The performance of a typical air source heat pump could be increased dramatically by a relatively stable air temperature with a high humidity, even during the peak heating months. In this short communication we show such conditions exist in the underground transport tunnels of the Glasgow Subway system, where we had conducted an annual survey of air flow, air temperature and relative humidity at thirty different points within the subway network. We found relatively stable temperatures and sufficient air movement inside the twin tunnels (average temperature during winter = 15 degrees C, annual variation = 2.6 degrees C; average air flow = 16.47 m(3)/h) indicating higher system efficiency compared to a conventional air source heat pump installation. Potential energy and carbon savings are discussed. (C) 2016 Elsevier Ltd. All rights reserved.