A liquid circulation heating system (1A; 1B; 1C) for performing air-heating by heating a liquid to produce a heated liquid and radiating heat of the heated liquid from a heating radiator, the system comprising: a heat pump circuit for circulating a refrigerant, the heat pump circuit having a refrigerant radiator for heating the liquid by radiating heat from the refrigerant; and a solar heating apparatus (4A; 4B) for heating the liquid by solar heat, wherein a first passage passing through the refrigerant radiator and a second passage passing through the solar heating apparatus (4A; 4B) are formed as passages through which the liquid flows to produce the heated liquid, the first passage has a first supply pipe for supplying the liquid to the refrigerant radiator, and a first recovery pipe for recovering the liquid heated in the refrigerant radiator, and the second passage has a second supply pipe for supplying the liquid to the solar heating apparatus (4A; 4B) and a second recovery pipe for recovering the liquid heated in the solar heating apparatus (4A; 4B),), the system further comprising: a main pipe having a first end connected to an upstream end of the first supply pipe and an upstream end of the second supply pipe, wherein the main pipe is provided with a pump for allowing the liquid to flow from a second end of the main pipe to the first end thereof, and the first supply pipe or the first recovery pipe is provided with a first open/close valve, and the second supply pipe or the second recovery pipe is provided with a second open/close valve; characterized in that the heating system comprises a bypass pipe for connecting an intermediate point of the first recovery pipe located on a downstream side of the first open/close valve and an intermediate point of the second supply pipe located on a downstream side of the second open/close valve and a switching member for determining whether the liquid heated in the refrigerant radiator is allowed to flow to a downstream side of the first recovery pipe or to flow to the bypass pipe. a heat pump circuit for circulating a refrigerant, the heat pump circuit having a refrigerant radiator for heating the liquid by radiating heat from the refrigerant; and a solar heating apparatus (4A; 4B) for heating the liquid by solar heat, wherein a first passage passing through the refrigerant radiator and a second passage passing through the solar heating apparatus (4A; 4B) are formed as passages through which the liquid flows to produce the heated liquid, the first passage has a first supply pipe for supplying the liquid to the refrigerant radiator, and a first recovery pipe for recovering the liquid heated in the refrigerant radiator, and the second passage has a second supply pipe for supplying the liquid to the solar heating apparatus (4A; 4B) and a second recovery pipe for recovering the liquid heated in the solar heating apparatus (4A; 4B),), the system further comprising: a main pipe having a first end connected to an upstream end of the first supply pipe and an upstream end of the second supply pipe, wherein the main pipe is provided with a pump for allowing the liquid to flow from a second end of the main pipe to the first end thereof, and the first supply pipe or the first recovery pipe is provided with a first open/close valve, and the second supply pipe or the second recovery pipe is provided with a second open/close valve; characterized in that the heating system comprises a bypass pipe for connecting an intermediate point of the first recovery pipe located on a downstream side of the first open/close valve and an intermediate point of the second supply pipe located on a downstream side of the second open/close valve and a switching member for determining whether the liquid heated in the refrigerant radiator is allowed to flow to a downstream side of the first recovery pipe or to flow to the bypass pipe. a main pipe having a first end connected to an upstream end of the first supply pipe and an upstream end of the second supply pipe, wherein the main pipe is provided with a pump for allowing the liquid to flow from a second end of the main pipe to the first end thereof, and the first supply pipe or the first recovery pipe is provided with a first open/close valve, and the second supply pipe or the second recovery pipe is provided with a second open/close valve; characterized in that the heating system comprises a bypass pipe for connecting an intermediate point of the first recovery pipe located on a downstream side of the first open/close valve and an intermediate point of the second supply pipe located on a downstream side of the second open/close valve and a switching member for determining whether the liquid heated in the refrigerant radiator is allowed to flow to a downstream side of the first recovery pipe or to flow to the bypass pipe.