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Many homes and buildings are heated using a hot water radiant heating. (Water is heated in a boiler in another room. It flows through pipes in the building to the radiator where much of the heat is given off; then the water returns via pipes to the boiler.)

Suppose one set a pan of food directly on a radiator, which is quite warm to the touch but would not burn your skin. What is the status of that food in regard to bishul? Is the dish a kli rishon? Is the radiator considered a heat source? Would it be prohibited to place a dish on a radiator on Shabbos?

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Since the hot water is inside the radiator, perhaps it is like putting the dish on a blech or on the top of a tea urn. – sabbahillel Apr 8 '14 at 3:27
Can you comfortably rest your hand on the radiator when it has been on for a while? – Tatpurusha Apr 8 '14 at 5:36
I discussed this problem with my LOR with respect to putting wet towels on the heated "towel rail" on Shabbos. He said he would not do it because there was a possibility of heating the water in the towel above yad soledes bo. – Avrohom Yitzchok Apr 8 '14 at 13:39
@AvrohomYitzchok, clothes have separate issues anyway; see Shulchan Aruch 301 (somewhere around s'if 40 or so?). – msh210 Apr 8 '14 at 14:45

It really depends.

Was the dish cooked before? If it was, and is a dry food, or is a liquidy food and is still warm (This is the Rema's opinion, the Mechaber requires it to be yad soledes), there is no problem of cooking on a biblical level. If it's a cool liquid, there may be a biblical prohibition (See chazon ish who assumes that Rema concedes that this is only a rabbinic prohibition...).

If there is no biblical problem, the only possible issur is chazarah, a rabbinic prohibition. This issur does not apply to places where one does not normally cook (See OC 253:5), such as on a radiator. Therefore, where it is halachically called previously cooked, it is permissible.

If it was not cooked, or is a liquid which has cooled off, then the biblical prohibition of cooking applies, provided that it will be heated to Yad Soledes Bo. If it will not be, then it is permissible (OC 318:14). The definition of yad soledes in terms of degrees is the matter of much debate (on which I'm sure you can find a lot of literature online), which I don't really want to go into....

If one already cooked the food in a prohibited manner, the food may be prohibited to consume. This depends on if the prohibition is unanimous (MB 318:2).

As always, be sure to ask your LOR. I apologize if not everything is sourced. All of the stuff on biblical cooking can be found pretty easily in Siman 318, and on chazarah in Siman 253).

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The Rambam in the Mishneh Torah, Hilkhoth Shabbath 9:2 writes as follows:

הנותן ביצה בצד המיחם בשביל שתתגלגל ונתגלגלה חייב: שהמבשל בתולדת האור כמבשל באור עצמה


"One who places an egg on the side of a kettle so that it will be [even] slightly roasted is liable [for a qorban be-shoghegh and kareth be-mezidh]. For the one who cooks with a derivative of fire is like one who cooks with fire itself."

Thus, a radiator with heated water being used as a heat source for cooking on Shabbath would be asur le-ghamrei, and since using derivatives of fire is like using fire itself, your dish would subsequently be a kli rishon. The food would be forbidden on Shabbath to benefit from, let alone eat.

Hope this helps. Kol tuv.

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How is a radiator derech bishul assuming it is not very hot – sam Jun 11 '14 at 1:04
How is using a small flame derekh bishul if it's not very hot? The question is clearly about bishul on Shabbath, even if it is slow or not "well-done." Either way, it is asur. – Maimonist Jun 12 '14 at 0:28
This is not correct. The fish is only assur if it was heated to yad soledes. A food is only assur bidieved if it was involved in a unanimous issur (MB in 318, note that he also prohibits it in some, nearly unanimous cases). As there is a machlokes as to what constitutes yad soledes, with some opinions requiring 160 degrees or higher, it would only be assur bidieved at that temperature. The questioner called it quite warm to the touch, which makes it sound like a lot cooler than that. – Ish Ploni ViKohen Jul 11 '14 at 1:06

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