The indoor climate in historic buildings is crucial in order to the preserve the building, interiors and objects. From a preservation point of view, the primary parameter is relative humidity. Too high values will promote biodeterioration, strong variations will cause damages on wooden objects. Many historic buildings such as churches, castles and large manor buildings are not used on a regular basis. In this case comfort is not a major consideration, heating and any other climate control would focus on maintaining a proper climate from a preservation point of view; i.e. a stable relative humidity according to the climate history of the building. Heating is a simple and efficient way to control the relative humidity, this practice is called heating for conservation. In the typical Nordic climate, heating for conservation means that the temperature is raised some 3 to 5 °C over the outdoor temperature. The indoor temperature will vary with the outdoor temperature while the relative humidity is kept more or less constant. Thus the heating load is remarkably stable over the year. The combination of a stable heating load and a low indoor air temperature makes this an ideal application for a heat pump. The present paper will investigate the potential and real improvement of COP with a lower indoor temperature and present a method for determining the heating load based on the climate history of the building. Also the design of mixed-mode heating systems, i.e combining conservation heating with intermittent heating for comfort will be dealt with. Results from tow on-going case studies in Sweden will be presented. The results will be evaluated with respect to energy efficiency and preservation aspects.