In a private home, it is possible to estimate in advance how much hot water will be needed for drinking on Shabbat. The water can be boiled in a hot water urn before Shabbat and the temperature then maintained by a heating element. But this is not good for large institutions, such as hotels, restaurants, and hospitals. In such places it is quite possible that the water in an urn will be used up before the end of Shabbat, leaving the guests without hot water.
For weekday use, there are commercial water heaters that are fed from the tap, which are automatically refilled when sensors detect that the water level has dropped below a specified minimum. When this happens, an electric valve lets in more water, which is heated to boiling temperature. However, this cannot be done on Shabbat, since any hot water that is removed is replaced by cold water which is then "cooked" by contact with the remaining hot water. In addition, removing water can cause the thermostat to turn on the heating element (if the water temperature falls below a specified minimum).
The Zomet Institute has solved the problem, providing a constant supply of hot water in institutions on Shabbat even according to the most stringent halachic requirements. This is an Overflow Water Heater. In this equipment, water is added to the tank every few minutes (the amount and cycle times can be set as desired). On weekdays, this cycle of adding water is repeated as long as there is room in the tank. On Shabbat, the level sensor is disconnected, and water is added during every cycle no matter if the tank is full or not. Any extra water is released to the sewage through an overflow channel.
It should be noted that what overflows to the sewage is hot water. Removing water for drinking thus has no effect at all on the operation of the equipment. A set amount of fresh cold water is brought into the tank at regular intervals, no matter how much hot water has or has not been removed.
The heater mechanism includes a weekly-daily timer so that the cycle of inputting fresh cold water can be turned off when the hot water is not needed. This can limit the extra expense involved in throwing away excess hot water.
Note: The Zomet Institute only manufactures the Shabbat controls for the equipment. A complete overflow heating system (including Shabbat control) may be purchased from: Herman Dispensing +972-54-7548563; Nirmetal,+972-8-6755586; Pal-Ram, +972-8-9243738.