Halachic Specification

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When operating an electric water heater on Shabbat, one may encounter two primary Torah-level prohibitions — in addition to the general prohibition against using electrical devices — namely: kindling (igniting) and cooking.

Electric boilers contain a metallic heating element that reaches a high temperature, transferring heat to the surrounding water. Heating metal involves the prohibition of kindling, even though there is no visible flame. In addition, boiling the water involves the prohibition of cooking.

When drawing hot water from an active electric boiler, there is no activation of an electrical device, and possibly no violation of kindling, since the boiler is already on. However, there is still a violation of cooking: for every drop of hot water drawn, a drop of cold water from the municipal water line enters the tank and becomes heated inside the boiler to a temperature at which the hand recoils ("yad soledet bo").

Furthermore, the thermostat periodically pauses the boiler’s operation — similar to a refrigerator, which cycles on and off when the desired temperature is reached. Drawing hot water when the thermostat is paused causes the heating cycle to restart sooner, and sometimes even triggers immediate activation.

When drawing water from an electric boiler that is completely off, there is no activation of an electrical device and no violation of kindling, but there is still a violation of cooking, since cold water entering the boiler becomes heated.

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In the Dolphin system, a “Shabbat Mode” control for electric boilers was developed, allowing automatic heating and water usage during Shabbat as follows:

To neutralize the issue of cooking, Dolphin is designed to heat the water only to a temperature below the threshold of “yad soledet bo” (the level of heat from which one’s hand recoils), thus avoiding any Shabbat prohibition. The halachic definition of “yad soledet” is set at 45°C (113°F), following the opinion of Rabbi Shlomo Zalman Auerbach and other authorities who agreed with his view, exactly or approximately.

To ensure a safety margin, the Dolphin system lowers the heating limit by an additional 5°C, so the maximum water temperature on Shabbat is 40°C (104°F).

To neutralize the issue of kindling, the boiler includes a “state continuation mechanism”: when the boiler is off, it automatically turns on for a few seconds every ten minutes (approximately). If the water has cooled and heating is needed, it continues operating; otherwise, it shuts back off automatically.

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Usage and Operation Instructions​

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In the app menu, under “Settings”, there is an option called “Shabbat Mode.”
When this option is enabled, the Dolphin system automatically enters Shabbat Mode during Shabbat and Jewish holidays.
During this time, it ignores all other programmed heating schedules and operates strictly according to the above halachic specification.

A. On Shabbat afternoon, if the water was heated by solar panels (a solar water heater), many halachic authorities permit using this hot water without temperature restrictions.

B. At the beginning of Shabbat, when water remains hot from Friday, those who are lenient rely on the assumption that the new cold water entering during Shabbat will not reach the level of “cooked” — since the cold water enters at the bottom of the tank, natural cooling occurs, and drawing water lowers the overall temperature.

Those who are stringent can ensure that by the time Shabbat begins, the water temperature is already below the “yad soledet” threshold.