Comprehensive Energy Diagnostics Carried Out In a Residential Building – A Case Study

The Polish National Center for Research and Development coordinated the implementation of the Strategic Research Project “Integrated System for Reduction of Energy Efficiency of Buildings”, under which Research Task no. 4. “The development of thermal diagnostics of buildings” was predominantly carried out in the Department of Heating, Ventilation and Dust Removal at Silesian University of Technology. A detailed description of the scope of the project is given in [1].

In the first steps, the project group has developed quick methods of building diagnostics for different building components, i.e.: building insulation [2], heating and domestic hot water systems [3], ventilation and air conditioning systems [4], the indoor environmental quality [5] and energy performance certification [6]. Each method includes on-site inspection, diagnostic and computational analyses. The final stage of the 3-year project has focused on the practical application and validation of developed methodology in real buildings, described in detail in the published Handbook Vol.5 [7] and presented in scientific/research articles [8, 9, 10, 11, 12].

This paper presents the insights on a comprehensive on-site diagnostics based on an exemplary multi-family building. It gives practical information on how to properly conduct diagnostic measurements for an energy performance certification..

The 5-storey residential building (Fig. 1) selected for the study is located in the western suburb of Gliwice, surrounded by buildings of similar height. It consists of a basement, ground floor and 4 upper-ground floors. Each storey has 4 apartments of the area varying from 50 to 74 m². The total heated area of each floor is approximately 1350 m². On the ground floor, the area of 235 m² is used by the Housing Cooperative. The remaining space is purely residential.

Figure 1.

The building has been built in the technology of a large slab (the pre-fabricated W-70 system) in the early 1980s. The three-layer construction has been used for 27-cm thick external reinforced concrete walls with wool insulation. Ceiling construction uses channel ceiling slabs with thickness of 24 cm. The ventilated roof is covered with a tar paper. Window joinery in the building is almost entirely replaced with a new one made of PVC–only two dwellings and a staircase do not have the listed windows. The heat transfer coefficients were identified based on measurements, and on the archival documentation in accordance with standard [13]. Table 1 gives a summary of the thermal specification of the building construction.

Detailed on-site heat measurement methodology, including equipment description, is shown in the Handbook Vol.1 [2].

The proposed value of the thermal bridge around the windows (Fig. 2) was ψ = 0.03 W/(m·K) and the linear bridge at the connections of wall panels (Fig.3) was ψ = 0.08 W/(m·K) (on the entire perimeter of the boards). The thermography studies on special places indicated local insulation discontinuities or a significant reduction in the insulation from slag wool on the entire height of the building. The value of the linear bridge was estimated as ψ = 0.04 W/(m·K). After taking into account thermal bridges, the proposed corrected value of the heat transfer coefficient for the entire side partition with 6-m. panels was U = 0.85 W/m²K.

Figure 2.

Figure 3.

The values of heat transfer coefficients for window and door carpentry were adopted based on the catalogue: new PVC windows in residential rooms: U = 2.4W/(m²K), old wooden windows U = 3.0 W/(m²K), glazed entrance door in metal frames U = 3.2 W/(m²K). Transparent and non-transparent glass barriers did not meet the criteria for thermal insulation requirements.

Central heating (CH) and domestic hot water (DHW) installations in the building are powered by a direct, low parameter heat centre located in the room in the building basement. The major modernization of the junction was carried out in 2005 despite the lack of thermo-modernization of the building.

This node is supplied with heat from a dual-function exchanger group node located outside the building. The nominal temperatures of the heating medium sent from the group node are 80°C/60°C, and the temperature of the heat source prepared for DHW is close to 58°C. The flow of heating medium in the supply lines of the CH system enforces circulation pump installed in a group node–CH installation is equipped with individual venting valves mounted on thide ends of the risers. A detailed description and equipment of the node together with the diagram can be found in Handbook Vol.2 [3].

Pipe network for CH installation was made of steel pipes, pipes for DHW installation and circulations lines were made of plastic lines. All pipes, including distributors, and valves, were thermally insulated with foamed polyethylene linings. Insulation thickness was ~15 mm, depending on the pipeline diameter. The insulation was in good technical condition. However, it did not meet the requirements given in [14] for all pipeline diameters. All branches within the room of the low-temperature node were hydraulically tight. The room has a window – no dedicated exhaust ventilation was in use. The inspection indicated that the direct node was in good technical condition and was appropriately operated (details in Handbook Vol.2 [3]). The mass flow rate of the heating medium deserves attention and correction – the flow of 0.75 kg/s is required, while the measured mass flow of the heating medium was 0.42 kg/s. Such a substantial difference in fluxes may indicate that the heating curve was incorrectly set for the CH installation. No damage was observed to the node elements that could significantly change the node’s operation in relation to the conditions of its proper operation.

As part of the diagnostics, the actual required maximum thermal power of the node for supplying heat to the CH system was estimated. The ordered power, taking into account the constant efficiency of control and heat transfer through the heating system, should be 61.8 kW. The power ordered for the studied building was equal to 63.8 kW, which confirms the correctness of the hitherto accepted order power. The calculations made for the measurement data on March 29, 2012 (temperature of water on the supply and return, room temperature, heat source power, heat loss of CH and DHW pipes together with the fittings) allowed to determine the average daily efficiency of the heat source for CH at 98.2%. The seasonal efficiency of the node was estimated at 97.5% for CH supply and 97% for DHW supply.

The central heating installation is a two-pipe system with a lower distribution, in a traditional layout–flats are supplied with multiple connection points. The supply risers of the internal installation are supplied from two main branches run from the manifold. The operation parameters are 80/60°C.

The residential rooms have cast-iron heaters equipped with Danfoss thermostatic valves and heating cost allocators. Radiators made of finned tubes are used in the basement (workshop, laundry, drying room). The branches of the installation at the manifolds and under the verticals are fitted with ball screw-in stop valves (under the risers, additionally with the possibility of draining the water). The installation has automatic vents mounted on the top of the risers.

The installation is made of welded steel pipelines, carried out under the ceiling in the basement, while the risers in the apartments run by the walls. The pipelines outside the junction room were insulated with a mineral wool wrapper with a thickness of 2 to 5 cm – the whole installation was wrapped with a cover made of aluminum foil. Approaches for risers (50 to 60 cm long) and valves were not insulated (Fig. 4, 5). In the building, the room temperature control used thermostatic valves on radiators. The hydraulic adjustment was made by means of thermostatic valve settings.

Figure 4.

Figure 5.

The heat meter was installed at the heat sources node. The supply and return lines are equipped with manometers and temperature sensors connected to the heat meter.

As part of the modernization of the installation, a heat meter has been installed and an individual system for heat consumption settlements has been applied (a system based on liquid heat cost allocators). The venting network has been removed, and automatic vents have been installed at the highest points of the risers. Radiator valves and orifices has been replaced with thermostatic valves with double control strategy, while the right and oblique valves under the rises were exchange for ball valves and the orifices.

Installation of domestic hot water in the analysed building was subjected to diagnostics according to the procedures described in the Handbook Vol.2 [3] and in [18]. Within the building, there is a central hot water installation with a circulating system. Installation of cold and hot water, and circulation is distributed in the basement in 8 plumbing divisions. In each apartment there are two installation sections – bathroom and kitchen. Cold and hot water pipelines in apartments are made of galvanized steel pipes and run in wall furrows. Just before the diagnostic took place, the building had underwent a modernization of the water installation and circulation, which included replacement of horizontal pipelines from galvanized steel pipes run in the basement with the PE pipes maintaining the original pipe diameters. The valves and sub-orifices in the circulation circuit has been replaced with TA balancing regulating valves (Fig. 6). In addition, three bathroom pipes has been changed into PE pipes with 1 cm thick polyurethane foam insulation. Other divisions of the DHW and circulation pipes carried out in installation ducts have not been replaced. They are made of galvanized steel pipes, which are uninsulated.

Figure 6.

The building is equipped with a natural gravity ventilation system. Each flat has 2 or 3 gravity conduits terminated from the side of the rooms with grates. In smaller apartments, one grille has been installed in the kitchen and one in the bathroom, while the larger apartments have an additional grille in the separate toilet. The number of discharge ducts is the same at the low and high floors. The apartments are equipped with rectangular or round grids of various sizes, i.e.: 16×16 cm, 17×17 cm, 10×12 cm, and Ø11 cm. Ventilation grilles are located under the ceiling of the rooms and are exposed.

Ventilation ducts in the building draw air out over the roof. Ducts are not equipped with chimney cowls and do not have inspection openings . Initial inspection showed that the ducts were unobstructed. With only a few exceptions, ventilation grilles were quite clean (Fig. 7). Small exhaust fans have been installed in few bathrooms and toilets (Fig. 8).

Figure 7.

Figure 8.

Most windows in the building were fairly tight and had micro-ventilation feature. Only in two apartments there were few decades old wooden windows. None of the windows was equipped with air inlets. The entrance door to the building was not very tight, while the doors to the apartments were generally tight. The door to all toilets and bathrooms has been provided with free flow – sleeves or grilles in the door. There were leaks in the form of cracks in building construction and poorly sealed verticals of central heating in apartments. In some apartments, there were also noteworthy leaks at the joins of the walls. Diagnostic measurements included direct measurement of the ventilation air flow in the outlet grilles and leakage evaluation with a blower door test. Measurements were made in apartments M1 and M2 on the second floor and M15 on the fifth floor on 17 February 2012. The outside temperature was about 3.5°C, and the day was moderately windy (wind speed about 2.5 m/s). The measuring sleeve of the balometer was used for the measurement according to the methodology described in Handbook Vol.3 [4]. The air flow was measured at the closed windows without micro-ventilation. The results are summarized in Table 3. The number of exchanges was calculated from the sum of ventilation air flows in individual grids in the apartment.

Diagnostic of the built environment was carried out in accordance with the guidelines described in the Handbook Vol.4 [5]. It consisted of long-term continuous measurements and survey study, on the basis of which the assessment of the quality of the thermal environment and the quality of the indoor air was carried out. Measurements of indoor environment parameters were carried out from February 11 to March 8 during the heating season. During this period the outside air temperature remained above 0°C, which did not meet the representativeness requirements of the thermal environment quality assessment for the heating period, The required outside temperature for this region is below -2.1°C. APAR AR 235 air temperature and relative humidity loggers, and SENSOTRON PS32 indoor air quality monitors were used for the measurements. The accuracy of the temperature measurement is 0.5°C in the range from 20°C to 30°C. In the remaining range it may vary in the range of 0.5–1.8°C. Sensors have been calibrated before the start of the measurements. Sensors have been installed in 5 residential flats, mostly in bedrooms and living rooms. Additional sensors were placed in the basement and in the stairwell for the monitoring purposes, but these readings were omitted in the analysis of the indoor environmental quality.

Figure 9 shows the results of the thermal condition measurements and their quality categorization based on standard PN-EN 15251:2012 [25] and defining category of indoor air quality. The air temperature for most of the evaluation period varied from 20°C to 25°C. In both bedrooms (in M2 and M5) and in the living room located in the south-east direction (M2), the air temperature values recorded are in the range of 22.5–25°C. The air temperature values recorded in living rooms in other apartments were ~3 K lower. The average daily temperature fluctuation of 1 K was observed within the whole facility. The relative humidity of the air during the measurement period fluctuated in the range of 20–60%.

Figure 9.

For the most part of the time the temperature in the rooms met the requirements of the second quality category, which corresponds to the recommended conditions for new buildings and after renovation. The questionnaire surveys also confirmed a high level of satisfaction with the thermal environment. Air quality assessed on the basis of indications of concentrations of metabolically discharged carbon dioxide indicates is shown in Figure 10. In the analyzed apartments, the concentration exceeded the level of the recommended category III for more than 40% of the occupied time, which indicates insufficient air exchange.

Figure 10.

Heat consumption for heating and ventilation in the standard heating season can be determined using the energy signature (ES) method described in the standard [25]. It applicates the linear regression to determine the empirical dependence of energy consumption on the outside temperature, and then to calculate the energy consumption in the heating or cooling season. Typically, this method assumes a constant value of the internal temperature, and that the outside air temperature is a parameter that has the greatest impact on energy consumption. In the analysis performed in this research task, the variability of the internal temperature values was taken into account in order to include heat depending on the internal heat gains, making heat demand dependent on the difference between the indoor and outdoor air temperature. This modified methodology is described in the Handbook Vol.6 [6] and articles [26, 27, 28]. In accordance with the adopted guidelines, the energy signature method can be used in the case of measurements of heat consumption in a period longer than one month–in our case, the measurement data from the whole heating season were available (212 days).

Discussed energy characteristics were based on measurements of heat demand and measurements of the external environment throughout the heating season of 2011/2012 (1.10.12-30.04.12) and measurements of indoor environmental conditions made in the period from March 14, 2012 to April 3, 2012. In order to apply the method, the following parameters were used:

hourly average outdoor air temperature values measured by the meteorological station in Gliwice,

The calculations were made for the standard climate date in Katowice where the average season temperature t _e,av,s.STD was equal 2.7°C.

Fig. 11 shows the dependence average daily heating power in reference to the internal and external air temperature differences (t_i,av-t_e,av) in the heating season 2011/2012 (212 days). Based on the linear regression equation (a correlation coefficient of 0.96), the average heating power was calculated for the difference between the internal air temperature and the outside air temperature of the standard season (dT _av,s,STD = 19 K). On the basis of the average heating power, the heat consumption in the heating standard season was calculated, taking into account the duration of the standard heating season L_s = 212 days (from October to April). The average seasonal indoor air temperature of the air was assumed 21.6°C (temperature calculated on the basis of measurements taken in the period of 14.03–3.04.2012.

Figure 11.

The final energy forecast for heating and ventilation in the standard heating season for Katowice is 368.4 GJ (102 330 kWh). This is the heat consumption for the standard outdoor climate conditions and for the actual conditions of use of the building, i.e., an internal temperature of 21.6°C with the actual infiltration rate and internal gains estimated for the whole heating season. The internal air temperature was higher than the normative value for residential buildings, while the ventilation air flows were lower than recommended in the ventilation standard [23].

Onsite diagnostic data has been used to extend the analysis, by which the energy performance evaluation was prepared in accordance with the regulations on the methodology of energy performance certification. The energy performance evaluation takes into account energy consumption for: heating and ventilation, hot water preparation and auxiliary energy. The data from measurements and indicators from the Regulation [17] were used:

the final energy used for hot water purposes was calculated based on measurements: DHW consumption 1.73 dm³/(m²·day) for the residential part and 0.4 dm³/(m²·day) for housing cooperative. The efficiency at level of 0.4 was assumed (η _W,d = 0.42, η _W,g = 0.97) based on the local visual inspection and the regulations [20],

Table 5 presents the final and primary energy consumption calculated on the basis of the carried out measurements, separately for the heating system, DHW system and auxiliary devices. The obtained energy indexes of 138 kWh/(m²yr) for final energy and 182 kWh/(m²yr) for primary energy assess the energy status of the building. As part of the analysis, the heat consumption for heating and ventilation was calculated using the calculation method described in Regulation [20]. However, the data obtained from the measurements was used for the calculation excluding heat demand. The calculated and measured heat transfer coefficients of the building partitions together with the identified thermal bridges and the designated heat sources efficiency were taken into account. The calculated efficiency of distribution and regulation of the CH and DHW systems were used together with measured actual consumption of hot water. Due to the insufficient number of measurements of ventilation air flows in the apartments, the results of these measurements were not taken into account. In addition, these values did not meet the comfort conditions and the normative (hygienic) use of residential premises. Due to the difference between the internal design temperature according to the regulations [20] (t_i,st = 20°C) and the temperature obtained from measurements (t_i,m = 21.6°C), both cases respectively were considered. Table 5 presents the calculated values of final and primary energy consumption divided into the needs of the heating system, DHW system, and auxiliary energy for heating and hot water preparation according to the Regulation [20].

When comparing the obtained energy performance results based on measurements and based on the methodology at the same indoor air temperature of 21.6°C, the final and primary energy indexes (202 kWh/(m²yr) and 265 kWh/(m²yr) respectively) are about 46% higher than measured values. On the other hand, the change of the calculation indoor air temperature to the nominal value of 20.0°C caused a further increase of 8%. It should be emphasized that the reason for such big differences is the increase in heat consumption for heating and ventilation purposes by almost 84%, and directly very poorly operation of natural ventilation, which does not provide nominal air exchange (as described in details in Chapter 6).

The comprehensive diagnosis of a residential building has indicated possible actions to improve the energy status of the building. The basic thermomodernization operation should include the insulation of external walls to the conditions that meet the current thermal insulation requirements. As part of the heat source diagnostics, CH and DHW installations should have adjusted heating curve and insulate the distribution pipes in the basement, along with insulating the fittings. Diagnosis of the DHW installation showed a 20% higher water consumption compared to the guidelines. Diagnostics of the internal environment showed above all that air temperature maintained in apartments is higher than the normative value and the air quality assessment indicates the problems with air exchange at the required level. This is confirmed by the diagnostics of the natural ventilation system, showing measurably too low air exchange in the flats and high airtightness. A malfunctioning natural ventilation system directly affects the designated energy performance of the building–measurements showed a 46% lower heat consumption for heating and ventilation than the value calculated according to the requirements. However, it should be emphasized that the presented method of determining energy performance based on measurements, in contrast to the method presented in [20], takes into account the estimation of energy consumption in relation to the standard season.