HVAC ducts should be secured to the floor structure and be installed at least 6″ from the crawlspace floor. This will prevent moisture from penetrating the insulation on the duct causing mold and an unsafe enviroment in the living space above. Often time when work is being performed under the house, the worker will climb over the duct breaking the mounting straps and sometimes bending and collapsing the duct preventing proper air flow to the structure. It can also be pulled loose from the main trunk. This is another important reason one should consider a home inspection prior to purchasing a home.