This paper presents original research of a model of water−water ejector whose aim is to mix a quantity of water of the return network with the water of the supply network of a central heating system. The water of the supply network at a certain pressure passes through the ejector nozzle, where the pressure energy in the nozzle is converted into kinetic energy, and consequently in the space around the nozzle vacuum gauge pressure is created which enables the absorption of a quantity of water from the network return, which is mixed with the water of the supply network. This water regulates the temperature of the water at the entrance of the central heating radiator. For the model ejector, characteristic equation was written and was analyzed in terms of pressure and mixing coefficient. The analysis was tested using nozzles of different sizes. To analyze the role of the diffuser as a part of the ejector, the ejector characteristics were analyzed without a diffuser. The characteristics of diffuser and non-diffuser ejectors are presented in the same diagram for comparative analysis of the pressure difference and mixing coefficients achieved by the ejector for different ratios f3/fr.