A control oriented model for an HVAC system is derived in this paper. The HVAC system consists of a room and a hydronic radiator with a temperature regulating valve (TRV) which has a step motor to adjust the valve opening. The heating system and the room are simulated as a unit entity for thermal analysis and controller design. A discrete-element model with interconnected small scaled elements is proposed for the radiator. This models the radiator more precisely than that of a lumped model in terms of transfer delay and radiator gain. This precise modeling gives us an intuition into a regular unwanted phenomenon which occurs in low demand situations. When flow is very low in radiator and the supply water temperature and the pressure drop across the valve is constant, oscillations in room temperature frequently occur. The model derived in this paper demonstrates that the oscillations are in part due to the large gain of the radiator in low demand conditions compared to the high demand situations. The simulation model of radiator is optimized in terms of approximating the small signal gain of radiator in all operating points accurately. The controller designed for high demand weather conditions is applied to the opposite conditions to illustrate the oscillatory condition more apparently. Suggestions to alleviate this situation are proposed.