Experimental investigation on the thermal performance of a domestic gas boiler for space heating with different heating load ratio

. This study explores the effects of heating demand and hot water temperature on energy efficiency in a domestic gas boiler (DGB). Mock-up experiments were conducted using two types of boilers with different capacities (i.e., 11.6 kW and 23.3 kW of non-condensing boilers, 16.3 kW of condensing boiler) for comparative analysis. As results, for space heating, the efficiencies of non-condensing boilers (i.e., 11.6 kW and 23.3 kW) and condensing boiler (i.e., 16.3 kW) decreased by 12-44%, 15-46%, and 2-22% respectively as the heating load ratios were changed from 100%-25%. When the supply hot water temperature settings decreased from 55 ℃ to 45℃, the efficiencies were reduced by 13.6% (11.6 kW) and 53.3% (23.3 kW). For the condensing boiler, an efficiency reduction rate was 16.7% as the hot water temperatures dropped from 45℃ to 35℃. Also, since the boiler capacity depends on domestic hot water (DHW) load, the daily heating demands of space heating and DHW load were analyzed. In the results, reducing the lower limit of heating output could be more effective in improving the boiler efficiency for space heating. To provide sufficient DHW demand, it is required to maintain the upper limit of the heating output.


Introduction
In South Korea, residential buildings account for 49% of total building energy consumption, which is equivalent to 55% of the total energy consumption that is generated by space heating and domestic hot water (DHW) [1,2]. On the other hand, the continuous attention to global energy crisis from fossil fuel shortage and environmental pollution makes that energy conservation and emission reductions are an urgent social issue. Therefore, the improvement of building energy efficiency is an excellent help to reduce national energy use.
A domestic gas boiler (DGB) as a heating system is commonly used in South Korea to provide a thermally comfortable indoor environment and supply hot water for human living. In general, space heating is controlled according to occupancy, the boiler, therefore, is operated much longer for space heating than the DHW supplement. With the development of insulation systems and airtightness technologies in buildings, the heating load of buildings is gradually decreasing. Even if a typical usage profile of the DHW is intensively used in a daily particular and short time, the boiler has to be always ready to provide a high temperature above 50℃. Therefore, the boiler should maintain a large capacity to meet the DHW demand.
To save energy consumption, it is important to select a boiler with the appropriate capacity to maintain system performance and energy saving. Nevertheless, in reality, the size of the DGB heating capacity is typially determined based on the heating energy use intensity multiplied by the heating area or the DHW demand of housed holds in residential buildings. The boiler capacity sizing using these conservative approaches would be oversized. Such sized boilers may operate under part load conditions in most cases, which would lead to short-cycling operation and performance reduction in the thermal performance of the DGB.
In this context, the experimental analysis was conducted to investigate the preliminary influence of the oversized DGB on the boiler performance for space heating and DHW. Two non-condensing DGBs with different capacities were chosen to analyze the influence of oversized capacity on boiler performance. At the same time, a condensing DGB was also selected to compare with the performance of the non-condensing ones under the same experimental conditions.

Experimental system configurations
A mock-up experimental system consisting of a heat source (i.e., LPG gas), DGB, a heating load model, and a cooler was established to analyze the thermal performance of the boilers, as shown in Fig. 1. In order to analyze the effect of oversized capacity on the DGBs efficiency, three DGBs sold on the market with different E3S Web of Conferences 396, 03017 (2023) https://doi.org/10.1051/e3sconf/202339603017 IAQVEC2023 heating capacities were selected. The three boilers are two non-condensing boilers with 11.6 kW and 23.3 kW capacity, respectively, while the other one was a condensing boiler with 16.3 kW capacity. An apartment house with four heating zones (i.e., 1 living room, 1 kitchen, 1 master bedroom, and 2 bedrooms) were modeled in the experimental system. To simplify the experiment, the heating load for each thermal zone was set to 2 kW. The cooler of the experimental system was set up to generate heating loads of the zones.

Experitemtal conditions and measurements
To analyze the effect of the oversized capacity of the DGB on the thermal performance for space heating, the efficiency variation of the three DGBs was analyzed separately by changing the heating load ratio. The experimental conditions of different heating load ratios were realized by adjusting the number of heating zones, as shown in Table 1. Whether the room is heated was controlled by an on-off valve of a heated water cycle for space heating.
Moreover, in order to understand the influence of supply water temperature on the thermal performance of the boilers during space heating, the supply water temperatures from the non-condensing boilers were set to 55℃, 50℃, 45℃, and 40℃, respectively. For the condensing boiler, return water temperatures to the boiler were set to 45℃, 40℃, 35℃, and 30℃, respectively. Then, comparative experiments were conducted. In real life, the size of the boiler also depends on the DHW demand. Therefore, the efficiency of the boilers under the practical application conditions was also analyzed. Based on the occupant schedule of residential buildings, the operation schedule of boilers was set as no heating during the daytime and continuous space heating at night. The DHW was used once in the morning and evening. All four thermal zones were heated simultaneously in this study. In addition, the supply water temperatures of non-condensing boilers for space heating were set to 55℃, 50℃, and 45℃, and the return water temperatures of condensing boiler were set to 45℃, 40℃ and 35℃, and the DHW temperature was set at 50℃. All four heating zones were heated during the space-heating period The boiler efficiency is defined as the ratio of the output heat from the boiler to the input heat to the boiler, and the efficiency can be calculated as: where η is the efficiency of boiler, Q is the heat transferred to the rooms by the heated water [kJ/hr], and G is the heat generated by the gas combustion [kJ/hr].

Boiler efficiency variations under partial load conditions.
Figs. 2 and 3 show the experimental results of noncondensing boilers, and the effect of different supply hot water temperature from the non-condensing boilers on the boiler efficiency was also analyzed. As the results, the efficiency of all boilers tends to decrease as the heating load ratio decrease.This refers that the efficiency of the boiler for space heating will decrease with the decrease of the heating load ratio. When comparing the 11.6 kW and 23.3 kW non-condensing boilers, the efficiency of the 11.6 kW boiler was relatively higher than the 23.3 kW boiler under the same heating load condition. It represents that it is necessary to select a boiler with a suitable capacity, matching the actual heating load to maintain the boiler operation efficiency. Moreover, in the same boiler, the lower the supply water temperature, the lower the efficiency of the boiler. In the case of the non-condensing boilers with different heating capacities, the one with a larger capacity showed a larger efficiency reduction rate according to the decrease in the hot water temperature.  Fig. 4 shows the efficiency analysis results of the condensing boiler. Similar to the experimental results obtained for non-condensing boilers, as the heating load ratio decreased, the efficiency of the condensing boiler also decreased. However, the efficiency reduction rate of the condensing boiler was relatively lower than the non-condensing boilers, and the condensing boiler showed relatively high efficiency compared to the noncondensing boilers under the same heating load conditions. In addition, the efficiency also decreased as the supply water temperature decreased. Therefore, in terms of energy saving, it is necessary to select a boiler with an appropriate capacity according to the actual heating load, and at the same time, it is necessary to consider the setting of the temperature of the heating supply water.
Figs. 5-7 represent the operating cycles of the boilers under different each experimental conditions. With the increase of the boiler capacities for the non-condensing boilers, the number of operating cycles increased under the same heating load conditions. In the case of noncondensing boiler, the number of operating cycles increased as the heating load decreased. On the other hand, the frequent operating cycles increased the energy consumption of the boiler system inevitably. Briefly, when using an oversized boiler, the performance of the heating system would be lowered, which would deteriorate the energy efficiency and savings. For the condensing boiler, the operating cycle, however, is more frequent than non-condensing ones under partial load conditions. The reason why the condensing boiler maintained high efficiency is that the condensing boiler absorbed the condensation heat emitted from the boiler to the outside and reused it to heat the supplied heating water.

Total boiler efficiency for space and DHW
When a residential household uses the DHW while space heating, the DHW heating then takes priority. For these reasons, the boiler efficiency can be divided into three cases: efficiency for space heating, efficiency for DHW, and total boiler efficiency.
As shown in Figs. 6-8, the heat generation of the gas source, the heat production and the efficiency of the boiler for space heating were analyzed. As results, a larger boiler consumed more gas and produced more heat, regardless of the type of boilers. For space heating, the boiler with a large capacity had lower efficiency than the one with a small capacity. That is because, while using a larger boiler, the boiler mainly was operated in a part load stage, causing the boiler frequently on-off.
Hence, it consumed much gas when heating, and the efficiency of the boiler was reduced. As mentioned in Section 3.1, the efficiency of the condensing boiler was significantly higher than that of the non-condensing one, regardless of the capacity.
Figs. 9-11 show the experimental results for the DHW. Since the setting value of the DHW temperature was the same, the gas consumption and heat production had to be the same when using the same boiler. However, in the efficiency comparison of the three boilers, the efficiency of the condensing boiler was higher than that of the non-condensing ones. In the case of noncondensing boilers, the boiler with low capacity showed higher efficiency than the boiler with high capacity. This also indicates that the oversized capacity of boilers may deteriorate the operation efficiency of the boilers. Based on the boiler efficiencies for space heating and DHW, the total efficiency of the boiler was calculated as follows: where the ηtotal is the total efficiency of boiler, Qsh and QDHW are the heat generation for space heating and DHW[kJ/hr], respectively, and Gsh and GDHW are the heat generated by the gas combustion for space heating and DHW[kJ/hr], respectively. Fig. 12 shows the total efficiency results for each boiler under different supply water temperature conditions in the space heating. As a result, in the case of non-condensing boilers, the total efficiency of boilers increased as the supply water temperature increased, and the boiler with low capacity was always more efficient than boilers with high capacity. For the condensing E3S Web of Conferences 396, 03017 (2023) https://doi.org/10.1051/e3sconf/202339603017 IAQVEC2023 boilers, the calculated total efficiency in the three experiments was similar, and almost all were higher than the other two non-condensing boilers when the supply water temperature for space heating was the same value. This provides findings that the condensing boiler can effectively maintain high working efficiency when the boiler is under a part load condition.

Conclusion
The objective of this study was to analyze the effect of the oversized capacity of the DGB and the supply water temperatures on the efficiency of the boiler systems. In addition, the influence of the boiler capacities on the efficiencies of the DGB during space heating and DHW was also explored. To this end, mockup experiments were conducted to evaluate the efficiencies of two non-condensing boilers and one condensing boiler.
The experimental results indicated that, in the case of non-condensing boilers, the efficiency of the boiler with a higher capacity was lower than the boiler with a small capacity under the same experimental conditions. Compared with non-condensing boilers, the condensing boiler showed the higher efficiency. This fact verifies the advantage of the condensing boiler when operating at low heating loads.
In addition, the supply water temperature has a noticeable effect on the boiler efficiency during space heating. If low-temperature heating is needed for energy saving, it is recommended to select a suitable capacity boiler.
This study evaluated the changes in boiler efficiency for space heating under partial load conditions. However, the scope of the study was limited to the analysis of the total boiler efficiency with heating in all heating zones. In real life, however, if the room is not occupied, the heating mode in this room will be switched to "offmode". Therefore, in further study, it is necessary to analyze the total efficiency changes of the boiler operating under partial load conditions.