Method for operating an air-conditioning system, the said air-conditioning system comprising an air-conditioning circuit through which there circulates a refrigerant and a secondary circuit through which there circulates a coolant, the secondary circuit consisting of: - a main line comprised between an exhaust of a first three-way valve and an intake of a second three-way valve and which comprises: • at least one coolant /electrical auxiliary exchanger, • a main pump for circulating the coolant from the exhaust of the first three-way valve to the intake of the second three-way valve, - a first secondary line comprised between a first outlet of the second three-way valve and a first inlet of the first three-way valve and which comprises: • a refrigerant /coolant first heat exchanger, • a secondary pump for circulating the coolant from the first outlet of the second three-way valve to the first inlet of the first three-way valve, passing via the first exchanger, - a second secondary line comprised between a second outlet of the second three-way valve and a second inlet of the first three-way valve and which comprises a coolant /ambient air second heat exchanger; the air-conditioning circuit comprising a first bypass and a second bypass, the refrigerant/coolant first heat exchanger being a constituent part of the air-conditioning circuit, a third heat exchanger exchanging heat between the coolant and the ambient air, characterized in that operation in air-conditioning mode involves opening the first bypass, closing the second bypass, closing the first outlet and the first inlet, opening the exhaust, the intake, the second outlet and the second inlet, and switching on the main pump or, alternatively, closing the exhaust and the intake, opening the first outlet, the first inlet, the second outlet and the second inlet, and switching on the secondary pump. - a main line comprised between an exhaust of a first three-way valve and an intake of a second three-way valve and which comprises: • at least one coolant /electrical auxiliary exchanger, • a main pump for circulating the coolant from the exhaust of the first three-way valve to the intake of the second three-way valve, • at least one coolant /electrical auxiliary exchanger, • a main pump for circulating the coolant from the exhaust of the first three-way valve to the intake of the second three-way valve, - a first secondary line comprised between a first outlet of the second three-way valve and a first inlet of the first three-way valve and which comprises: • a refrigerant /coolant first heat exchanger, • a secondary pump for circulating the coolant from the first outlet of the second three-way valve to the first inlet of the first three-way valve, passing via the first exchanger, • a refrigerant /coolant first heat exchanger, • a secondary pump for circulating the coolant from the first outlet of the second three-way valve to the first inlet of the first three-way valve, passing via the first exchanger, - a second secondary line comprised between a second outlet of the second three-way valve and a second inlet of the first three-way valve and which comprises a coolant /ambient air second heat exchanger; the air-conditioning circuit comprising a first bypass and a second bypass, the refrigerant/coolant first heat exchanger being a constituent part of the air-conditioning circuit, a third heat exchanger exchanging heat between the coolant and the ambient air, characterized in that operation in air-conditioning mode involves opening the first bypass, closing the second bypass, closing the first outlet and the first inlet, opening the exhaust, the intake, the second outlet and the second inlet, and switching on the main pump or, alternatively, closing the exhaust and the intake, opening the first outlet, the first inlet, the second outlet and the second inlet, and switching on the secondary pump.