Investigation of Building Passive Thermal Storage for Optimal Heating System Design

Heating and cooling load calculations are critical to size Heating, Ventilation and Air conditioning (HVAC) systems and determine energy use of their operations. The ASHRAE (2009) model, which is most commonly used for heating load calculations, adopts a simplified approach by considering only steady-state instantaneous conductive heat transfer and ignoring internal heat gains and thermal storage effects. Those assumptions evaluate the worst case conditions which can reasonably occur at nights when the outdoor air temperature is lowest and with no inputs from solar, occupants, lights, or any electronic devices. However, due to thermal storage effect, heat generated in daytime can be still stored in buildings. Such ignorance leads to significantly over-sized heating system, high initial cost and a higher cost of energy uses. On the other hand, by considering passive thermal storage of buildings and allowing space air to drift to reasonably lower values, buildings need to be warmed up in the morning before being occupied. The worst case conditions might happen in the morning warm-up period, when heating is needed. This study therefore examines the thermal response of different constructions (heavy, medium, and light) of the building envelope and investigates the effect of their passive thermal storage on the size of the heating system. Results show tremendous opportunities for downsizing of the heating system while still maintaining thermal comfort requirements. As such, this paper is a fundamental study of building thermal characteristics in order to investigate the potentials of establishing a new heating device design standard.