Question: Can I place my cold food on the hotplate on Shabbos if the timer is set to heat it up later? If not, can I do so before Shabbos?
Answer: The Shulchan Aruch (OC 253:5) writes that one may heat up a davar gush, dry solid food, on Shabbos by placing it on top of another pot. One may place such food onto a hotplate, regardless as to whether it is on or will later be switched on with a timer. Some are particular to put an upturned tray, etc. down first (See Yechave Daas 2:45; Tzitz Eliezer 8:26:5; Meor Hashabbos 10:4).
While one mustn’t place a davar lach, a boiled food with liquid, on the stove or hotplate on Shabbos, R’ Ovadia Yosef (Yabia Omer OC 10:26) allows one to place such a pot on the hotplate while it’s off even though it will later switch on through a timer.
Most poskim, however, disagree. R’ Tzvi Pesach Frank (Har Tzvi OC 136) compares using a timer to later heat food to placing food on a stove that will be lit soon, which he argues is assur mideoraisa. The Chazon Ish (Shabbos 38:2-3), R’ Benzion Abba Shaul (Ohr Letzion 2:30:18) and R’ Yehoshua Neuwirth (Shemiras Shabbos Kehilchasa 1:26) write that while it isn’t assur deoraisa, one still can’t do so on Shabbos because it is a problem of gerama, causing something to happen (though one could ask a non-Jew to do so while it was off). R’ Eliezer Waldenberg (Tzitz Eliezer 2:7) adds that one cannot even place such food before Shabbos on a hotplate that is scheduled to come on as we are concerned that one may come to do so on Shabbos. R’ Yehoshua Neuwirth (ibid.) disagrees, however, allowing one to place even cold food on a hotplate before Shabbos, providing it is fully cooked.In conclusion, one may put dry solid food onto a hotplate on Shabbos that will come on later. While some sefardim may allow placing liquid food on, too, ashkenazim mustn’t do so. One may place food before Shabbos onto a hotplate that was off providing the food is fully cooked.