The condensing boiler technology has been known for at least two decades and its energy saving capabilities are nowadays acknowledged. In recent years its application has spread, due to an increased sensitivity to energy and economic savings in HVAC plants and a raised interest in low temperature systems. Taking advantage from the additional heat recovered from the condensation of the water vapour contained in the exhaust flow, the condensing boilers achieve, during test operations, efficiencies up to 108% on the LHV of natural gas. However, the yearly global performance of such boilers on one side strongly depends on the kind of heating system installed downstream, and on the other is affected from the climatic conditions outside the building. The present study, by means of a transient numerical simulation model (TRNSYS), investigates the energy performance of condensing boilers in residential applications, considering different heating systems (radiant system, radiator, fan coil) and several climatic areas. The results are then compared to those achieved by ordinary boilers.