This paper describes a perturb-and-observe (P&O) control aiming to increase the heat exchange between solar heat stored underground and the ambient of a single conditioned room without any heat pump. This P&O control increases or decreases the water flow rate through an underground hosepipe heat exchanger. With this purpose, two power converters were used to activate, respectively, a low power water pump and a fan coil so as to keep the room within the limits of a reference temperature range (between 18 °C and 24 °C). Outside these limits, the P&O control searches for the best heat exchange between the ambient room and the underground soil and, when inside these limits, the water pump and fan coil are turned off. Two identical experimental rooms, referred in this study as “reference” and “test” rooms, had their temperatures measured every 1-min during winter and summer. For comparison purposes, the reference room was left at its natural conditions without any air conditioning. The experimental results show a remarkable improvement in the heat exchange and a considerable reduction in power demand when using the P&O control. As a result, it was obtained an energy saving of approximately 45% in one summer day and 22% in one winter day. It is important to point out that this paper refers, strictly, to the description of a P&O control for heat exchange systems involving solar heat stored underground in a single room.