How do you seal gas flue from furnace?

To seal a gas flue from a furnace, first ensure the flue is properly disconnected from the furnace. Use high-temperature silicone sealant or a metal flue cap to cover the opening, ensuring it is airtight to prevent gas leaks. Additionally, inspect the area for any cracks or gaps and seal them with appropriate materials, such as metal tape or a chimney cap, to ensure proper sealing and safety. Always follow local building codes and regulations for gas appliances.