This paper explores the simulation of the thermal performance of a radiant floor for heating and cooling that is connected with an underground heat exchanger installed under the concrete floor of a house. In the heating season, an electric boiler is used to maintain the operative temperature at the set point value by varying the supply water temperature to the radiant slab. In the cooling season, the water from the radiant floor is circulated through an underground heat exchanger installed under the concrete slab. This water is then mixed with the return water, and it is supplied to the radiant slab. The system does not use energy for mechanical cooling. The paper presents also the sensitivity analysis of the model to the efficiency of underground heat exchanger and the mixing ratio between the water flow rate circulated through the underground heat exchanger and the water flow rate circulated through the radiant floor.