In recent years, boilers have fully exhibited and proven their green conscience, so that they are now used in sustainable homes or buildings that do not sacrifice the environment and its resources to the comfort and well-being of the inhabitants. The demarcation line is often very narrow and this is precisely where the heat pump expresses its raison d'être, namely the ability to combine excellent performance with more and more use of renewable energy sources. An efficient machine that exploits the renewable resources present in nature ensuring heating levels in line with those obtained through more traditional systems, but with considerable energy savings. Air-to-water heat pumps use outside air as a source to heat the water, water-to-water systems draw groundwater as a heat source, and ground/water solutions use geothermal energy from the ground as a heat source. How does one choose between the available technologies? It is necessary to carefully assess the thermal needs, analyze the available renewable sources, know the performance of the building envelope, whether the project involves a new construction or a renovation.