As construction of low-energy buildings becomes a rising issue, small-size energy systems can deal with loads in heating, domestic hot water(DHW) supplying, ventilation, and so on. A single apparatus containing all the system components is recently developed particularly in Europe. Since a single heat pump inside the apparatus must meet different energy needs, dynamic analyses are important for such a system. In this work, a simulation model of a selected multi-functional apparatus is developed in TRNSYS, one of the well-known dynamic simulation tool. Each energy component is defined using experimental results, and a final combined TRNSYS module is validated with experimentation carried out for the entire system. Results show that error is under 10% for all the testing cases. Transient simulations are run with the developed model to analyze the thermal behaviour of the apparatus. As the heat pump is operated in a way that the control priority is on DHW, auxiliary heaters are frequently operated when DHW signals are active and particularly for a not-well insulated building and when outdoor temperatures are low. Analysis results also show potential of connecting a solar thermal collector to the apparatus as proposed in an option by the manufacturer.