A hydronic heater comprising: a main refrigerant circuit formed by annularly connecting, to one another through a main refrigerant pipe, a compressor, a refrigerant-water heat exchanger, a supercooling heat exchanger, main decompressing means and an evaporator in this order; a bypass circuit including a bypass pipe which branches off from the main refrigerant pipe extending between the refrigerant-water heat exchanger and the main decompressing means, and which is connected to the main refrigerant pipe extending between the evaporator and the compressor through the supercooling heat exchanger; bypass decompressing means provided in the bypass pipe located upstream from the supercooling heat exchanger; outside air temperature detecting means which detects an outside air temperature; water temperature detecting means which detects an entering-water temperature of the refrigerant-water heat exchanger; and control means being adapted to operate a bypass control operation for heat exchanging of a refrigerant carried out by the supercooling heat exchanger, characterised in that the control means starts a bypass control operation if the outside air temperature detecting means detects a state where the outside air temperature is lower than a given value and if the water temperature detecting means detects a state where a water temperature exceeds a given value, the control means sets a target value of a supercooling degree of a refrigerant at an outlet of the refrigerant-water heat exchanger at the time of the bypass control operation, such that it judges as to whether or not the outside air temperature detected by the outside air temperature detecting means is more than a threshold value, and the supercooling degree becomes greater, rather when the outside air temperature is less than the threshold value rather than the outside air temperature is more than the threshold value, the control means is configured to control such that when the supercooling degree of the refrigerant at the outlet of the refrigerant-water heat exchanger at the time of the bypass control operation is smaller than the target value, an opening degree of the bypass decompressing means is adjusted toward its close side while adjusting an opening degree of the main decompressing means toward its open side, thereby increasing the supercooling degree, and the control means is configured to control such that when the supercooling degree of the refrigerant at the outlet of the refrigerant-water heat exchanger at the time of the bypass control operation is greater than the target value, the opening degree of the bypass decompressing means is adjusted toward its open side while adjusting the opening degree of the main decompressing means toward its close side, thereby reducing the supercooling degree. a main refrigerant circuit formed by annularly connecting, to one another through a main refrigerant pipe, a compressor, a refrigerant-water heat exchanger, a supercooling heat exchanger, main decompressing means and an evaporator in this order; a bypass circuit including a bypass pipe which branches off from the main refrigerant pipe extending between the refrigerant-water heat exchanger and the main decompressing means, and which is connected to the main refrigerant pipe extending between the evaporator and the compressor through the supercooling heat exchanger; bypass decompressing means provided in the bypass pipe located upstream from the supercooling heat exchanger; outside air temperature detecting means which detects an outside air temperature; water temperature detecting means which detects an entering-water temperature of the refrigerant-water heat exchanger; and control means being adapted to operate a bypass control operation for heat exchanging of a refrigerant carried out by the supercooling heat exchanger, characterised in that the control means starts a bypass control operation if the outside air temperature detecting means detects a state where the outside air temperature is lower than a given value and if the water temperature detecting means detects a state where a water temperature exceeds a given value, the control means sets a target value of a supercooling degree of a refrigerant at an outlet of the refrigerant-water heat exchanger at the time of the bypass control operation, such that it judges as to whether or not the outside air temperature detected by the outside air temperature detecting means is more than a threshold value, and the supercooling degree becomes greater, rather when the outside air temperature is less than the threshold value rather than the outside air temperature is more than the threshold value, the control means is configured to control such that when the supercooling degree of the refrigerant at the outlet of the refrigerant-water heat exchanger at the time of the bypass control operation is smaller than the target value, an opening degree of the bypass decompressing means is adjusted toward its close side while adjusting an opening degree of the main decompressing means toward its open side, thereby increasing the supercooling degree, and the control means is configured to control such that when the supercooling degree of the refrigerant at the outlet of the refrigerant-water heat exchanger at the time of the bypass control operation is greater than the target value, the opening degree of the bypass decompressing means is adjusted toward its open side while adjusting the opening degree of the main decompressing means toward its close side, thereby reducing the supercooling degree.