*2.3. Measurements*

Measurements were carried out using the devices listed in Table 2. Air temperature, relative humidity, and carbon dioxide concentration in the rooms were recorded. Temperature and consumption of tap water and DHW (if any) were measured in each apartment. Detailed information on electricity consumption in the apartments was also obtained. Thermal energy consumption measurements varied depending on the heat source. In the case of premises equipped with district heating, readings were obtained from individual energy meters. In the case of flats with heat sources powered by natural gas, gas flow was measured, if possible, or consumption was noted manually at daily intervals.


**Table 2.** Measuring Devices.

In the case of apartments heated by electric heaters, each heat source was connected to an individual electricity meter. In flats heated with solid fuel stoves, the weight of fuel used for heating was registered and the temperature of the tiled stove surface was measured. Additionally, thermovision studies were carried out to better investigate these heat sources, and the values of ambient temperature and insolation were measured.

## *2.4. Computational Models*

For each apartment, a calculation model of final energy demand for heating and DHW preparation was developed, based on the guidelines of the standard [27] and Polish regulations [28]. The model development was preceded by a detailed site inspection. During the field visits, the area of the premises, the level of thermal modernization of the building, the construction of building partitions, and the technical condition of windows and doors were determined. Weather conditions and conditions of use of the apartments were assumed according to the guidelines for engineering calculations [28]. Assessments made in strict accordance with these recommendations are described as Simulation 3.

To increase the accuracy of the calculations, the data of weather observed during the tests were used. It should be noted that in the analyzed period, the average ambient temperature was higher than in previous years. Figure 2 compares these values against the average values from the last 10 heating seasons in Wroclaw and the average temperature from the 30-year period, which are used in the calculations for energy certification of buildings [29].

**Figure 2.** Average daily ambient temperatures in Wroclaw in the selected period.

The average ambient temperature during the study period was 4.4 ◦C, while the average for the last 10 years in the same period was 1.9 ◦C. An even lower value of −1.0 ◦C is reached with the data used in calculating audits and energy certification. Therefore, there is as much as a 5.4 ◦C difference in relation to the period of research. It should be emphasized that the period of the last 10 years is characterized by much milder temperatures in the heating season than in previous years. The average ambient temperature in the last 10 years in Wroclaw was 1.72 ◦C higher than for the same period in 2000–2009 [30] and 3.15 ◦C higher according to the climatic data prepared for energy audit and certification [29]. As a result, the computational energy demand for heating is overstated in relation to the real values. The scale of these discrepancies is shown in the energy gap values described in this paper.

The calibration process was based on the recognition of users' habits. In order to determine actual internal heat gains, data on the time for basic housework, home furnishings, and the use schedule of the premises were obtained. The level of ventilation was estimated based on measuring the concentration of carbon dioxide and obtaining information from residents on the organization of air exchange in the rooms, i.e., the presence of ventilation grilles and trickle vents. In the calibration process, the internal air temperature was measured in every room of the apartments, as well as in staircases, basements, and attics. As a result, the actual temperatures of the surrounding spaces were determined. For calibration of the energy consumption calculation model for DHW production, actual measurements of domestic hot water consumption were used. The most important data used in the calibration process are shown in Table 3. The calibration process was performed until the simulation result was equal to the real final energy use (qH, qW), also described in Table 3.


**Table 3.** Results of the measurements: conditions during measurements and standard of use of apartments.

Internal temperature is a very important factor that a ffects energy consumption for heating in residential premises. In the case of the tested apartments, a correlation between the average internal temperature and the type of heating system used was observed, as shown in Figure 3. In flats supplied from central heat sources (A8–A11) and individual gas boilers (A12–A15), this profile was correct and no significant di fferences in temperature were observed in di fferent rooms. The temperature of the examined apartments oscillated between average and maximum values. The inhabitants were able to maintain thermal comfort in the rooms without much e ffort. Indoor temperature is not only based on thermal comfort needs, but is also an indicator of problems related to energy poverty or the inability to provide adequate temperature [31]. In some of the apartments, the average interior temperature during the research period was about 16 ◦C, and the lowest recorded in the bathroom was 13 ◦C. This is mainly due to the heating characteristics of solid fuel stoves (A1–A4) and the most expensive electric-powered systems in Poland (A5–A7). In apartments heated with systems powered by solid fuel or electricity, the interior temperature profile is not correct. Significant di fferences in the measured air temperature in di fferent rooms in the apartment were observed, and temperature changes oscillated between average and minimum observed values.

**Figure 3.** Average indoor air temperatures in apartments during the tests: 14 January to 9 March 2020.
