A waste heat source (100) is used to heat a high temperature heat transfer fluid which is used to heat an absorption heat transfer machine (10) having a generator (20), an absorber (30), a condenser (40), and an evaporator (50) operatively connected together. The high temperature heat transfer fluid can also be used to heat a load (190) such as a room space or a process. The waste heat source (100) can also be used to heat an intermediate heat transfer fluid, which can be used to heat a second load (175) such as a space, a process, or an absorption heat transfer machine. Novel flow control devices (70, 60) for controlling the flow of weak solution from generator (20) to absorber (30) or of refrigerant from condenser (40) to evaporator (50), respectively, are also described.