Heatex exchangers have a very large contact surface (approx. 250 m2 per m3 exchanger) which results in very effective humidification if water is sprayed into the exchanger. Close to 100% humidity can be achieved. In combustion plants for combined production of gas and electricity this function is optimised together with preheating of incoming air. The condensation from the cooled gas fumes is used for spraying. The total boiler efficiency approaches 115%.