Air heating unit to condition the inside environment or premises by heating air from outside the premises and leading it into said premises, and/or heating the air from inside the premises and taking it again into said premises, comprising at least one combustion hot-air generator provided with an exchanger for heating said air flow, wherein the heating unit, in addition to the hot air generator, has at least one heat recovery device, characterized in that said recovery device has at least: - an air inlet system into the heat recovery device from inside (i) the premises; - a duct or chamber for air inlet from inside (i) the premises that starts at the air inlet system and leads to a preheating exchanger; - a preheating exchanger having an air circuit starting at the duct or chamber for air inlet from inside (i) the premises and which leads to the outside (e), which exchanges its thermal energy with a circuit of air from the outside (e) into the preheating exchanger, and where the circuit of air from the outside (e) leads to an air inlet system into the hot air generator for its subsequent inlet into the premises, wherein the inlet into the exchanger, at least from the duct or chamber for air inlet from inside (i) the premises can be opened or closed selectively; wherein between the heat recovery device and the hot air generator there is at least: - a connection system for inlet of combustion gases from the hot air generator into the duct or chamber for air inlet from inside (i) the premises, with selective opening and closing to allow or prevent the passage of the gases generated by combustion into the duct or chamber; - a connection system for recirculation of air from inside the premises that passes from the duct or chamber for air inlet from inside (i) the premises to the hot air generator, with selective opening or closing, for recirculation into the premises; and wherein the heating unit has control means that act, at least, on the hot air generator, on the air inlet systems (111, 121), and on the connection systems (113, 114), allowing and/or regulating the air flow through these in an automatic and/or manual manner. - an air inlet system into the heat recovery device from inside (i) the premises; - a duct or chamber for air inlet from inside (i) the premises that starts at the air inlet system and leads to a preheating exchanger; - a preheating exchanger having an air circuit starting at the duct or chamber for air inlet from inside (i) the premises and which leads to the outside (e), which exchanges its thermal energy with a circuit of air from the outside (e) into the preheating exchanger, and where the circuit of air from the outside (e) leads to an air inlet system into the hot air generator for its subsequent inlet into the premises, wherein the inlet into the exchanger, at least from the duct or chamber for air inlet from inside (i) the premises can be opened or closed selectively; - a connection system for inlet of combustion gases from the hot air generator into the duct or chamber for air inlet from inside (i) the premises, with selective opening and closing to allow or prevent the passage of the gases generated by combustion into the duct or chamber; - a connection system for recirculation of air from inside the premises that passes from the duct or chamber for air inlet from inside (i) the premises to the hot air generator, with selective opening or closing, for recirculation into the premises;