Can a solar-powered water heater (a dud shemesh [דוד שמש] in Hebrew) be used on Shabbat? These are quite common in Israel as a way of heating water without electricity on a sunny day.

Essentially, the water is in a glass tank and gets heated by the sun's rays. When you turn on the faucet, hot water leaves the tank and cold water enters.

I see two possible problems here:

  • Is the water being heated directly from the sun (חמה), which is permitted, or from an implement warmed by the sun (תולדת חמה), which is forbidden?
  • When new cold water enters the tank, does it get cooked by the hot water already in the tank?

What do the different poskim say about this?

2 Answers 2


All sources are from Yeshiva.org.il's detailed article about this. Note that i am relying on them being correct -- i did not check them.

As we said in the question, there are two possible issues:

  • Is it being warmed directly by the sun or not?
  • Does the cold water get cooked upon entering?

It is most definitely a machloket about whether or not it can be used, so make sure to ask your rabbi before using it to get a personal psak.

Let's start with the first issue.

Some write that the water gets heated directly from the sun, and that the glass and pipes do not absorb enough heat to actually heat the water, making it permissible. These include the Tzitz Eliezer (7:19), Rav Moshe Tzvi Neriah, Rav Tzvi Pesach Frank, and Rav Shlomo Zalman Auerbach (הערות על ספר יסודי ישורון, עמוד לא; הו"ד בספר שולחן שלמה שיח ג יג).

On the other hand, Rav Binyamin Yehoshua Silber (שו"ת אז נדברו מלאכת אופה 13) writes that without the dud, the water would never heat up that much, and since it is the dud that makes the water get so hot (by focusing the sun's rays), it is considered toldat chamah, and is forbidden.

(Sorry, the article didn't bring Sephardic poskim on this matter.)

This is only regarding heating it initially. What about the second question, the new [cold] water that enters when you turn on the faucet?

Most poskim agree that this is a psik reisha d'lo neicha lei by an issur d'rabbanan.

Rav Ovadia Yosef writes (Yabia Omer part 4 OC 34) that it [a פרדלנ"ל by דרבנן] is permissible, but it is still good to be machmir, and if possible, stop the cold water from entering.

Ashkenazi poskim don't generally allow a פרדלנ"ל by a d'rabbanan, but this is a special case. We also have grama here -- you don't actually heat the cold water -- it happens on its own. As such, it's now a פרדלנ"ל plus two issurei d'rabbanan, and that is enough to be meikel and allow it, at least according to Rav Tzvi Pesach Frank (his shu"t OC 188), Harav Or Letzion [unsure who this is], and the Tzitz Eliezer (7:19, 20:16).

However, the Minchat Yitzchak (4:44) and Shevet Halevi (1:94) disagree with defining this as a פרדלנ"ל + two d'rabbanans, saying that this is not grama, and it is also a psik reisha d'neicha lei.

Shmirat Shabbat K'hilchata (1:45) uses the term "טוב להמנע" -- it is preferable to abstain. He also brings Rav Shlomo Zalman Auerbach that [despite his ruling that it is heated by the sun itself,] there are some systems that will automatically switch to using an electric boiler if there isn't enough sun, which is definitely forbidden, so it's better to avoid it.

The article sums it up: If one wants to be lenient, he has upon whom to rely, but as for one who is stringent -- תבוא עליו ברכה.

  • No sources, but I've seen the tradition to run the hot water right before shabbos until the water starts becoming tepid. That way you know for sure that any new water going into the boiler will not become hot (until the next day). This allows you to use the remaining warm water on Friday night (e.g. to wash dishes).
    – Nic
    Commented Jul 14, 2015 at 15:00
  • FYI, the Or Letzion is Rabbi Ben-Tzion Aba-Shaul Z''TL, one of the late sfardi rabbi's of our generation.
    – shmuel
    Commented Jan 8, 2017 at 18:50

I think there is another problem that was not addressed, and that is, that there is cold water in the pipes from before hand.

If you ever turned on the hot water faucet, it is cold for a few seconds until the hot water comes through.

This being the case, it will be forbidden to turn on the hot water because at the meeting point between the hot water and the cold water, the cold is being heated by the hot water.

  • Why is that a problem?
    – Double AA
    Commented Jan 8, 2017 at 18:57
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    – mevaqesh
    Commented Jan 8, 2017 at 19:00
  • it is a problem because the hot water is toldes haor and it is heating the cold water, which is forbidden
    – shmuel
    Commented Jan 15, 2017 at 18:35

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