Over the last two decades, electrical energy generation has become more sustainable (photovoltaic, wind energy, etc.), but also more distributed, less predictable, and less controllable. Besides storage and flexible production, Demand Response (DR) offers great opportunities to help stabilizing the electrical grid. This paper presents how the flexibility of space and domestic hot water heating in existing residential buildings can be controlled for grid services. It focuses on the Internet of Things (IoT) framework including both hardware and software to connect existing buildings to a central Virtual Power Plant (VPP) intelligence. It also presents field experiments that were performed during the European FP7 SEMIAH project.