Halachic Debate on Using Solar-Heated Water and Opening Hot Water Valves on Shabbat

This week’s discussion focuses on two key halachic questions: whether it is permissible to wash with hot water heated by a solar water heater on Shabbat, and whether opening a valve that lets cold water enter the solar heater during Shabbat is allowed.

The Torah commands Shabbat observance, including refraining from work, and traditional Jewish law forbids bathing in hot water heated on Shabbat due to concerns about cooking (bishul) and related prohibitions. However, washing with cold water is generally permitted, and exceptions exist for those in discomfort or infants needing a bath. Classical sources differentiate between cooking by fire and cooking by sunlight (bishul be-toldot hama), with the latter traditionally permitted since it was not a common cooking method.

Solar water heaters heat water via sunlight, but halachic authorities dispute whether this constitutes cooking by sunlight or by indirect heat. Some, like the "Tzitz Eliezer," argue the water is heated directly by the sun and thus permitted for washing the entire body. Others, including Rabbi Ovadia Yosef and Rabbi Eliyahu, contend the water is heated indirectly through black absorbers, constituting cooking by indirect heat, which is forbidden.

A further complication arises when opening the hot water tap on Shabbat, as cold water enters the heater and heats up, potentially violating cooking prohibitions. Rabbi Ovadia Yosef permits this leniently, reasoning that since the prohibition is rabbinic and the person does not intend to cook, it is allowed. Others, like the "Minchat Yitzchak," disagree, viewing this as a direct violation.

Additional halachic discussions reference the Talmud and later authorities debating cooking on rooftops heated by the sun and the nature of indirect causation (grama) in Shabbat law. Some authorities permit leniency when the cooking is clearly by sunlight and not fire, while others caution against relying on such leniencies.

In summary, washing with solar-heated water on Shabbat is subject to differing opinions based on the method of heating, and opening the valve allowing cold water to enter the heater is also debated. Those wishing to be stringent may avoid these actions, but lenient opinions exist allowing their use under certain conditions.