Energy storage is a very active area of research in recent years as it provides a sustainable solution to energy demand fluctuations and increases energy efficiency. Different energy storage methods such as mechanical energy storage (gravitational energy, flywheels); electrical storage (e.g. batteries); thermal storage (sensible, latent) and thermochemical heat storage [1] can be used. Thermal energy storage (TES) is particularly suited to buildings because ahighpercentage oftheir energydemandrelates toheating and cooling needs. Sensible TES utilizes the heat capacity properties of materials while latent TES uses heat exchanges via the phase change of materials, usually between solid and liquid for building applications. Latent thermal energy storage (LTES) can provide more energy per volume than a sensible thermal storage system, making LTES a promising solution for buildings either integrated into building envelope (passive LTES) or in ventilation systems (active LTES) to reduce cooling demand [2] or reduce heating demand [3]. Active LTES integrated in mechanical ventilation systems has received attention during the last two decades. A room ventilation system incorporating heat pipes embedded in PCM thermal battery was tested experimentally for applications in the UK 20 years ago [4,5]. Heat transfer rates of up to 200W were measured under simulated UK summer conditions comparing the system favorably to conventional air conditioning and other technologies such as cooled beams. Since then, many investigations through experiments and simulations followed. A recent review [6] critically discusses experimental studies of PCM applications in buildings dividing them in free cooling passive and active methods, active and passive heating methods and hybrid applications. It describes developments in ventilation and air-conditioned systems based on PCMs as well as nano-enhanced PCMs. The extensive literature review has revealed that active LTES incorporated within the ventilation system can overcome heat exchange limitations of passive systems because of the increased heat transfer by convection and LTES is an appropriate solution to increase energy efficiency of cooling systems in buildings. A review [7] focusing on cooling LTES applications summarises experimental results of active LTES and discusses the importance of PCM selection according to cooling needs due to internal heat gains and climatic conditions. The PCM melting temperature is one of the most influencing parameters for the success ofthe application. Such recent reviews have also highlighted that limited analysis has been published from operational buildings with commercially installed LTES systems; such results are important to accelerate inclusion in designs for new and refurbished buildings. This paper attempts to fill part of this gap by presenting field measurements from two operational case-study rooms ventilated and cooled through a commercially available mechanical ventilation system incorporating an active LTES. Section 2 describes the ventilation unit, the two case-studies and how the system works. Section 3 presents the operational performance of the system in terms of thermal comfort, indoor air quality and energy use. Section 4 presents validated thermal and CFD models which were used to carry out parametric analysis for performance improvements. Finally, Section 5 includes the conclusions.