Is an oven with Cook-time or Stop-time Mode acceptable for Shabbos use?
There are many ovens on the market, and it is impossible in this short space to investigate and address every single model. The best we can do is provide a summary of the many issues to look out for. It is important that you read through the manuals of any model you are considering for purchase, in order to determine whether, and to what degree, the issues outlined below apply.
The main problem with using a oven on Shabbos is the thermostat, which is affected by opening or closing the oven while the oven is on. When the oven door is opened, the thermostat will detect the cold air that rushes in and will activate the heating element if it is off at that moment. It is forbidden to cause this to occur, even if all the food will be removed all at once before the oven door is re-closed. [If the oven door is re-closed with some of the food remaining inside, there may be additional concerns of Bishul on Shabbos, depending on the circumstances.] The thermostat is not a concern for Yom Tov use, but there are additional problems with a oven on Yom Tov as detailed below.
If the oven is off, like after the cook-time or stop time has finished this would not be an issue.
the problem with using such an oven on Yom Tov is primarily a practical one; most modern ovens have an auto shut-off feature that turns the oven off after around 6-12 hours. Yet another problem, applicable on both Shabbos and Yom Tov, is that many models have lights or fans that are activated and deactivated as the door is opened or closed. In summary, a modern oven without Sabbath Mode may be completely unusable on Shabbos and Yom Tov.
Ovens with “Sabbath Mode”:
Buying an oven with Sabbath Mode will alleviate many of these problems. Sabbath Mode will generally disable the oven’s display panel and any electronic controls, as well as disengage the fans and lights from being activated by opening and closing the oven door. The Sabbath Mode feature will also disable the auto shut-off feature. However, one very big problem, at least with regards to most models, is that the thermostat usually remains operative. [Some ovens have a light which illuminates when the heating element is on, in which case the oven could theoretically be opened when the light is on. However, it has been demonstrated through rigorous testing that these indicator lights are generally inaccurate, and may be illuminated even when the heating element is off.]
The thermostat does not present a problem for Yom Tov use, but makes it completely forbidden on Shabbos, according to most opinions. The outcome is that, ironically, the Sabbath Mode oven remains forbidden for Shabbos use, and is permitted only on Yom Tov. Perhaps it would have been more apt to name this feature “Yom Tov Mode”.
There is a practical workaround for Shabbos use that is easy and simple: Estimate about what time the main course will be served, and make sure that the oven turns off before then. This is generally easy to achieve, as the Sabbath Mode feature can usually be set to turn the oven off in any increment amount of time up until 72 or 96 hours. So, you can easily set the oven to go off after 2 or 3 hours, for example. Once the oven is off, opening the door does not create a problem with regards to the thermostat.
However, this method will usually create a new problem, which requires its own workaround. Now that the oven is off, this means that Sabbath Mode is now deactivated as well, and opening the door will turn the light on (if the oven has a light). This can be circumvented before Shabbos by manipulating the magnets or levers that detect whether the door is open. The specific manner of circumvention will vary from model to model, and in the case of magnetic detectors, may take some trial-and-error. Some oven doors are designed to be taken apart to allow proper cleaning of all the glass panes, and the magnet can be physically removed from the door. In other models, it may work to place a strong and thin magnet on the oven wall at the correct place.(The location of the oven’s magnet can usually be detected by sliding a paperclip around the top of the oven door.) These options may or may not work, depending on the model, so you would have to try before Shabbos, when you have the time and patience to troubleshoot it. [Note: Disabling the magnetic sensor only helps with regards to the functions controlled by the door sensor, such as the oven light and fans. However, the thermostat is not controlled by the door sensor, which is why the oven needs to be off before opening the door on Shabbos.]
Following the advice above, when Shabbos is followed by Yom Tov, one would have to choose whether to reserve the oven’s use for Shabbos or Yom Tov, as using it on Shabbos would require the oven to be set to turn off on Shabbos, in which case it will remain off for the duration of Yom Tov as well.
The Issur of Sheiyah with regards to an oven:
The above answer focuses purely on using the oven itself. However, there are also restrictions about which foods may be placed in an oven before Shabbos without a Blech, and much discussion about whether, and how, a Blech can be facilitated in an oven. Here is a brief summary of the issues:
From the Torah perspective, one may place food over an open flame before Shabbos in order that it cooks on Shabbos. Nevertheless, Chazal forbade this due to the concern that a person may come to stoke the fire on Shabbos in order to speed up the cooking process. This prohibition is knownas Shehiya.
One method of avoiding Shehiya is to cover the flames, typically with a Blech. Covering the fire indicates that one does not desire a strong flame, it diverts his attention from the fire, and it serves as a reminder not to increase the flames. With regards to an oven, there is a dispute whether – and how – a Blech can be adequately created. In practice, a Blech in an oven should be situated so that it covers the heating elements, which may be located either at the top of the oven, the bottom or the backwall. In many ovens, there are heating elements in all of these places.
Another way of addressing Shehiya, and to sidestep the need for a Blech, is to ensure that the food is cooked prior to Shabbos. It is sufficient if the food is already half cooked. This fraction does not represent time, but rather, is qualitative and measured against the degree of cooking most people would regard as fully cooked. Nevertheless, for a number of reasons, it is best that the food be fully baked before Shabbos.
There is yet a third possible way to circumvent the prohibition of Shehiya. Chazal allowed one to put completely raw meat on an open flame just before the onset of Shabbos. They were not concerned that one might come to stoke the flames, because doing so would not serve any productive purpose – it would not ready the meat in time for the Friday night meal, nor would it be necessary to ready the meat in time for the Shabbos day meal. This leniency is known as “Kidra Chaysa” – literally: raw pot. (Indeed, some people may wish to put raw chicken and rice right before Shabbos to slow bake in the oven for the daytime Seudas Shabbos.)
In order for this leniency to apply, two criteria must be met. First, the food item must be one that takes a relatively long time to cook properly (such as meat). Second, the food must be put on the fire just before Shabbos, late enough that it will not begin cooking before Shabbos comes in. Some contemporary Poskim therefore assert that this leniency seldom applies nowadays, because most modern cooking appliances are able to cook in a relatively short amount of time. As a case in point, meat put up to cook rightat the onset of Shabbos would usually be fit for consumption by the time the meat course arrives. Even if the heat is set to low, this does not diminish the oven’s overall ability to bake quickly, because the seat heating can be easily raised. Thus, according to these Poskim, this option is not applicable to a modern oven.
Other associated issues:
1. It should be pointed out that even when the oven is off and cold, if it will be turned on with a Shabbos clock or a timer (see next point), one may not place inside any food which is not completely cooked. Furthermore, even completely cooked food that has cooled down may not be placed inside either, unless the food is completely dry (and will remain dry when heated). The same applies to an oven which will remain off but has not yet cooled down.
2. With regards to Shabbos clocks in general, the Rebbe writes in a letter (Igros Kodesh Vol 12 page 192): “There are those who are extremely stringent and are even sceptical about the permissibility of using a Shabbos clock… Therefore, I am abstaining from giving this idea in an official capacity.” Even so, it is reported that the Rebbe instructed some individuals that they may be used (See Shulchan Menachem Volume 2 page 173). At the same time, many authorities who permit Shabbos clocks in general nevertheless rule that they should not be used with regards to cooking specifically, for a number of reasons.
For further discussion of these issues, including sources, please see the following links: