Shiur #03: Using a Solar Water Heater

  • Rav Yosef Zvi Rimon









For decades, many people have used a solar water heater.  Is one allowed to use this device on Shabbat? 


Solar Heaters


We will first explain how a solar heater works.  On the roof are collectors - black tubes that absorb the heat of the sun, wrapped in glass that traps heat.  Water flows from the heater into these tubes, and the water is heated up as it travels.  After it has been heated, the water returns to the heater, and in its place more water from the heater flows into the tubes.



First of all, we must analyze how we relate to the water being heated up in the tubes: is this cooking in the sun (permissible) or cooking with solar byproducts (forbidden)?


Permissive View: Cooking in the Sun


The Tzitz Eliezer (Vol. VII, ch. 19) writes that one should be allowed to use the water from a solar heater, since the water in it is considered to have become hot by the agency of the sun itself.  He explains in the name of Rav Yosef Kafih that there is a difference between hot sand (considered a solar byproduct, and therefore rabbinically prohibited) and a solar heater.  Hot sand, even if it is moved to another place, will stay hot for some time, and therefore one might come to mistake it for something heated by fire.  In a solar heater, on the other hand, if the tube is moved elsewhere, it will cool immediately.  Therefore, it is clear to everyone that this tube was heated directly by the sun.


Heating by Magnifying Glass — Permissible  


The Tzitz Eliezer adds that if we explain that the glass helps to focus the rays of the sun, it is considered the sun itself.  This is what he cites from the words of the Shevitat Ha-Shabbat (Mevashel, Be’er Rechovot, ch. 44), whose inclination is to allow heating by a magnifying glass that focuses the light of the sun: [1]


One cannot compare tubes to sand and dirt from the road… because sand and dirt from the road have already absorbed a lot of heat from the sun, and they can now heat and cook even in the absence of the sun: if one moves this sand to a shady part, its heat is still sufficient to cook food.  It is only in this case that they have banned solar byproducts because of igneous byproducts, because they are interchangeable.  However, if these substances could only make something hot while they were in the sunlight, what worry would there be of interchanging them?  We would see that the sun is the sole actor, and without it no action can take place.


Now, in our case, should we take one of these collecting tubes to another place, it could not heat up anything at all, and the thickness of its metal is not sufficient to heat other substance with its own heat, and it turns out that the water is heated directly from the sun.


In such a case, we certainly view it as being heated by the sun directly and not by solar byproducts, and it should be permissible.  Thus we should know that the glass panels on the collector do not act at all to heat the water, and they have no influence upon it, aside from preventing the wind and the cold air from cooling the water that passes over the collector tubes.  This is the view of the learned.[2]


This is what is appropriate to say about this matter.  I have seen something even greater than this in the book Shevitat Ha-Shabbat; that author is inclined to say that even when a metal vessel is heated by glass (and no flame is ignited, for if so, it would be true fire), which focuses the energy of the sun, and through this the food is cooked in it, this may be like the sun itself [and permissible]. 


Forbidden: Cooking with Solar Byproducts


However, this is not simple.  According to the Minchat Yitzchak (Vol. IV, ch. 44), [3] the collectors are considered solar byproducts because they absorb solar energy and afterwards heat the water.  On the contrary, this is actually a more sophisticated type of solar byproducts.


Additional Problem: Ancillary Water in the Heater


There is another problem, particularly in modern solar heaters.  When one opens the hot-water tap, hot water comes out of the heater, and new cold water enters from the main.  This water does not directly reach the collectors; rather, first it goes through the central heater (see the above diagram), and there they are heated by the water that is already in the heater, and that water has already been heated by the collectors.[4]  The hot water in the heater certainly should be classified as a solar byproduct (because they have been cooked by the sun), and if so, the cold water that is heated by this heated water should be considered to be cooked by solar byproducts.  According to this, it would be unanimously forbidden to use a solar heater on Shabbat because the cold water is heated by the hot water.


Is a Rabbinical Pesik Reisha Permitted?


Even so, there are those who permit using a solar heater on Shabbat.  Rav Ovadya Yosef (Yabbia Omer, Vol. IV, OC, ch. 34) struggles with the issue of a pesik reisha, an inevitable, albeit unintentional, result: [5]


There is a question whether one is allowed to use hot water on Shabbat if it was heated by the solar heater, because at the time that one opens the hot-water tap in one’s residence, cold water immediately comes into the heater, and this is like cooking on Shabbat…  Even though one opening the tap has no intent to heat the cold water (which flows immediately into the heater when the tap is opened), nevertheless, it should be a pesik reisha, and this is like cooking on Shabbat, because it is forbidden to cook via solar byproducts on Shabbat…


However, in our case, that one does not intend to heat and boil the water, we have come to a dispute among the halakhic authorities about the law of pesik reisha as it applies to rabbinical prohibitions, whether it is permitted or not. The Terumat Ha-deshen (ch. 64) writes that for a rabbinical prohibition, one should distinguish between full intent and a pesik reisha of an unintended consequence…  However, the Magen Avraham (314:5) has argued strenuously and impressively, overwhelming every occupant of the study hall with his brilliant and convincing proofs from a number of sources that a pesik reisha of a rabbinical prohibition is prohibited…  Nevertheless, the words of our master the Beit Yosef indicate that, halakhically, he believes that one should different between biblical and rabbinical pesik reisha


The determination of the law with which we began and with which we conclude is that, by the letter of the law, one may open the tap for water that was heated in a solar heater and use it for washing dishes, washing hands, drinking or anything of the sort (and, similarly, it is permitted to close the tap after one has finished using it).  There is no need to be concerned with the flow of cold water into the heater and the tubes that come out from it where the water gets heated …[6]  However, one who wishes to act stringently and avoid using water that was heated in the solar heater because of the concern of boiling the cold water is praiseworthy; nevertheless, those who act leniently have upon whom to rely for practical purposes.  This is how we rule, God willing, for the tendency to permit must always be greater.  May the Holy One illuminate our eyes in His holy and pure Torah, amen. 


The main reason for Rav Ovadya Yosef to permit this is the fact that the person does not intend to heat the cold water that comes into the heater (it has enough hot water for his needs, so he does not need to heat additional cold water).  The assumption is that the water will indeed be heated, as a pesik reisha; however, because this is a rabbinical ban (against solar byproducts) we may, in his view, be lenient for a rabbinical pesik reisha. 


Magen Avraham: Rabbinical Pesik Reisha Is Forbidden


However, there is no consensus to allow this.  The Magen Avraham (314:5) argues and forbids even rabbinical pesik reisha (even if it is unwanted).  This is how Ashkenazim rule, following the Rema’s view (314:1, Mishna Berura ad loc. 11; Rema 340:3, Mishna Berura ad loc. 17; et cetera).  A number of Sephardic authorities also rule this way.


Perhaps There Is No Pesik Reisha


However, many times there is no certainty that the cold water will be heated by the hot water, since the cold water enters the solar heater from below, and many times the water that is already there in the bottom of the heater is cold (and that water will still need to be heated more by the collectors, so that it may be that it is considered like the sun and not like solar byproducts).  If so, it may be that there is no pesik reisha but rather a doubt (or chance) of a pesik reisha, and the Mishna Berura (316:16) permits a doubtful pesik reisha of rabbinical prohibition.


No Concern of Confusing with Igneous Byproducts


There are halakhic authorities who want to allow the use of a solar heater for another reason, based on the view of the Maharshal (Responsa, ch. 61). The Maharshal claims that the entire prohibition of cooking via solar byproducts applies only when there is a concern of confusion or error — i.e., when one may confuse a solar byproduct with a byproduct of fire.  However when it is clear to all that the heat comes from the sun, there is no prohibition in using the substance, even though we are talking about a solar byproduct.  Based on this, the Maharshal allows cooking food directly on a hot roof that has been heated by the sun, because it is obviously visible that the source of the heat is the sun.  Based on this allowance, there are those who believe that it is permitted to use a solar heater, since the heating is always being done by the sun, and the matter is similar to a hot roof, concerning which there is no concern for error (Shemirat Shabbat Ke-hilkhata, 1st ed., 1:29, n. 67).



Rav Auerbach: Nowadays, a Concern Exists


However, this approach is also the subject of some dispute.  First of all, the Magen Avraham (318:10) writes that the words of the Maharshal are not conclusive; he cites Acharonim who dispute this view and forbid to cook on a hot roof.  Secondly, according to Rav S.Z. Auerbach (Shemirat Shabbat Ke-hilkhata, ch. 1, n. 127), even when it comes to a solar heater there is a concern for error.  Nowadays, the solar heater generally contains a heating element (even if there is no such element, generally the tap in one’s home is also attached to a water source heated by electricity).  Thus, there is a concern that a person may open the tap at home and think that the water is coming from the solar heater, when the water is in fact being heated via electricity (which has the halakhic status of fire).  The concern is very significant, since even if the boiler is currently off, if before Shabbat the water had been heated by the electricity (and it is still hot due to this), the water remains an igneous byproduct, and there is a concern of violating the biblical prohibition of bishul.[7]


In Practice


Practically, it is best to avoid using a solar heater on Shabbat because of the concern that this device has the status of a solar byproduct, as well as the concern that one will confuse the water heated by this device with water heated by a boiler.  However, in a time of need (e.g., for the ill or for minors), one who is lenient about this has upon whom to rely (particularly when we are talking about a rabbinical doubt).[8] 

Translated by Rav Yoseif Bloch



[1]      Furthermore,

Rav S.Z. Auerbach (Minchat Shlomo 12:2) allows one to heat using a magnifying glass. 

However, the allowance is limited to one who is heating it directly by the sun. If one uses the magnifying glass to ignite paper, then one is definitely violating the prohibition of kindling, and then it is forbidden to cook with such a fire.

[2]     As we have said, these are the words of Rav Yosef Kafih, and this responsum is addressed to him.

[3]      This is the view of the Shevet Ha-Levi, Vol. I, ch. 94.

[4]      We should note that there is no barrier between the cold and hot water, because the cold water ends up on the bottom due to its weight.

[5]    This responsum is remarkably long, including the clarification of many important laws of Shabbat, the law of pesik reisha among them.  From those familiar with Rav Yosef, I have heard that it took him half a year to write this responsum.

[6]     Nevertheless, even according to him, it is preferable to close the pipe leading the cold water into the heater:

However, if it is possible to close the pipe of cold water and prevent its flowing into the heater, it is proper to do this in order to avoid any question.  Nevertheless, when it is impossible to do so, it is permissible to use the hot water.  (If the water is very hot and one wants to open both of the faucets simultaneously, the hot and the cold, one may be lenient about this too.)

Indeed, in a practical sense, it is impossible to shut off the cold water from entering the heater, because this will also prevent the hot water from coming out, as this would create a vacuum and prevent any water from coming out.


[7]     However, the Tzitz Eliezer (Vol. VIII, ch. 14) writes in a similar context

that one should not add on to the Sages’ decrees:

Now, I will tell him that I recently received a letter from Rav Moshe Zvi Neria, instructor in Yeshivat Kfar Haroeh.  He told me that he once had a question about solar heaters, and he consulted the great sage and halakhic authority, Rav Tzvi Pesach Frank.  His first thought was that it is ostensibly like that which is heated by the sun and therefore permitted.

Furthermore, [Rav Neria] had reminded [Rav Frank] of Rashi’s words, Shabbat 39a, s.v. De-sharei: “For this is not its way of cooking, and no one would ever confuse fire with the sun, so there is no reason to decree against the latter because of the former.”  This raises the possibility that, at this time, when it is very common and this is derekh bishul, one should include also that which is cooked in the sun itself in the rule of the decree.

However, [Rav Frank’s] answer to [Rav Neria] was straightforward: it has already been stated in the Yerushalmi that we do not add on to decrees; since they did not make the decree in their time, let us not add on to it!

[8]We might have argued that since this is the regular way to heat water it involves a biblical prohibition, as we suggested in the previous lesson concerning a microwave.  However, generally the halakhic authorities have followed the path of Rav S.Z. Auerbach (discussed in the previous lesson), which dictates that only bishul by fire is biblically forbidden.  (See Rav Asher Weiss, “Bishul Be-tannur Microgal Le-inyan Shabbat U-vasar Be-chalav”, Oraita 18, pp. 159-163.  Also see Rav Yisrael Rozen, “Bishul Be-Shabbat Le-lo EshTehumin 17, pp. 15-24; Professor Ze’ev Lev, Ma’arkhei Lev, ch. 11.)  In addition, since we use a solar heater only to heat water for washing and not for cooking, it may be that this is not considered the standard derekh bishul (Az Nidaberu, Vol. I, ch. 34).