Heating system with biomass furnace and heat pump

The invention relates to a heat generating system (15) and a method of controlling a heat generating device (15) for heating and / or cooling a building, wherein the heat generating plant comprising: a biomass (18), in particular a pellet boiler or stove, the biogenic solid for combustion is fuels and formed for heating a first heat transfer medium, in particular water and / or air; and a heat pump (32, 51) for heating a heat transfer medium, in particular water and / or air, said method comprising the steps of: detecting an air temperature; Determining a switching point; and controlling the biomass (18) and the heat pump (32, 51) is operated in function of the determined air temperature and the changeover determined using either the biomass (18) or the heat pump (32, 51).