When a large enclosed space is heated from a localised source, the space may become vertically stratified in temperature. We develop a model for this stratification in the case of constant mixing ventilation, in which there is a fixed high level supply and extract of air and a point source of heating at low level, testing our model predictions with some new laboratory experiments. We demonstrate that with large ventilation flow or a small heat flux a strong two layer stratification develops in the space, whereas with low ventilation flux or large heat supply the stratification is weak. The presence of thermal mass weakens this stratification. One important consequence of the stratification is that in order to achieve comfortable temperatures in the occupied zone near the floor of the space, temperatures become elevated higher in the space. With mixing ventilation this increases the heat loss associated with a given ventilation flow in comparison to that in a well-mixed space, in which the temperature is maintained at the comfort temperature through-out. Using our model, we quantify the difference in heating load, accounting for the role of the thermal mass in reducing the strength of the stratification. (C) 2012 Elsevier B.V. All rights reserved.