The rationale for the design and operational mode of a simple, direct, solar thermal mass (STM) domestic heating system is presented. The benefits of using the building foundation as the heat store over alternative, more common, wet or phase change thermal storage media are discussed. It shown that the Laing Ecocirc pump, connected directly to a BP 30 Wp PV module provides an effectively constant collector outlet temperature over the operating range of irradiance. Through comparison of measured collector thermal output and thermal mass temperature rise, it is seen that there is good agreement between predicted and measured system performance.