I agree about the lack of info. Also confusing to mention two brands of combi-ovens to describe ONE oven (Rational AND Vulcan?). I'm not sure what's meant by it “releasing all water through the drain pipe.” How is that known that it's ALL water?
Beings that it's not steaming and there's water going down the drain (ALLOT…I guess), I'd first check to see if the fill valve is stuck open. If it is, that would temper the water in the steam generator and can prevent it from reaching its boiling point – AND would subsequently send more water down the drain than what's typically observed when there's the standard & normal quench system function.