Thermal analysis of an HVAC system with TRV controlled hydronic radiator

A model for an HVAC system is derived in thispaper. The HVAC system consists of a room and a hydronicradiator with temperature regulating valve (TRV) which hasa step motor to adjust the valve opening. The heating systemand the room are simulated as a unit entity for thermalanalysis and controller design. A discrete-element modelwith interconnected small scaled elements is proposed forthe radiator. This models the radiator more precisely thanthat of a lumped model in terms of transfer delay andradiator gain. This precise modeling gives us an intuitioninto a regular unwanted phenomenon which occurs in lowdemand situations. When flow is very low in radiator and thesupply water temperature and the pressure drop across thevalve is constant, oscillation in room temperature occurs.One reason could be the large gain of radiator in lowdemand conditions compared to the high demand situations.The simulation model of radiator is optimized in terms ofapproximating the small signal gain of radiator in alloperating points accurately. The controller designed for highdemand weather conditions is applied to the oppositeconditions to illustrate the oscillatory condition moreapparently. Suggestions to alleviate this situation areproposed.