Energetic Impact of Temperature Gradients in Heat Re- covery in Ventilation in Dwellings

Abstract

The installation of air-to-air heat exchangers in ventilation systems can contribute to reducing the heating demand of the building by recovering the heat from the exhaust air. When there is an active heating demand, the recovered heat becomes useful to reduce that demand. However, as the heat exchanger works independently of the heating needs of the dwelling, the recovered heat may not contribute to the reduction in the heating demand. Furthermore, the in-door temperature gradients in the various rooms/zones of the dwelling can influence the heat recovered by the ventilation system and, consequently, the useful recovered heat. A suitable method to evaluate the effect is based on dynamic multi-zone building energy simulations. In this work, a residential building typology has been modelled with eleven ventilations systems. Some of them work with fixed airflows and others are commercial smart ventilation systems with var-iable flow rates controlled by CO2, VOC and humidity sensors. Various systems do not have a heat exchanger due to their ventilation type. Still, the study was extended to those, considering that the unwasted heat as a consequence of using a smart control would be equivalent to the presence of a heat exchanger. The building was heated with two heating strategies: uniform heating in all the zones except for the attic and non-uniform heating with a specific setpoint for three different types of zones. To evaluate the impact of the heat recovery, the annual heating demand and the recovered heat are compared between scenarios with different effectivenesses for the heat ex-changer and, to evaluate the effect of the temperature gradients, the results are compared be-tween the two different heating strategies: uniform and non-uniform. The results for the case presented in this study show that centralized heat recovery systems perform better in the sce-narios with uniform heating, while distributed ventilation systems perform similarly in both heating scenarios.