## 3 Application in an Existing Building  
The case studied in this paper is a single-family residential building built in 2015. The building is located in the village of Zielonki Wies near Warsaw in Poland. The building consists of two main blocks. The shape of the garage section is parallel to the sides of the plot, while the residential part is rotated by 19.5\({}^{\circ}\) so that the wall with the largest amount of glazing is oriented south (Figure 1). During the winter only the residential part is air-conditioned by a continuous, heating system. The assumed indoor temperature as set on the thermostat, is 20 \({}^{\circ}\)C. The only available energy carrier at the site was electricity. For this reason, as well as for economic reasons, the designer used electric cables placed in a 5-cm thick concrete floor as surface heating. The building was designed using traditional masonry techniques. It is made of materials with a large thermal capacity: masonry walls made of lime-sand blocks, a monolithic reinforced concrete ceiling above the ground floor, and a concrete floor with an epoxy resin layer. The heat transfer coefficient of the external partitions is between 0.09 and 0.10 W (m\({}^{2}\)-K). The building is mechanically ventilated, and heat is recovered from both the supply and the exhaust. The building's air-tightness assumed in the calculation was n\({}_{50}\) = 0.6 h\({}^{-1}\), while the tested air-tightness of the building envelope was n\({}_{50}\) = 0.35 h\({}^{-1}\). Parameter n\({}_{50}\) describes the amount of air volume exchanges through the building envelope at a pressure difference of 50 Pa. The air-tightness of the existing building was tested by performing the Blower Door Test according to PN-EN ISO 9972:2015-10. All the windows have external blinds, additionally the windows on the south side on the ground floor were protected from the sun by an external canopy. The usable area of the residential part of the building is 187 m\({}^{2}\), the garage and the garden room are 47 m\({}^{2}\). The study involved a thermal zone which consisted of the rooms located on the ground floor. These include the kitchen, the dining room and the living room, with large areas of glazing on the south side. The total area of the rooms analysed is 55 m\({}^{2}\), with an internal (net) volume of 140 m\({}^{3}\). The plan of the ground floor with the rooms considered in this study (marked in red) is shown in Figure 2.  
The heating energy demand obtained with the steady-state method was 12.2 kWh/m\({}^{2}\)a. The calculated final energy demand for heating amounted to 2974.6 kWh per year. The air tightness n\({}_{50}\) at a level of 0.6 h\({}^{-1}\) was assumed in the calculations. Figure 1: The case study—a single-family low energy residential building. Photo by the author. A BMS (building management system) was installed in the building in order to monitor climate parameters and energy consumption in the building. The measurements of the BMS system were registered from September 2016, and were recorded in 10-min intervals. The BMS system records weather parameters such as: external temperature (\({}^{\circ}\)C), average wind speed (m/s), maximum wind speed (m/s), dominant wind direction (N, E, S, W, NE, SE, SW, NW), light intensity (lx), precipitation (logic value y/n), air relative humidity (%) and air pressure (hPa). In addition to the weather data, the following parameters were recorded: the consumption of electricity for external and internal lighting, energy consumption to supply the heat recovery unit, heating (separately for individual rooms), domestic hot water (DHW) and the internal temperature in individual rooms. An exemplary series of data recorded from 15 February, 2018 to 2 March, 2018 is presented in Figure 3. Heating load (W) indicates that the actual temperature in the room was less than the temperature set on the thermostat (assumed indoor temperature) and thus the heating system was activated.