This paper describes the experimental result of heating operation mode using building mass for thermal storage to make use of a stability at room temperature of the outside insulation building. As heat load per hour of this building was reduced by extension of heating operation time, so the hot water temperature produced by a heat pump and the supply air temperature for heating could be set lower than usual operation. As a result, the heat load per day didn’t so increase and the coefficient of performance of the total heating system were on an equality with usual operation.
