The article presents the findings of the tests conducted for the forced air heating unit of a supply ventilation system. The supply air system is designed to feed clean air into the industrial facility of the oil-gas field of the Yamalo-Nenets Autonomous District. The tests were performed in five modes at the ambient air temperatures of -5, -10, -15, -20, -25 °C. The mode throughout the tests was steady, i.e. the temperatures of water and air remained unchanged within 30 minutes before the tests. Since the capacity of the heater unit was proved to be 24 - 41.5% according to the test results, recommendations were made to improve the capacity. With the said measures having been taken, control measurements of test parameters were carried out, the actual U-value was calculated and the capacity of the heater unit was determined. The performed work is relevant in view of theoretical and practical significance for engineers and employees of industrial enterprises servicing the ventilation systems.