A temperature and hydraulic-balance control strategy has previously been proven effective for improving the operational performance and achieving significant energy-savings in air source heat pump (ASHP) heating systems. In this study, a frequency-conversion technique was added to the control strategy for the pump operation, evolving into an updated control strategy. To investigate the performance and evaluate the energy-saving benefits when using the updated strategy, a field experiment was conducted in two almost identical ASHP heating systems in a teaching building, one with the updated strategy (system A) and the other without (system B). Results showed that the updated strategy can ensure an appropriate indoor temperature without overheating. The working number and hours of ASHP compressors in system A were lower than those in system B. Moreover, the savings of heat and electricity of system A were 46.6% and 43.0%, respectively. Electricity-saving of the pump using frequency-conversion technique reached 20.2%, and the effect is more remarkable in the high-frequency range. The percentage of pump's electricity consumption of system A was only 16.4%, which was significantly lower than the previous experiment. Additionally, the COP of ASHP units of system A was 3.23 and was not affected by the introduction of frequency-conversion technique. (C) 2019 Elsevier Ltd. All rights reserved.