Energy efficiency analysis of water heating system gas condensing boilers

Abstract

Increasingly relevant is the issue of using energy-saving technologies. This is due to the increase in the consumption of energy carriers, which reserves are limited. The heat supply system plays a significant role in heat consumption in the heat energy sector. The main consumer of thermal energy from the heat supply system is the water heating system. Any heating system consists of three main elements: a heat source, pipelines and heating devices. In a private residential building, a gas heating boiler is most often used as a heat source, where the coolant heating results from the fuel combustion. According to the standards adopted for determining the efficiency of boilers, admitted is that the boiler’s maximum possible heat output is equal to the lower heat of fuel combustion. The reason for this situation was the lack of technical possibility to use the latent heat of condensate without harm to the boiler design. In the 1980s, condensing boiler technology began to develop. Condensing boilers are able to achieve additional efficiency by using the energy of atmospheric moisture condensation in flue gases. Thus, the emergence of condensing technology influenced the change in the efficiency calculation rate and amounted to more than 100%. The design of condensing boilers with the use of an additional heat exchanger and condensate removal allows to obtain additional thermal energy released during the phase transition. Unlike conventional boilers, condensing units use most of the latent heat of vaporization. For efficient operation, not the lowest, but the highest heat of combustion is used, which is 10…11% higher for gas. The fuel efficiency in condensing boilers will be higher when operating in condensing mode. This mode is possible when the coolant is heated to a temperature never exceeding 57 °C. If the heated coolant temperature is higher, the water vapor will not condense from the combustion products and the condensing boiler efficiency will decrease. Thus, the condensing boiler maximum efficiency is achieved in the low-temperature heating mode. When this boiler operated in the 80…60 °C mode (normal water heating mode), the operation period is just slightly more than 30% of this one in the heating season: the efficiency is significantly reduced, to increase the efficiency of operation, indirect heating boilers or plastic heat exchangers and accumulator tanks are used. However, with this operating mode, the heating devices surface area increases by 2 times. The gas fuel economy during the operation of a condensing boiler compared to a traditional one per 1 kW of thermal energy is 0.023 m3/h, that is it allows 19% less gas consumption.