The development of boiler technology for space heating purposes introduced into the market machines which can operate at very high efficiency at part load. This feature gives excellent results in detached house heating plant and in centralised heating systems. The optimisation of the plant’s efficiency can be achieved by splitting the heating capacity into several boilers, even of different rated power, thus allowing the system to reach high efficiency in a range of load spacing from 5 to 100%, especially in summer mode, requiring only the domestic hot water production. The present work evaluates, by means of TRNSYS dynamic simulations, different strategies for the sizing and the operation of the plant.