When the heat balance of a building is assessed, the heat needed for the ventilation air is usually calculated according to the intended ventilation rate. However, in order to calculate the air change rate accurately several aspects have to be considered. One important parameter is the ventilation system. Whether the building has a mechanical exhaust-only, supply-only, balanced exhaust-supply or natural ventilation system will influence the air infiltration rate through cracks in the building envelope. High infiltration rates lead to an increase in the heating demand and can result in an inadequate capacity of the designed heating installation. In this paper, computer simulations of the air change rate for a detached single-family house are presented. The house is simulated in different topographical surroundings, equipped with a mechanical exhaust-only, or a balanced exhaust-supply, ventilation system. In addition, the airtightness of the building is varied, from very tight, I air change per hour (ACH), to quite leaky, 6 ACH, when pressurized to 50 Pa. Results from the simulations show that the same house has quite different air change rates in different surroundings with different airtightness. (c) 2005 Elsevier Ltd. All rights reserved.