A domestic microwave oven is provided having an oven cavity, a water reservoir, a piping system coupling the reservoir and the oven cavity, and a pump for pumping a predetermined amount of water into the oven cavity. The pump is a mono-directional positive displacement pump and the piping system includes a by-pass coupled in parallel to the pump and an on/off electro valve configured to allow a drainage of the piping system when the pump is idle.