Determination of pre-heating time for room based on predicted room air temperature.

Abstract

The purpose of this study is to provide a method for predicting pre-heating time for rooms whose air temperature is controlled by on/off heating cycle in relation to its occupancy. By calculating the pre-heating time accurately, the operating hours of heating can be minimized while occupants still experience thermally comfortable room conditions. Currently, the determination of pre-heating time is largely by trial and error. The study proposes that the predictive room air temperature is the best indicator for determining the pre-heating time. A mathematical model is formulated based on two assumptions: first, it should predict room air temperature as dependent upon the thermal performance of a given room, and secondly, it should predict room air temperature for sufficiently short time intervals so as to clearly reveal the pattern of the temperature changes. The model satisfies both of these conditions. It consists of heat balance equations based on the Crank-Nicolson Finite Difference method. These equations provide the temperature on the nodes in the wall cross-section and the temperature of the room air under unsteady state heat transfer conditions. Because a large number of complex and repetitive calculations are required to solve these equations, it became necessary to implement a computer program for the model. A series of experiments is conducted utilizing a Thermal Test Chamber in the Building Technology Laboratory, College of Architecture and Urban Planning, to obtain data with respect to the pre-heating period. Validation tests are performed by comparing the data calculated by the model with the data gained from the experiments. The results of the test prove that the model simulates with a high accuracy the actual thermal performance of the test chamber. The simulation model is further applied to calculate the pre-heating time for rooms in an office building with variable exterior and interior wall types and heating conditions. Also, the simulation model is used to investigate the effects of the physical parameters of rooms on the pre-heating time. The simulation findings indicate that thermophysical properties of enclosing materials considerably influence the pre-heating time. The order of the layers in a composite wall also affects the pre-heating time. This research concludes that the model is adequately sensitive and complex for predicting room air temperature for various rooms under unsteady state heat transfer conditions. The model can be used as a decision-making tool for determining the pre-heating time for a given room. Further research, especially related to whole and complex buildings, is recommended.