A non-atmospheric pressure vapor oven system that utilizes a controllable pressure zone to facilitate fast phase change heat transfer at any desired temperature to heat or cool flat substrates, and to level temperatures across different locations of the substrates. The system enables the use of a heat transfer fluid, such as water, without being limited to a particular temperature, such as the fluid's natural boiling point at atmospheric pressure. The system includes a vapor oven (hermetic enclosure) defining a pressure chamber having sealed entry and exit ports for transferring an object (e.g., a sheet of paper) with added material (e.g., toner) through the pressure chamber, and a pressure regulation apparatus for setting the saturation temperature (boiling point) of heat transfer fluid inside the vapor oven to an optimal heating/cooling temperature by selectively controlling the pressure inside the hermetic enclosure.