Transient Effects in Intermittently Heated Buildings-Measurements Versus Calculation

Abstract

Transient effects in the buildings energy balance are very important and could be dominant in some cases, especially for long setback schedule in the heating system operation. Thermal behaviour of insulated and intermittently heated buildings requires advanced calculation tools for proper estimation of the room/building heat balance. This is especially important for buildings where the heating system is on for up to 10 hours per day. In these buildings the transient effects are dominant and inner walls, due to their significant thermal capacity, strongly influence the variation of the room air temperature in time. In this paper inner walls are divided into side and top/bottom walls and are modelled with transient heat conduction equation each, instead of lumped-thermal-capacity method. Model for simulation is made by using Modelica Buildings Library in Dymola environment. Temperature measurements are recorded in one representative room and measured values are compared with results of simulation. Two cases are analyzed, one with variation of the air room temperature when the thermal inertia of the inner walls is taken into account, and the other where this effect is neglected. The results showed significant variation of the room temperature in the case where heat exchange between the inner walls and room air is neglected compared to the case where thermal inertia of the inner walls is taken into consideration. In case with no influence of the inner walls, due to small thermal inertia of the room air, the analyzed room overheats up to 27 °C during the heating period, while it drops to 9 °C when the heating system is off. Also, internal wall heat fluxes from and towards the room are quantified and analyzed for situations when the heating system is on and is off. It is shown that, for massive and thermally insulated buildings, these heat fluxes can be dominant in the room heat exchange.

Keywords

• intermittent heating
• inner walls
• Modelica
• simulation
• thermal inertia