(Revision of the concept of inertia in the use of heat gain. Night cooling). Revision del concepto de la inercia en el uso de las ganancias de calor. Ventilacion nocturna..

The utilization factor is a relative well-known concept used in some simplified calculationmethods as CEN EN-13790, in this regulation this factor depends on the thermal inertia and the ratioheat gains/ heat losses. In this paper we will define the utilization factor for cooling. The present paper starts analysing the energy stored and released in building components as walls. In this step we will establish a load period and a discharge period. In the load period the component will release heat cool load- while in the discharge period the component will storage heat cool discharge-. Theduration of each couple of charge/discharge periods is 24 hours. If we repeat this period a sufficientnumber of times we will obtain a cuasi-steady-state. In this way we will be able to assess the absorbed energy by the component. Up to this point we have quantified how much heat is absorbed from the indoor space to the component as a consequence of the existence of night ventilation. Further from here we will explain how the heat absorbed is used to diminish the cooling requirements revisiting the concept of utilization factor. Next we will analyse all the concerning parameter making special attention to the inertia of the component. Finally we will determine the influence of the inertia of each component in the total inertia of the room.