The demand for energy is growing faster than the world population. In particular, electricity, which will be increasingly produced by gas or renewable sources, to the detriment of coal and heavy oils, as well as thermal energy for district heating and district cooling networks. However, one of the best ways to have more energy available is not to waste it, increasing the efficiency of each appliance, but also recovering, recycling and reusing it. Condensing economizers applied to boilers and gensets, as well as cogeneration and trigeneration systems keep usable and available energies that were previously wasted.