This paper discusses the strategy of improving the efficiency of air-source heat pumps by adding a small amount of heat to the sensor of the thermostatic expansion valve (TXV). TXV heating retards the closing of the valve and boosts energy efficiency in heating mode. Test results demonstrate that appropriate TXV heating achieves an improvement in coefficient of performance (COP) and thermal comfort. The required heating power is no more than 40 W and the additional equipment cost is less than $20 at manufacturer cost (2006). Thus, the strategy of TXV heating is both technologically practical and low cost. (C) 2009 Elsevier Ltd. All rights reserved.