Abstract: This invention provides a Refrigerant Side Economizer cycle and a fuel-heat based heating cycle for air conditioners, heat pumps, and chillers. The built-in refrigerant side economizer cycle can be substituted for an air-side economizer cycle or a water-side economizer cycle. It can provide an economizer cycle for split system air conditioners, chillers, coolers, and heat pumps. Also, it makes possible an economizer mode for variable refrigerant flow units. With this invention, the units can distribute heating from a fuel-heat source without compressor operation. During refrigerant side economizer mode, the refrigerant pump in the units maintains circulation without compressor operation, with a fraction of energy consumption. To allow for longer distance between split-system units, the refrigerant pump if needed can operate during compressor operation.