Heating Ceiling System Efficiency in Different Climate Zones

For the purposes of heating system design, the countries specify climate zones and the characteristic outdoor (design) temperature values [14]. The territory of Ukraine is divided into 2 climate zones according to the DBNV.2.6-31 standard [15], in Poland there are five, given in the standard PN-EN 12831:2006 [16]. The climate zones of both countries are displayed in Fig. 1. Exemplary cities located in the considered climate zones were pointed out and presented in Table 1 together with the mean and design air temperature values. In the case of Poland, assigned outdoor air design temperature values were determined about 65 years ago [17]. Table 1 also presents the indications of the climate zones and design temperature values given by ASHRAE for the year 2021 [18]. Calculation conditions according to ASHRAE are determined for weather stations registered by the World Meteorological Organization and have long-term hourly or three-hourly observations with a minimum measurement period of at least 12 years [14]. When comparing the values from the literature sources, it is visible that design air temperature values vary greatly whereas yearly mean ones vary slightly.

Figure 1.

A residential building belonging to Social Housing Associations, which provides affordable apartments with lifelong leases, is considered in the analysis. This is three floor building with an area of one floor equal to 637.4 m², without a basement and with two staircases. There are 13 flats on each floor, 7 in the left segment and 6 in the right one, so 39 flats are in total in the building. There are one-, two- and three-bedroom flats. The layout of the building was adopted on the basis of technical documentation and the model for heat loss calculations is shown in Fig. 2. The stairwells do not require a heating system, as they are heated by the gains from neighbouring rooms. The heated area is equal to 1912.3 m². The partition constructions were assumed so that the heat transfer coefficient values do not exceed the maximum values given in the standard [15] in the case of Ukraine and in the regulation [20] in the case of Poland. The minimum thermal insulation requirements for Poland are much stricter than for Ukraine, which is visible in Table 2 where the required values of the heat transfer coefficients are presented together with those assumed in the analysis.

Figure 2.

The indoor air temperature values according to the standards [15, 20] maintained by the heating system in the rooms are presented in Table 3.

The heat loss for the residential building was calculated according to the EN 12831 standard [16] using the Audytor OZC 7.0 Pro software package. Heat demand is determined based on the introduced building model (Fig. 2) and climate data for the assumed location of the building (Tab. 1). For the calculation of spaces heat loads the heat exchange between units (flats) was considered, whereas heat load for the entire building was determined without this heat exchange. The package allows for estimating the heating demands of all rooms in the building and assessing if the amended parameters of the medium ensure the required needs.

Based on the information provided in the manufacturer's brochures [4,21], the specific parameters of the ceiling panels are listed in the upper part of Table 4. The heating capacity given there is only an approximate value and is not suitable for direct use in heating system design or thermal performance assessment. To design and analyse radiant heating/cooling systems in detail, heat transfer coefficients are used [22]. The performance characteristic q_h of 1m² of thermo active ceiling panels was obtained at the RWTH Institute Aachen [21] based on EN 1264-2 [23]. It depends on the difference Δt_av between the average medium temperature (supply t_s and return t_r) and the temperature inside the room t_i, which takes various values depending on the function of the room and is given by the formula: (1) qh=4.64⋅Δtav, {{\rm{q}}_{\rm{h}}} = 4.64 \cdot \Delta {{\rm{t}}_{{\rm{av}}}}, where: (2) Δtav=(ts+tr)/2−ti \Delta {{\rm{t}}_{{\rm{av}}}} = \left( {{{\rm{t}}_{\rm{s}}} + {{\rm{t}}_{\rm{r}}}} \right)/2 - {{\rm{t}}_{\rm{i}}}

In the lower part of Table 4 is given the ceiling heating capacity depending on Δt_av and its thermal performance q_h for the indoor air temperature values (Table 3) valid in both countries and assuming a medium temperature of 35/33°C.

The data presented in previous sections were used to perform the calculations. Research was carried out according to the procedure shown in Figure 3.

Figure 3.

The heat load of the residential building, calculated according to the requirements valid in the countries, in all climate zones, is shown in Table 5. In both climate zones in Ukraine, the heat load of the building is higher than in a building located in Poland (regardless of climate zone). Furthermore, in the I climate zone in Ukraine and in the IV in Poland the heat losses differ significantly, although the design temperature values are the same (Table 1). This is caused by both: the weaker insulation of the building partitions and the higher mean indoor air temperature in the apartments in Ukraine. It is worth noticing, that the reduction of the indoor temperature setpoint by 1°C in each room could reduce the heat load demand and energy consumption even by 16% [24].

The specific heat load was calculated as the mean value for the entire building and is also displayed in Table 5. When comparing those values with the thermal performance given in Table 4 it seems that in the case of Poland, the thermal performance should be sufficient in all climate zones. However, in Ukraine, the system may be deficient because the thermal performance of the ceiling heating system in rooms with indoor temperature +25°C is lower than the specific heat load.

For particular apartments in the residential building, the specific heat demand given in [W/m²] was evaluated and compared with the specific heat output q_h of the ceiling heating system which was calculated according to formula (1) with respect to the mean indoor temperature Θ_int in the apartment. Outcomes are presented in Table 6 where: No is the number of flats, A_h is the area of flat [m²], and I, II, III, IV and V are climate zones.

The values in Table 6 highlighted in green show the apartments with the lowest heat load, while the red colour indicates the apartments in which the ceiling heating efficiency is not sufficient to maintain the inner conditions required when the heating medium temperature values are equal to 35/33°C.

The results of the calculated heat demand for individual apartments in Ukraine indicate that 15 apartments in zone I and 6 in zone II are deficient in heat demand. In Poland, the ceiling system heating capacity is sufficient to meet the average heating demand in all apartments.

In Ukraine, apartment A62 (climate zone II) was identified as the one with the lowest heat load and A11 (climate zone I) as the one with the highest heat load. In Poland, apartments A63 (I climate zone) and A12 (V climate zone) were identified as those with the lowest and highest heat load, respectively. It is interesting that the apartments are not located in the same place in the building layout in both countries. It is caused by the differences in mean design indoor temperature Θ_int shown in Table 6.

The apartments A11 and A12 with the highest heat loss are located on the ground floor of the building while A62 and A63 are on the floor I. They are shown on the repeated floor plan in blue and orange in Fig. 4.

The thermal power shortages in apartment A11 in Ukraine in both climate zones are significant and cannot be omitted. Raising the medium temperature allows the heating capacity of the ceiling panels to increase. The minimum temperature of the medium, which allows the heat loss to be covered, was determined based on the relationship (2) and knowing the calculated heat load of the apartment from Table 6. The results of the estimation are shown in Table 7. In the case of the I climate zone, the minimum supply temperature of the medium must be increased by 3°C and is established at 38/36°C. For the II climate zone, the minimum supply temperature of the medium should be 1°C higher.

Figure 4.

In the next step, the heat demand in the individual rooms was analysed to verify if even though the results in Table 6 suggest that in Poland the amount of heat delivered to all apartments would be sufficient, the thermal performance of the ceiling heating will be high enough to cover the heat demand in all rooms. Detailed analysis showed that in certain cases power shortages in the rooms can be noticed. It refers to, for example, bathrooms in all buildings in five climate zones. In the V climate zone, there are also thermal power shortages in the kitchen and living room. In zone IV, the problem only affected the bathroom, and in the other two rooms, the heat demand was only negligibly higher (< 20 W and < 3%) than the amount of heat supplied by the ceiling heating.

The layouts of the apartments with the highest and lowest heat demand (A11, A62, A12 A63) with the thermal loads ϕ_HL and the indoor design temperature values t_i are shown in Fig. 5. Table 8 presents the values of the specific heat loads of the rooms q_HL and the capacities of the heating system q_h, as well as the thermal power shortages ΔQ.

Figure 5.

In Fig. 6 the percentage values of thermal power shortages in the best and worst case apartments, according to Table 8, are given and illustrated by shadings. These illustrations and Table 8 allow to recognise in detail the differences between apartments in particular countries and climate zones. In Ukraine, in zone I, rooms with a required temperature of 22.0°C had insufficient ceiling performance up to 524 W (Table 8). In the climate zone with a temperature of −19°C, in apartment A62, insufficient heat will be delivered to the bathroom and bedroom. The shadiness in Figure 6 highlighted that the heating system is the most insufficient in the bathrooms due to the highest values of the required indoor temperature.

Figure 6.

In apartment A11, in the living room and bathroom, the values of the heat load are higher than the average value in the entire apartment (Table 6). The necessary medium temperature should be 42/40°C to cover heat loss in these rooms, which is not allowed because the maximum temperature of the ceiling surface will exceed 35°C. In apartment A12 with the highest heat loss, in the V climate zone in Poland, to provide sufficient heat to the living room and kitchen, the medium supply temperature should be 36/34°C, while the heat losses in the bathroom need the delivery of the medium with a temperature too high for the system, which is 40/38°C.

When considering performance shortages of ceiling heating systems, it should be noted that heating systems are generally sized with a thermal power surplus resulting from the design assumptions discussed earlier. Thermal power is increased due to heat exchange between apartments, as required by standard PN-EN 12831 (assuming that adjacent apartments may periodically have reduced temperatures due to, for example, longer absences of tenants). Additionally, the outdoor design temperature values (Table 1) are very low compared to the ASHRAE data. Such low temperatures occur extremely rarely, only for a few hours, during the heating season, and the power calculations do not consider either internal and external heat gains or the thermal accumulation of the building.

Oversized heating systems do not operate efficiently, they require larger and/or additional heat sources, increasing the negative impact on the environment. Systems should operate efficiently, according to actual heat needs. The ongoing revision of the EU Building Efficiency Directive [25] will also force changes to national regulations. This creates an opportunity and space for the design of the heating system methodology to be revised and upgraded according to climate changes and conditions related to room usage.

When designing the heating system for the building according to the requirements given in Sections 2.1 and 2.2 certain thermal power shortages in rooms are negligible and may be omitted. However, when the heating ceiling system does not meet the heating demands, specific solutions can be proposed:

designing the building envelope (walls, floor on the ground, roof and windows) with reduced heat transfer coefficients, and elimination of thermal bridges,

In existing buildings, while planning a deep thermo-modernisation process [27], the ceiling system will have the possibility to be also implemented because the heat demand would be reduced by improving the performance of the building envelope and reducing the heat transfer coefficients. Furthermore, during this process ensuring proper air tightness of the building will be significant because leaky windows cause higher air velocity in the rooms causing thermal discomfort for the residents [28, 29].