BAX is a condensing heat recovery unit built from AISI 316 Ti stainless steel that must be located after the flue gas exit output of boilers running only on natural gas or LPG. The flue gases passing through the recovery unit undergo a considerable reduction in temperature, and, if the system return temperature is below 57°C, the water contained in the gas condenses by exploiting the latent heat of condensation. The recovery unit must be hydraulically connected in series to matching boiler; the increase in water temperature is only a few degrees centigrade and it is therefore necessary to verify the minimum working temperature allowed by the boiler.