The combination of boilers of high efficiency is presented which contribute to energy saving in domestic heating plants and solve the problems of delivery of hot water in the favourable terms. They present the following features: conventional or condensing instantaneous boiler, flame modulating, and with electric ignition. A natural gas-fired high efficiency conventional boiler is considered: maximum output 35kW; combined instantaneous boiler to produce heating and hot water; with a hermetically sealed gas combustion chamber, fan assisted, ready to be connected to flue-gas and external air ducts. Experimental efficiencies and cyclic values, calculated by simulation at partial load, are compared with those of other three different high efficiency boilers, for heating and tap water services: - ON-OFF, conventional boiler with tank; - ON-OFF, conventional instantaneous boiler; - flame modulating, conventional instantaneous boiler.