By now, most conscientious builders and designers know that ducts, furnaces, and air handlers belong inside a house’s conditioned space. Researchers have shown that ductwork in unconditioned attics or vented crawlspaces wastes about 20% of the output of a furnace or air conditioner. If the duct system is unusually leaky and poorly insulated—or as occasionally happens, if some of the ducts are crushed, ripped, or completely disconnected—the energy waste will be far higher.
The furnace or air handler’s location determines where ducts should be installed. The best locations for ducts are in insulated basements, sealed crawlspaces, or unvented conditioned attics. If these places won’t work, ducts can be installed in openweb floor trusses or in some type of soffit, dropped ceiling, or chase.