While metal pipes and ductwork are great heat carriers, additional outer wraps or treated coatings help keep more heat en route to the vents where air comes out to heat rooms. Adhesives also block air from escaping from seams where air flow systems are joined to the furnace or to more lengths of ductwork. As with gaskets and caulking, products with asbestos were common when reinforcing ducts and pipes because they'd contain the heat without deteriorating under the high temperatures [source: CPSC].
Old systems of steam piping and even some hot water plumbing are wrapped in asbestos-containing "blankets" that pose serious risk when removed or cut without the help of a professional who uses protective measures to damper the release of particles. Any systems in older homes, even if they appear intact, can be evaluated for cracks or deterioration by a trained asbestos inspector. Disturbing the materials or self-testing is not advisable. If wear is found, ductwork and piping may be covered, or encapsulated, and sealed in [source: EPA].