## 6 Conclusions  
This study investigates the possibility of storing internal heat gains to stabilise internal air temperatures during the heating or cold season. A method for defining excess heat gains (temporary TEHG and seasonal SEHG) on the basis of internal temperature measurement data from a thermal zone with controlled environment parameters was proposed. The proposed technical solution was a small peripheral heat storage tank connected to the air ventilation circuit using phase change materials. The energy potential of this solution was defined on the basis of measurement data from a passive, ultra-low-energy building, with a heavy construction in continental climate conditions.  
The proposed peripheral heat storage tank connected with a heat exchanger contained 0.027 m\({}^{3}\) of phase change material, RT24. The analysis of energy efficiency showed that a storage tank designed in this way could effectively store short-term internal heat gains from the building's zone. Charging and discharging of the storage tank occurred practically in a 24-hour cycle, and in the winter season it achieved full charge on multiple occasions. The house was designed to minimize overheating of the building in the summer, for example, the southern canopy is used to protect the windows from direct solar radiation. Due to the change in the sunlight angle of incidence during the year, direct solar radiation penetrates into the interior in winter, which is utilised as an additional heat gain. In such a situation, the indoor temperature may increase and users open the window causing losses of unused energy. For these reasons, in today's homes it is helpful to encourage residents to appropriate behaviour of so that they have an impact on final energy consumption.  
The paper analysed an ultra-low-energy building under continental climatic conditions. Based on the measurement data from the BMS, the yearly energy consumption for three rooms was 156.4 kWh/a (563.0 MJ/a). By using the proposed heat recovery system combined with a thermal storage tank, the yearly energy savings are 51.9 kWh/a (187.0 MJ/a), or around 33%. This device has the additional task of ensuring proper thermal comfort. In building with high air tightness and low energy demand for heating, any additional heat gains, e.g., resulting from the recreational use of the fireplace, result in excessive temperature increases. In the summer, this type of device complements the systems that increase the protection against overheating.  
The proposed solution is related to a specific case, however, it shows the potential of employing this type of storage tank, especially in energy efficient and passive buildings where temporary (even during the heating/cold season) heat gains significantly exceeded heat losses. The expected energy effect could be much higher in the case of timber frame constructions with a much lower thermal inertia. Based on the simulation analysis, the achieved differences in SEHG values between a heavy and a lightweight building were about 30%. It is important to keep in mind that a building with different energy characteristics might require adjusting the parameters of the heat storage tank, including its size and the amount of PCM used.  
Although similar solutions have been applied to the storage of thermal energy from solar collectors, this paper proposes an innovative method for their future use.  
Concept formulation, literature review, D.H.; evolution of overarching research goals and aims, D.H. & A.W.; conducting measurement and data analysis, M.P.; resources, investigation, calculation, data curation and visualisation, M.P. & A.W.; development of heat storage tank concept and construction, A.W.; writing--original draft preparation, review and editing, ALL.  
This research received no external funding. The APC was funded by Politechnika Warszawska.  
**Conflicts of Interest:** The authors declare no conflict of interest.