According to the invention, an adapter is provided to receive and hold a bag-in-box container on top of a conventional water cooler and to control the flow of water from the bag-in-box container into a sealed water reservoir of the water cooler to maintain a desired level of water in the water reservoir. Level control of water in the reservoir is provided by controlling the venting of the sealed reservoir to the atmosphere and/or by controlling the flow of water into the reservoir from the water supply line. Venting control can be through use of a hydrophobic membrane or through use of float valves in the vent, and control of flow of liquid into the reservoir from the water supply line can be by a special float valve that allows high flow capacity at low pressures.