The vacuum hot water unit is equivalent to a hot water boiler with a built-in heat exchanger. The furnace body is divided into upper and lower parts; the upper part is a vacuum chamber (negative pressure steam chamber), and a tubular steam-water heat exchanger is installed in the space to heat the circulating water flowing through the heat exchanger. According to user requirements, different heat exchange areas can be allocated to meet the demand for simultaneous heat supply to heating equipment under different parameter conditions. Full membrane wall, full condensation, ultra-low nitrogen vacuum hot water unit.