This paper exposes a HVAC system installed and tested during the Solar Decathlon Europe 2012 competition at the SMLsystem prototype. This system is focussed to improve the performance of a heat pump based HVAC system with two thermal storage tanks using phase change materials (PCM). A cold tank is used to take advantage of the low outside temperatures at night to cool the PCM with a high COP and it is used later to cool the building when the outside temperature rises. The second tank is operated as an alternative hot reservoir which provides the system the flexibility to dissipate the heat to the tank at a constant temperature granting never to reduce the COP from a minimum value. Different functioning strategies to evaluate the efficient use of the HVAC system for cold accumulation and heat dissipation are shown and implemented. (C) 2014 Elsevier B.V. All rights reserved.