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Haus M is a multi-family building composed of twentynine dwellings and a kindergarten. The building and its energy systems have been modelled in Modelica considering a typical occupancy profile based on the habits of the tenants and questionnaires. The limited information about the building combined with its highly-unpredictable use and the simplistic controls of the systems in Haus M conducted to a shallow validation of the model. The results of simulations show an energy use for heating of about the 77% of measured energy. The building is heated using an underfloor heating system that is coupled to a geothermal heat pump. After validation of the model, the estimated energy use was compared with the energy use of a baseline heating system composed of a natural gas boiler and radiators for all heated zones.

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According to the official statistical reports, gas-fired boiler units still remain to be one of the main equipment types for meeting the space heating and daily hot water demand of the residential dwellings across the European Union. Due to the prevalence of the natural gas grid and performance stability, gas-fired boilers are considered to remain as one of the standard energy sources. On the other hand, even though gas-fired water heating technology is a well-known concept, existing numerical models found in the literature are often case-specific with poor reusability mostly reflected in fitted efficiencies. Algorithms behind these models usually require the input of large amount of hardly attainable design characteristics of the units. In this paper, a modelling method for acquiring the performance of a heating gas-fired condensing boiler unit will be shown. The model is based on the limited input data available in the official characteristics of the units issued by the relevant manufacturers. The simulations are programmed by using the programming language Modelica and the software tool Dymola. The model is based on the fixed natural gas intake which combusts into a stable mixture of the combustion gases that further heat the circulating water. During the heat transfer process inside the condensing boilers there is a possibility for condensate formation out of the water vapour of the combustion gases which increases the efficiency of the unit. The formation of condensate, however, is depending on the return water temperature of the unit which has to be lower than the dew point temperature of the combustion gasses. The goal of this research is to determine how accurate can performance indicators of gas-fired boilers be attained with the use of a limited amount of available input data together with clearly defined assumptions that follow the modelling methodology.