To provide a vacuum water heater which eliminates risks due to explosion by efficiently collecting combustion heat of combustion discharge gas discharged from a combustion burner provided to a water heater to supply desired warm-up water and preventing increasing temperature and pressure in the water heater and warm-up water overheating.

The vacuum water heater includes a body unit 10 and a heat exchange unit 20. The body unit includes a combustion chamber 1 equipped with a combustion burner 1a and an upper water pipe group 1b, a vacuum steam room 4 arranged over the combustion chamber 1, and a lower heating medium room 5 arranged under the combustion chamber 1. The heat exchange unit includes an upper heat exchange space 8 formed by communicating with the vacuum steam room 4 for filling a heating medium therein in a gas phase and provided with a second heat exchange part 6 and a lower heat exchange space 9 formed by communicating with the lower heating medium room 5 for filling a heating medium therein in a liquid phase and provided with a first heat exchange part 7. In addition, the cross section C of the upper heat exchange space 8 is larger than the total cross section in the upper water pipe group 1b.