I thought of a question over Yom Tov. If one uses a blech of Shabbos, are they allowed to use it on Yom Tov? The reason for using a blech on Shabbos is shema y’chate, but if one uses a blech when it is muttar to cook (on Yom Tov), what is the hekker to remind us not to adjust the fire when it is assur (on Shabbos? Does it make a difference if one adjust the knobs or not on Yom Tov? Thanks in advance for the answer.
This is an interesting question. It is true, part of the reason we use a blech is as a heker, a sign not to stir the coals, however the main reason is that by placing the blech on the fire we are showing that we don’t want to stir the coals. (Obviously he does want the heat of the coals, otherwise he wouldn’t have put the pot there, but he is showing that he isn’t desperate for them, that he might come to stir them.) That action is part of the heker, and if the person showed that he doesn’t want the coals for actual cooking. In general people don’t use the blech for cooking on yom tov, but only for heating up the food.
Regarding adjusting the knobs, it is true that if we adjust the knobs when the blech is there we are showing that we do want the heat even though the blech is there, which defeats the purpose of the blech. It is for this reason that R’ S. Z. Auerbach says that on erev shabbos we should preferably not adjust the knobs or the fire after putting he blech down. Never the less it isn’t going to disqualify the blech.
M:B 253-14, Igros Moshe O:CH 1-93, Minchas Shlomo 2-34 (14), Shmiras Shabbos Khilchoso 1 ftnt 54 ( old edition).