In the paper the padding investment costs and decrease of the consumption on a thermal energy are reviewed at salvaging a heat of exhausting air under the scheme with intermediate heat carrier in systems of mechanical ventilation. The nature of relation of the data of costs from a factor of temperature efficiency of heat recoverer is parsed. The expression for optimum temperature efficiency is offered by minimization of cumulative discounted costs during given computational term. The analysis of the obtained outcomes is given and the nature of their change at different parameters of an outside climate, and also operational price level on the equipment and fares on power supplies also is justified. The comparison of conclusions of the investigation to the data of the previous author’s researches is conducted. The presentation is illustrated graphically and by an example of numerical calculation.