The Torah prohibition of cooking refers to cooking using the heat of a fire (esh) or using something that was itself heated by fire (toldot ha-esh). In contrast, one may cook using the heat of the sun (ĥama). Therefore, one may leave an egg in a place where the sun is beating down hard enough to cook it. However, it is rabbinically prohibited to cook using something that was itself heated up by the sun (toldot ha-ĥama). This is due to the concern that if one cooks in such a pan, he may mistakenly come to cook in a pan heated by fire, and will thus transgress a Torah prohibition (SA 318:3; MB 17).
In practice, then, one may only cook in the heat of the sun itself, but cooking using toldot ha-ĥama is rabbinically forbidden. Thus the question of a solar boiler (“dud shemesh”) is dependent on how we categorize it. Is the water in a solar boiler considered heated by the sun itself, or by toldot ha-ĥama?
According to several poskim, one may not use water that comes from a solar boiler, because the water is heated up with the help of receptors and black pipes, which are toldot ha-ĥama, and removing water from its tank causes the cold water that enters it to cook (Minĥat Yitzĥak 4:44; Az Nidberu 1:34). Another reason to prohibit this is that since many solar-powered boilers have the option of being powered by electricity by flipping a simple switch, there is a concern that if people use solar-heated water, they might also come to use electrically heated water. Therefore, it is better to avoid using hot water from the solar boiler (SSK 1:51 in the name of R. Shlomo Zalman Auerbach).
Others maintain that one may use hot water from the solar heater, because it is considered heated by the sun itself. The receptors simply help to focus the sunlight and aid in its absorption. If so, there is no problem of cold water flowing into the boiler as hot water flows out, because one may cook this water on Shabbat in the sun’s rays (Har Zvi, OĤ 188; Tzitz Eliezer 7:19; R. Qafiĥ; Yabi’a Omer 4:34; Or Le-Tziyon 2:30:2).
In practice, since this disagreement is on a rabbinic level and we are generally lenient in such disputes, one may be lenient and use hot water from a solar boiler on Shabbat. Those who wish to be stringent should be commended. However, when it comes to bathing babies, one should not be stringent.
Today there is an additional type of boiler, mainly used in very tall buildings. In this boiler, the hot water in the receptors remains in a closed system of pipes that descend to the water tank, and the cold water is heated upon contact with these pipes. Thus, the cold water is heated via toldot ha-ĥama. Hot water from this kind of boiler may not be used on Shabbat, because turning on the hot water tap causes the cold water flowing in to become cooked by toldot ha-ĥama. But on Friday night one may use hot water that was heated before Shabbat, as explained in the note.
. One does not need to take into account that the cold water that will flow into the boiler will be heated by contact with the hot water that was heated through toldot ha-ĥama, because there is no certainty that the water entering the boiler will be heated to yad soledet bo. The incoming cold water can only be heated in this way if the tank is completely filled with very hot water. But if the water at the bottom of the tank is still not very hot, it will not have the ability to heat the cold water; it will only be heated up by the receptors, i.e., directly by the sun. When one is in doubt about how the cold water will be heated, it is a davar she-eino mitkaven. And even when it is certain, there is room to be lenient since this case is a psik reisha de-lo niĥa lei in the case of a double rabbinic prohibition, for which we are lenient. The two rabbinic factors are: 1) cooking using toldot ha-ĥama is a rabbinic prohibition, and 2) the entry of the cold water is done via grama. (See Harĥavot here, 24:2 and 25:1.) The unintentional result here is undesired, because it would be better for the person if the water is heated by the receptors rather than by the water in the tank; the latter case would cool down the hot water in the tank.. In a normal system, when one turns on the hot water tap, first all of the cold water in the pipe that descends from the tank on the roof comes out of the faucet. In contrast, in this new system, the hot water stays in the apartment’s tank and reaches the faucet immediately, saving time and water. Additionally, less scale accumulates in it. However, since the cold water is heated by the pipes that pass through the tank, it is cooked through toldot ha-ĥama, which is rabbinically prohibited. One cannot maintain that this is a psik reisha de-lo niĥa lei in a rabbinic prohibition, because it is to his advantage for the cold water to enter the tank and heat up. On the other hand, on Friday night it is not to his advantage for cold water to enter the tank, because it will cool down the tank’s hot water; and since there is no sun then to heat the water flowing through the pipes, the water in the tank will remain cold through the night. Therefore, on Friday night one may use the hot water from the tank. However, during the day it is beneficial to him for the cold water to enter the tank, because the sun will quickly heat the water in the pipes descending from the receptors on the roof. In a case of necessity, it may be that even during the day one may use the hot water, because of the combination of two rationales: 1) Perhaps we can consider the action grama (see Har Tzvi §188; Tzitz Eliezer 7:19). 2) According to a minority of poskim, psik reisha is not forbidden if the prohibition involved is rabbinic (above 9:2 and Harĥavot 9:5:4).