your house will not be heated if the blower belt is defective or broken. Therefore, before starting the furnace, inspect the blower belt to confirm that is it not worn out, cracked or loose; tighten the belt if it's loose or replace it if it's damaged.
The furnace exhaust and vents are responsible for channeling out waste gases outside the home. These waste gases may not only be chock full with particles that reduce indoor air quality, but they may also be poisonous. Unfortunately, they may sometimes be blocked by dead tree leaves, bird droppings, and even pests, among other debtors. Therefore, it's advisable to ensure these openings are clear before running the furnace.
Lastly, you should also confirm that your safety detectors, mainly the smoke and carbon monoxide detectors, are not malfunctioning before starting the furnace. This is because one of the risks of furnaces is that they may leak carbon monoxide or start a fire. Test the detectors to ensure they are operational and have good batteries; otherwise, you may not be as safe as you think.
You may want to get a heating tune up to be sure your heating is in top shape.