Abstract: An auxiliary device for a hot water device includes a compressor, a condenser, a drier, an expansion valve, an auxiliary system, and an evaporator. The auxiliary system is connected with the evaporator as integral, and heat source is sent into a pipe route of the auxiliary system by a pump. So when lower pressure gas coolant is evaporated in the evaporator, the coolant is heated by the heat source in the pipe route of the auxiliary system. The coolant is in advance heated, so the compressor can reduce electricity for raising the temperature of the coolant. The auxiliary device can be operated regardless of weather condition.