Question:
Can one use Sternos to keep food warm on Shabbos? Is there an issue of hatmana or shehiyah (i.e., a non covered flame)? Is there any concern with taking food from the pans when they are sitting above Sternos, with pans of water beneath the food trays?
Answer:
There is no issue here of hatmana, as hatmana applies only when an item is wrapped or surrounded by an insulating material, which is not the case here. There is likewise no issue of shehiyah, provided that the food was at least half-cooked (ma’achal ben Drusai) before Shabbos.
Even if the top food tray is resting in water, which itself is on the Sterno, it is still not considered hatmana, so long as the majority of the walls of the tray are not covered or submerged in the water. In practice, this is usually the case, as the tray merely sits on top and the water does not reach up along its sides.
However, there is a separate concern of magis (stirring). Accordingly, it is forbidden to remove food directly from the tray while it remains on the Sterno. Rather, one must first remove the tray from the Sterno and only then serve the food.
Sources:
The Shabbos Kitchen [Goldstein] Chapters 2 Halacha 3B, Chapter 3 Halacha 3, Chapter 6 Halacha 5; Admur 253:8-9; Shabbos Kehalacha Vol. 1 p. 226; Shabbos Kehalacha Vol. 2 page 14-24; Piskeiy Teshuvos 257:11