Based on the volume of calls that the Star-K Kashrus Hotline has received about warming drawers, it appears that many consumers are unaware of how to properly use a warming drawer on Shabbos. As mentioned on the Star-K website (http://www.star-k.org/kashrus/kk-cooking-ovenshabbos.htm), one may not put food into a warming drawer on Shabbos. Apparently, the term “warming drawer,” a misnomer, is misleading consumers into thinking that they can warm up their food in it on Shabbos.
What exactly is the problem? When one opens the warming drawer, one is letting cool air into the drawer. A thermostat will sense the loss of heat and make up for this loss by turning the heating element back on. Similarly, when one opens the oven door, one indirectly causes the burner or electric element to turn on. This is gram havarah, which is prohibited.
If one leaves food in a warming drawer or oven and wishes to open either of them one time to remove all of the contents, this may be done, because normally one does not want the element or burner to go back on if the oven or warming drawer is empty. However, if one leaves food in the oven after the door or warming drawer has been opened, they obviously want the oven to go back on. This is prohibited. It follows, therefore, that if one accidentally opens the oven door or warming drawer, they must remove all remaining food. Therefore, the oven door or warming drawer cannot be opened to check on the food. If the door or drawer was accidentally opened and closed and no food was removed, the food is still permitted to be eaten provided that it was completely cooked before Shabbos.
In conclusion, it is crucial to understand that the same halachos that apply to using an oven on Shabbos apply to warming drawers. Just like you wouldn’t put food into an oven to warm it up on Shabbos, you may not put food in a warming drawer on Shabbos. And, similarly, as with ovens, you must also cover your warming drawer controls if you are leaving it on over Shabbos and not change the setting.