As part of a project to simulate the energy flows within urban districts, this paper describes a simplified model to simulate heat flows in buildings. Taking a standard two-node resistance-capacitance model as a starting point, we discuss developments to improve the handling of external surface radiant and convective energy exchanges, internal convective exchanges and the placement of capacitance in multi-layer walls. We also describe the extension of the model to solve for a building with an arbitrary number of zones, accounting for the capacitance of separating walls. Comparisons with ESP-r for a range of building configurations show that in general the model reproduces well both aggregate energy demands and their temporal characteristics (likewise with internal temperature). The paper closes by discussing the computational implementation of the model and its coupling with the integrated urban energy modelling tool. (C) 2006 Elsevier B.V. All rights reserved.