A "minibar" is an appliance which provides a steady supply of hot or cold purified water (from the tap or from a source of mineral water). What problems are there in using this equipment on Shabbat?
(1) Removing hot water from the minibar involves a Torah prohibition, in that any new water that enters the hot water tank will immediately be "cooked," no matter whether the water source is the tap or bottled mineral water. In addition, incoming cold water might cause the thermostat to turn on the heating element, violating the prohibition of "burning." These two reasons are enough to prevent use of the minibar, in order to avoid any chance of mistakes.
The only way to have unlimited amounts of hot water available is to use the principle of constant overflow, but this is usually restricted to commercial and not home use.
(2) Electrical valves: In some equipment the valves are electrically operated, even though they appear to be mechanical (often an electrical pump is turned on as a result of the pressure change when a mechanical valve is opened). It is easy to check if this is the case – if there is an pump, no water will flow out if the wire of the minibar is unplugged.
(3) Water purification: Units that are fed from tap water usually have a purification system. Mechanical filtering is permitted on Shabbat (since the water is suitable for drinking even before being filtered), but purification systems that use electronic means (an ultraviolet light to destroy the germs) only turn the light on when water is removed from the minibar. It is easy to check for this too.
(4) Low-temperature thermostat: Even if the consumer is very careful not to touch the hot side of the minibar, there is also a thermostat on the cold side of the machine. The cooling mechanism is similar to that of a refrigerator, which is used by most people on Shabbat, without worrying that their actions will indirectly affect the thermostat. However, a minibar might be more troublesome than a refrigerator for two reasons: First, the tank is usually so small that there is a greater probability than in a refrigerator that removing water will trigger the thermostat. (As an example, a typical cold water tank in a minibar might hold 2 liters, while a glass of water is 200 cc. Thus, filling up two glasses of water means that one-fifth of the cold water in the tank is replaced by water at room temperature!) In addition, adding tap water to a tank of cold water is a more substantial action than allowing warm air to enter by opening a refrigerator door.
(5) A mehadrin (stringent) mechanism: The Zomet Institute has developed a mechanism that can be installed in a minibar, allowing it to be used on Shabbat. An electronic card in the control mechanism causes the compressor to operate using the technique of "continuing the existing status." The cooling mechanism is turned on for a few seconds every ten minutes, no matter what the state of the thermostat. If water is removed from the cold water tank and the temperature has risen, nothing will happen at first. Only at the end of the ten-minute cycle will the cooling mechanism start to cool the water. In this way, the person does not have any effect on the electric compressor, not even an indirect one. All he or she does is to allow it to continue operating, and this is permitted on Shabbat.
Equipment that has this mehadrin Shabbat mechanism is provided with a "Shabbat/weekday" selector. In the "Shabbat" position, the special electronic card is engaged, the heating element for hot water is disconnected, and if there is an electric purification system the UV bulb will be turned on constantly, without a stop.
In order to guarantee that the minibar includes a mechanism for Shabbat, ask for a "certificate of technological kashrut from The Zomet Institute." The Zomet Institute can add a Shabbat mechanism to a minibar that was not approved if the conditions outlined above can be met.