Can you use a timer on a warming tray so that the warming tray will go on and off on Shabbat?
Rav Moshe Feinstein was against using timers on Shabbat, as he considered them to be a ziluta di’Shabbat, a denigration of Shabbat, and possibly a type of amira li’goy—having a melakha done on Shabbat by someone else, a non-Jew, or in this case, something else, a timer (Iggrot Moshe OH 4:60.2). The one exception he made was for electric lights, since the practice in Europe was to permit having non-Jews extinguish lamps on Shabbat, and sometimes to even relight them. The widespread practice in the Orthodox community, however, is to permit timers for all electrical appliances, with the possible exception of those that generate a lot of noise (a possible problem of hashma’at kol, making noise that many will hear, see Shulkhan Arukh OH 252:5 and Yabia Omer, OH 1:20.12).
Thus, there is no problem using a timer for a warming tray from the perspective of turning it on or off. The warming tray would then be used as it would be if it had always been on from the beginning of Shabbat.
It is possible that you are asking a different question, though. If one has his or her warming tray on a timer, would this allow a use that is normally forbidden on Shabbat? For example, could you put a cold liquid on it when it is off, knowing that it will turn on soon? The answer is no. To heat a liquid to the temperature of yad soledet bo (approximately 110˚ Fahrenheit, see Iggrot Moshe OH 4:74, Bishul, no. 3) is a Biblical violation of cooking on Shabbat (see Gemara Shabbat 40b, Rambam Laws of Shabbat 9:1, SA OH 318:14). If a liquid has already been cooked or heated, but is now cooled down, it is still a violation—either Biblical or Rabbinic—to reheat it to this temperature. (Shulkhan Arukh OH 318:4 and 253:2 rules that bishul achar bishul, cooking something that has already been cooked, when done with liquids is a Biblical violation. Rema’s position is less clear. See Rema, OH 318:15, and 253:2 and Mishne Brurah 253:68. See also Iggrot Moshe OH 4:74.2 and Magen Avraham 253:31).
The restriction against reheating a liquid that is cold or has cooled down also applies to food dishes that have both liquid and solid in them. According to most poskim, even a food that is primarily dry, such as meat balls with sauce, is considered to be a “liquid” if the liquid component is not trivial and incidental (see Iggrot Moshe, OH 4:74, Bishul, no. 7, and Yabia Omer OH 7:42). Questions about what exactly falls into the liquid category and the dry category should be addressed to a local halakhic authority.
What about putting a cold liquid on a fire that will turn on later? This scenario is addressed by Rema. Rema (OH 253:5) rules that one can have a non-Jew place cooled down stew on a stove top, even if another non-Jew will come later and heat up the oven for the purpose of warming up the house. This parallels our case—putting a cold or cooled down liquid on a surface that will later become hot. However, as Mishne Brurah (no. 99) makes clear, this is only permissible when the food is placed on top of the oven not for the expressed purpose of it becoming heated up later. Moreover, Rema rules explicitly that it would be forbidden for a Jew to place cooled down stew on a stove top that would later be heated by a non-Jew. Rav Tzvi Pesach Frank rules that this might even be a Biblical transgression of cooking on Shabbat (Har Tzvi, 136). Thus, you cannot place a cold or cooled-down liquid on a warming tray that you know will turn on later, if the liquid can get to the temperature of yad soledet bo.
It goes without saying that you could not put uncooked food on the warming tray, even when it is off, if the food can get to the temperature of yad soledet bo once the warming tray turns on.
Could you take dry food out of the fridge and put it on the warming tray when it is off, knowing that it will turn on? The answer is yes.
A warming tray—in contrast to a slow cooker or a hot plate—is only used for heating food and not for cooking food. Thus, according to a number of poskim, it is permitted to put dry food on a warming tray even straight from the fridge, especially if it does not have knobs to adjust the heat. This is not considered derekh bishul, to have the appearance of cooking, since most people never cook on a warming tray, and as there are no knobs (or if they are covered), there is no concern that the heat will be adjusted (See Yechave Daat 2:45, and Tzitz Eliezer 8:26.5. Many compare this to the case discussed in Mishne Brurah 253, no. 81). It is my understanding that widespread practice in Israel in the Dati Le’umi community is to put food from the fridge onto a warming tray that does not have adjustable knobs, although some people will only use it by placing an upside pan between the food and the warming tray.
However, many poskim forbid this, especially if in theory food could be cooked on the warming tray, even if this is not the normal use of the tray (see Iggrot Moshe OH 4:75, Bishul, no. 35, Shevet HaLevi 1:91, and Rav Shlomo Zlaman Aurbach, as quoted in Shmirat Shabbat ki’Hilkhata 1:30, although in the most recent edition he defends the lenient position). In the States, many people are strict, and do not put dry food directly from the fridge onto a warming tray. For an extended discussion on this topic, see Rav Shmuel Kadar, Tosefet Ohel, Shabbat, 253:3.
Even following this more stringent approach, it would nevertheless be permitted to put dry food on a warming tray when it is off, even if it will turn on soon. Putting food on a warming tray that is off is definitely not derekh bishul, and the act right now is completely unproblematic.
This is unlike putting liquid or uncooked food on a stove which will turn on or be lit by a non-Jew, which is forbidden. There, the focus is on the food, and one may not do any action now which will result in it being cooked in the future. In the case of cooked, dry food, there is no concern that it will be cooked in the future – halakhically, it can never be recooked. The only concern relates to the current action, that one may not put cold food on a fire, and when the warming tray is off, one is not doing an act of placing food on a fire (See also Tosefet Ohel,above, section 5, who is lenient as well).
It should be noted that if there is a knob for adjusting the temperature, it would have to be covered or removed to prevent accidental adjusting of it. This no different than what you must do when you leave food on a warming tray or fire from before Shabbat. Food cannot be left on a fire (or source of heat) where there is a concern that you will adjust the heat during the duration of the time that the food is on the fire.
- A warming tray can be put on a timer and used as it would normally be used if it were on for the entire Shabbat.
- One may put food from the refrigerator that is dry and fully cooked on the warming tray when it is off, even if one does not normally do this when the warming tray is on. If there are knobs, they must be removed or covered.
- Food that is a liquid or contains liquid, even if it has been cooked, may not be returned to the warming tray when it is off.