The present paper deals with an energy audit of a residential building served by two central systems for heating and domestic hot water production respectively. The heating service is obtained by a low pressure and natural circulation steam system. These types of plants show lots of problems: low energy efficiency due to high operating temperatures and to the poor insulation of pipes, lack of comfort within the building units due to the high inertia and to the inefficiency of the regulation. In order to calculate the energy performance of the building, a model based on the UNI/TS 11300 standards and validated on the basis of the real consumption has been used. The current building state has been simulated and different actions to improve energy performance have been examined. For each of... them, using cost-benefit analysis, the simple payback time has been rated.