Rabbi Mendel Agranovich
The device discussed in this article consists of two parts: an array of pipes enclosed in a glass casing and painted black, and a tank into which water from the array flows.
Water flows into the house from the top of the tank, while water from the mains enters the bottom and then flows from there into the array to be heated. As the sun heats the water, it flows into the tank, while another portion of water enters from the bottom. This continues until all the water in the tank has heated up, or until we begin using it, at which point another portion of water flows from the mains into the array.
Regarding the use of this device on Shabbat, several observations must be made and several questions discussed:
1. When the water in the tank is hot (1) and cold water (2) from the mains mixes with it, the water (2) is cooked by the water (1). By rabbinic decree, this is considered cooking (“Orchot Shabbat,” ch. 1, section 102), since the Sages prohibited cooking by means of things heated by the sun, even though cooking directly in the sun is permitted, because one cannot determine from a heated object how it was heated.
2. Should the tank in this case be classified as a “first” or “second” vessel, given that the water is heated in the array and only afterward enters the tank? In response, we can say that the tank is a first vessel because it is connected to the heat source, the array. Only a vessel that receives no additional energy can be called a second vessel (ibid., note 201).
3. Is the water in the array heated by the sun or by the pipes? Let us examine this in a little more detail. The Talmud states that water may be “cooked” (heated) directly by the sun.
A reasonable question arises: how can this be done? What does the Talmud mean when it states that cooking in the sun is permitted? Does it mean only direct heating by the sun’s rays, or also a situation in which we pour water into a black pot with its lid on, standing in the sun? At first glance, one might say that since the Talmud specifies no conditions, the permission applies to all possible cases... But there is one caveat: the example given in the Talmud speaks of placing water in the sun to heat up, whereas if we pour water into a heated pot, this is already cooking with a secondary heat source, i.e., heat from the pot, and it is then forbidden, since the water (in our case) often enters pipes that are already hot from the sun. We can also note that there is a difference between an ordinary pot and a special device designed to collect and concentrate solar energy, which is more logically compared to the sources of heat derived from the sun described in the Talmud, such as sand or a heated roof. No definitive statement can be made, but most opinions hold that cooking in the solar collector involves heat derived from the sun, since the water is heated, at least in part, by the pipes.
4. Another type of “solar water heater” is designed so that the water is heated in a household tank containing two coils: an electric heating coil and a coil carrying water heated in the solar collector and continually circulating through it. Simply put, the same water is constantly heated on the roof, flows down through a pipe into each individual tank, heats the water in the tank, cools, and is pumped back (returned) to the roof. In other words, the water in the tank is heated by a pipe containing water heated in the sun. With this design, the heat is unquestionably derived from the sun, and using this water means cooking the water entering from the water supply.
This is precisely the kind of water heater I have at home, according to my electrician. Incidentally, Rav Shlomo Zalman Auerbach considered a very important reason for prohibiting the use of a “dud shemesh” to be precisely the fact that the electric water heater and the solar tank are combined into a single appliance and can easily be confused through lack of knowledge. This is the central idea behind the Sages’ prohibition on heat derived from the sun. Although in the first edition of “Shemirat Shabbat Kehilchatah,” Rav Neuwirth wrote that a “dud shemesh” could be used, in later editions Rav Shlomo Zalman, on whose opinion Rav Neuwirth based his writing, agreed with his opponents and reached the conclusion described above: one may not heat water in a combined electric and solar water heater, even if it is disconnected from electricity.
5. Even if we assume that the water is always cooked only by the pipes in the first device as well, and not only in the second, we know that the prohibition on cooking “in the sun” is a rabbinic decree, and a person opening the tap does not intend to heat the newly entering water. Although this will unquestionably happen, a question remains: did the Sages prohibit it in this case as well, or only when a person acts consciously and intentionally?
Although there is considerable disagreement among the Sages on this subject, the opinion of the “Mishnah Berurah” is binding for us. It holds (based on the opinions of the “Magen Avraham,” Rabbi Akiva Eiger, and the Vilna Gaon) that the Sages’ prohibition applies even in this case (314, 11). But all this applies to a direct action, whereas in our case a law of physics is at work: water enters the solar collector through the circulation of water in the system, the law of communicating vessels, and the fact that hot water is heavier than cold water.
6. When the hot and cold water are turned on simultaneously, if the resulting water temperature is above 45 degrees Celsius, the cold water will be cooked by the stream of hot water. Therefore, the only possible option is to turn on a strong stream of cold water and add hot water to it.
Practical conclusions:
1. One may not use a “dud shemesh” on Shabbat.
2. If the water in it is below 45 degrees Celsius, the incoming water will not be cooked in the tank, but only in the solar collector. On this point, the authors of “Orchot Shabbat” stated: “One who is lenient has authorities to rely on.” In other words, leniency is possible, though it is not recommended.
3. May a “dud shemesh” be used for a child or a sick person? In this situation, the authors of “Orchot Shabbat” suggested asking a rabbi.
4. If water is not entering the tank (the supply has been shut off with a special valve), the authors of “Orchot Shabbat” are prepared to be lenient if the temperature of the water at the tap is below 45 degrees Celsius. In this case, it is preferable to turn on the cold water first and then open the hot tap slightly. This would then resemble the ruling we cited above from the “Mishnah Berurah” (314, 84).