Abstract: A control system and method for controlling an indoor heating system for an indoor environment of a building in accordance with a desired indoor temperature, comprising a sensor (18) for detecting an outdoor temperature, a sensor (17) for detecting a in return temperature T.sub.return of a heat carrying medium circulated in a heat distribution system with a flow rate, and a controller (13). The controller is adapted to determine a required heating power P.sub.req to be delivered by the heat distribution system to maintain a heating power balance according to P.sub.req=P.sub.loss-P.sub.source, where P.sub.loss is an approximation of heating power losses from the building, and includes a dynamic approximation of heating power transfer through walls of the building, based on at least the desired indoor temperature, the detected outdoor temperature, a heat transfer coefficient of the wall, and a heat capacity of the wall, and P.sub.