Your furnace will eventually need to be replaced because it is old or inefficient. This job should be handled by an HVAC professional who is skilled and not a DIYer. Installing a furnace requires knowledge in electrical, sheet metal work, and plumbing. 

Installing a new furnace requires knowledge and experience in all areas of skilled trades. HVAC technicians are licensed and may need to have permits in order to legally install a furnace in a home. Before starting any work, the new furnace installer will ensure that all gas and electrical supplies are completely turned off. 

SOURCE:GOOGLE

It is dangerous to allow gas and power to flow to the furnace unit during its removal. This could lead directly or indirectly, potentially, to serious injury. The old furnace can be taken out of service once the gas and power have been turned off. 

It is possible to disconnect all wires and gas supply, as well as vent pipes and ductwork. Most likely, the HVAC professional will wear gloves and be careful not to be cut by sharp edges of sheet metal. It is important to clean the area between the old furnace and the new furnace. 

You should sweep away dirt, dust, and other debris. A homeowner who has spent a lot on a furnace should not have it fail or malfunction due to excessive dust contamination. The only thing homeowners should worry about is what type of furnace they will purchase. The HVAC pros will handle the installation and removal. It will be done correctly, safely and quickly.