**5. Conclusions**

Carefully analyzing the results obtained it can be noted that the building under consideration is characterized by the final space heating energy consumption being lower than the space heating energy needs and lower than the primary energy consumption. At the same time, primary energy consumption is less than the heating energy needs (if heat recuperation from ventilation systems is included in the calculations of the final energy consumption). This low consumption of final energy results from the use of a heat pump for which a seasonal coefficient of thermal performance (SCOP) is used instead of standard efficiency or effectiveness of devices and systems. So the use of a heat pump should always be recommended to achieve a small final energy consumption.

In standard buildings with standard thermal energy systems supplied by fossil fuels the primary energy consumption is always the highest, then the final energy (which is lower than primary energy) and the lowest being the energy heating needs (in this case for the space heating). In energy efficient buildings the application of a heat pump reduces the final energy consumption, which is lower than energy needs (what is also the case in the energy system described in this paper). Such a lowering of final energy can be also achieved and fostered through application of a ventilation heat recovery system (real operational effectiveness of the recuperative heat exchangers is 70% in the considered system, theoretical one accounts for 85%). However, it can be noted why the heat recovery of the

ventilation system should be considered as final energy consumption, when the decision to use such a system is made during elaboration of the architectural concept of the building (ventilation ducts for forced airflow must be considered at this stage). So when the space heating of the low energy solar house is considered in this paper and if we assume the heat recovery of the ventilation system at the stage of determining the space heating energy needs, then comparing all three forms of energy, i.e., energy needs, final and primary energy consumption, the final energy consumption is the lowest, and the energy needs are lower than the primary energy consumption. Next, when application of renewables is taken into account then the primary energy consumption is reduced. How much this energy is reduced depends on the share of renewable energy sources used to fulfill the energy requirements. All these considerations show how di fficult it is to determine the energy performance of a building. The problem is that in the o fficial requirements for determination of energy performance of a building there are no limits (indices) for the heating energy needs of the building, so perhaps if a heat pump and the heat recovery ventilation system are to be applied, then the space heating energy consumption can just be at the limit of o fficial regulated indices, even if not much has been done to reduce the energy needs.

Another problem is connected to o fficial national regulations, which do not require (or even do not allow) inclusion of the electricity use by electrical lighting and appliances, when determining the energy performance of a residential building. Consequently, when any renewable energy system, like photovoltaic or wind energy, is used, then it can be considered if such a system supplies energy to drive the heating device, like a heat pump or electric heater. This means that any energy produced by such renewable energy systems for electrical appliances during the whole year cannot be included in calculations for determination of the energy performance of buildings. Thus the air heat pump used all year round to supply heat for DHW and seasonally for space heating can turn out to be a much better solution than using solar collectors for heating energy demand, mainly for DHW demand. This paper describes a ground source heat pump operated during the space heating season. Such a heat pump operates in a very e fficient way, because out of heating season the ground can recover and come back to its natural thermal state very quickly. As a result the thermal conditions of using the ground source throughout the space heating season are very good (SCOP is quite high). In a case of an air heat pump the SCOP is much lower, because of using an ambient air source during winter.

It turns out, that determination of the energy performance of a solar low energy house based on official regulation does not allow for showing how energy e fficient and smart the building really is. The main problem is that there are no limits on the energy demand of the building.

The basis for ensuring the energy e fficiency of the building is primarily ensuring significantly reduced energy needs for the energy used in the building, i.e., the heating energy (energy for DHW, for space heating or cooling) and electricity. The biggest energy saving is the lack of demand for it. In buildings, significant savings can be obtained through the appropriate architectural concept of the building, designing the compact shape of the building, opening the southern side of the building to the impact of solar radiation and closing it tightly from the north to limit the impact of the external environment, especially in winter. A suitable concept for the interior of the building, e.g., as described in this paper, is the concept of using bu ffer zones, including the introduction of a southern bu ffer zone into the interior of the building, allowing the use of energy from the environment, including primarily solar radiation, and thus significantly reducing heat demand for heating. The architectural concept should ensure a natural temperature zoning of the interior of the building, including high thermal living comfort zones for permanent residence of people, zones for periodic residence (e.g., in transition seasons like spring and autumn) and non-residential zones (e.g., for cold or hot auxiliary facilities). Such natural zoning ensures a significant reduction in final energy consumption, and determines the smartness of the building. One could say it is an innate, inborn intelligence because the smartness is achieved naturally, passively, and not through the use of complex energy managemen<sup>t</sup> systems in the building.

As has been presented in this paper, a low energy solar building is smart through its architecture, construction, energy efficient devices and systems applied, and through the well managed operation of all components based of utilization of renewable energies. It can therefore be concluded that the building should have the built-in (embodied) smartness of reducing energy demand and consumption, achieved naturally, passively, as well as through the use of energy-efficient devices and installations, planning appropriate operating priorities and the use of efficient low-carbon energy sources, preferably renewables.

**Author Contributions:** Conceptualization, D.C.; methodology, D.C. and M.C.; software, M.C.; validation, D.C. and M.C.; formal analysis, M.C.; investigation, D.C. and M.C.; resources, D.C. and M.C.; data curation, M.C.; writing—original draft preparation, D.C.; writing—review and editing, D.C. and M.C.; visualization, M.C.; supervision, D.C.; project administration, D.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** Open Access and Article Processing Charge are covered by the Institute of Heat Engineering, Faculty of Power and Aeronautical Engineering of Warsaw University of Technology.

**Conflicts of Interest:** The authors declare no conflict of interest.
