Abstract: A parallel-bivalent apparatus functioning as an absorber heat pump and as a boiler for heating a heat carrying medium, includes a condenser, an evaporator, an absorber and an expeller forming the boiler. The heat carrying medium is passed through the absorber. The apparatus further includes a heat exchanger for poor and rich solutions. The heat exchanger receives poor solution from the expeller and applies the poor solution to the absorber through an expansion valve. A bypass conduit circumvents the heat exchanger and is arranged for carrying one of the solutions between the expeller and the absorber. A valve is contained in the bypass conduit for opening the bypass conduit when the flow temperature of the heat carrying medium falls below a predetermined value. Further, heat exchanging surfaces in the absorber are so dimensioned as to effect thereon condensation of flash gas generated by expansion of the poor solution upon passage through the expansion valve.