Inspect the venting from the furnace or boiler to when it exits from the home. It must be sized correctly, it can go up in size as it leaves the home, but it can never go down in size. It must have continuous pitch. It must be free of holes and corrosion. If it is Class C venting it must have at least 6 inches of clearance to combustibles. If it is class B venting it must have a minimum of 1″ clearance to combustibles. The termination of the venting must comply with the manufactures specification.