Modelling the energy performance of night-time ventilation using the quasi-steady state calculation method

Many European countries assess the heating and cooling needs of buildings using the quasi-steady state calculation method described in EN ISO 13790. The energy need is calculated by establishing the monthly balance of heat losses and heat gains, and the dynamic effects are taken into consideration by introducing correlation factors. The difficulty of evaluating night-time ventilation with such a method comes from the strong influence of the dynamic effects. In this paper, two different calculation methods for modelling night-time ventilation have been developed and tested: one is based on correcting the convective heat transfer by ventilation using the factor (method proposed in the standard), and the other one introduces an adjustment factor on the relative heat gains. 288 simulations of a typical Danish office building have been performed using different boundary conditions: level of thermal mass, level of insulation, orientation, internal heat loads, duration and air change rate of night-time ventilation. For both methods, the derived correction factors are highly dependent on the thermal mass of the building. An influence of the period of activation of night-time ventilation has also been observed. Correlations have been developed and an improvement of the accuracy has been observed:  the uncertainty on the annual energy consumption obtained with adjustment factor has been decreased from around 10 % down to 5%.