The energy efficiency of the heater depends on the U-value at certain energy costs. The article considers the calculation method for the heater performance efficiency in the inlet ventilation and air curtain of the garages for 100 and 75 motor vehicles of the largest industrial enterprise rendering transport services in transportation of loose cargoes, including ores and metal-containing products, large-sized and heavy cargoes, hazardous cargoes etc. The ventilation systems of the surveyed garages are combined with air heating. In order to determine the efficiency of the air handling units, the heat carrier flow through the heater, heat carrier temperature and the air temperature at the inlet and outlet of the air heater, the flow of heated air and the phase currents through the fan motor have been measured. Basing on the instrumental measurements carried out, the calculation results for the air heater thermal performance and the air consumption have been presented, and the determination method for the real U-value and the thermal efficiency of the examined heaters has been provided. Recommendations have also been made to improve their performance. This method is an important hands-on recommendation for the engineers serving air handling units and for the departments developing energy-saving activities for industrial enterprises.