The paper considers the issue of relocation of heat loss between apartments in pre-war multifamily buildings. The in-situ studies that form the basis for further simulations and analytical considerations were conducted in three apartments located in a pre-war tenement. It was found that the value of the energy performance gap (EPG) in these apartments was +29%, –45% and –75%. It was found that heat loss relocation was an important component of it, and was responsible for the increase in heat consumption by 49% in one of the apartments and its decrease in the other two by 25% and 47%. The second significant factor affecting the EPG value was weather conditions other than those assumed for the calculations, responsible for 34% and 33% reductions in heating consumption relative to design consumption. Simulation studies were carried out on apartments in a pre-war tenement house, representative of the urban fabric under study, being in poor technical condition. It was found that changing the air temperature parameters in four of the twenty apartments affects the heat balance of all units. In the case of a non-thermally upgraded building, lowering the temperature in the selected apartments to 16.5°C results in a decrease in their energy intensity from 69% to 90%. This is reflected in an increase in the heat consumption of adjacent apartments, ranging from 12% to 44%. Analyses for the thermo-modernized building showed that, if underheated, the four units result in a reduction in their energy intensity of 55% to 69%. This translates into a smaller increase in the heat demand of adjacent premises, ranging from 6% to 29%. Based on the study, it was concluded that without thermal modernization of the buildings, it will be very difficult to effectively solve the problem of excessive heat flow between apartments in pre-war tenements, especially if there are energy-poor households in the building. This is because reducing the energy demand for heating will increase the chances of less affluent residents maintaining higher indoor temperatures, and any heat flow between apartments will be less burdensome for neighbours.