Energy-efficient facilities programming involves making a trade-off between construction costs and operating costs. In general, energy-efficient construction costs more to build but less to operate for heating, cooling, and lighting. Facilities, especially continuously occupied residential structures, with thermal loads primarily dominated by the envelope are comparatively simple and straightforward to analyze and to adapt to climate. A passive solar heating system is one in which the thermal energy flow is by radiation, conduction, or natural convection. This is distinguished from an active solar heating system, which uses fans or pumps for forced distribution of heat. Direct gain is a passive solar heating system type consisting of a facility with south-facing windows that admit slanting rays of winter sun. In order to examine the potential of energy-efficient nonresidential facilities through a process of careful design, construction, and field testing, in 1979 the US Department of Energy (DOE) instituted a large design development and field test program in response to the questions.