Cooling in traditional AC systems is accomplished using the vapor-compression cycle, which uses the forced circulation and phase change of a refrigerant between gas and liquid to transfer heat. The vapor-compression cycle can occur within a unitary, or packaged piece of equipment; or within a chiller that is connected to terminal cooling equipment (such as a variable refrigerant flow indoor unit or a fan coil unit) on its evaporator side and heat rejection equipment on its condenser side. An air source heat pump has shares many components with an air conditioning system but includes a reversing valve which allows the unit to be used to heat as well as cool a space.