Although it is prohibited to cook on Shabbat, this should not be considered a recommendation to eat cold food on Shabbat. On the contrary, one should eat hot foods on Shabbat, in order to honor Shabbat and enjoy the holy day. The Sages criticized one who refrains from eating hot food on Shabbat, saying that such a practice is overly stringent. It is therefore necessary to know the relevant halakhot, in order to ensure that the food will be heated in permitted ways.

The simplest way to eat hot food on Shabbat is to cook the food before Shabbat and place it before Shabbat begins on a source of heat, which will keep it warm until the meal. The Sages prohibited leaving food on a regular fire that is used for cooking, lest one forget that it is Shabbat and, out of habit, increase or lower the intensity of the flame under the pot. They therefore instituted that food should be placed on a source of heat in a manner that does not enable a change in the intensity of the fire.

For this purpose, one may use an electric hot plate that does not have an open burner, and on which it is impossible to intensify or moderate the heat. One can also place cooked dishes on top of a stove, provided that the fire is covered and is not visible. This can be accomplished by means of a metal sheet (often called a blekh in Yiddish) that is placed over the flames and bent in such a way that it also covers the control knobs of the stove. This serves to avert the possibility of adjusting the flame on Shabbat.

There is a problem with leaving food in a preheated oven at a low temperature, because opening the door lets in cold air, and as a result, in an ordinary oven the heating mechanism is activated, which means that the one who opened the door is in effect the indirect cause of this heating. Some ovens have a special switch enabling a “Shabbat mode.” This switch ensures that the activity of the heating system will remain constant, and will not be affected at all by the penetration of cold air. One may place food in an oven of this type on Friday and remove it from the oven before or during meals on Shabbat. However, some are stringent and do not use an oven even in such a manner.

Putting food before Shabbat into a slow-cooking pot, such as a crock pot, so that it will cook during Shabbat, entails halakhic complications, and one should ask a rabbi if and how it is permitted to do so.

Many people use electric warming trays to keep their food warm on Shabbat. Before Shabbat, many people place a covering over the pots that are on the warming tray, to better preserve the heat. This is permitted only if the covering is not wrapped around the pots on all sides.

It is permitted to move a pot from one place to another on a warming tray on Shabbat.

When the pot is on a heat source (warming tray or a covered stove) one may not mix the contents with a spoon, a ladle, or the like, or even use these utensils to remove food from it. This action is prohibited because it speeds and improves the cooking process. One should remove the pot from the heat source and then take out the food.

Soup or any other food that is on a heat source such as an electric warming tray, is classified as being in a primary vessel, and it is therefore prohibited to add salt or other spices to it. If one accidentally did salt or spice the dish, it is permitted to eat the food.

In the event that the food has been transferred to a secondary vessel that is not on the heat source, spices may be added to the food. However, some are stringent even with regard to the secondary vessel while it is still hot, if the spices have never been cooked before.

When preparing hot water for use on Shabbat, one must be careful not to violate the prohibition against cooking. One option is to boil the water on Friday and place it before Shabbat on a hot plate (or a covered stove, as explained above). It is also permitted to use an electric urn that is appropriate for Shabbat use. Nowadays, there are electric urns with regulated temperatures that have halakhic certification for use on Shabbat.

On Shabbat, one may not use ordinary electric water dispensers that provide hot and cold water for drinking, as pushing the button immediately activates the heating or cooling system. There are devices today whose operating system is specially adapted for Shabbat use. These devices are marked with appropriate halakhic certification.

Sometimes one might want to remove food from a pot and then return the pot to the heat source, e.g., if not everyone is eating at the same time. This is permitted if the following conditions are met: (1) At the time one removes the pot from the heat source he intends to return it there; (2) by the time one returns the pot, it has not cooled below 45° C (113° F); (3) one holds the pot while it is off the heat source, or at least does not place it down in a manner indicating that he has decided not to put it back on the heat source; (4) the heat source is covered rather than an open fire or an oven. This includes a metal sheet over a flame. Similarly, an electric warming tray is acceptable.