Inside furnaces that run to forced hot air systems, there are fire doors. These fire doors are held open by lead weights which melt if there is a fire and seal off the vents and ductwork.
Well, these nice pieces of lead also can snap. And then you have to take apart ALL the ductwork and find the darn fire door and hoist it up so your heat can work again.
Did I mention the entire thing is lined with fiberglass??