If one already had a pilot on prior to a yom tov beginning, and they turn the flame up or down, are they allowed to use that pilot for cooking? If they do use it to cook, are they allowed to receive any benefit from the food on the yom tov?
Regarding the halacha of lowering or increasing a flame on Yom tov:
The Mishneh Halachot in Chelek 11 Siman 421 says that one may not increase or decrease the flame.
In Shemirat Shabbat Kehilchata Perek 1 Seif 27 and haraa 75 he also forbids increasing and decreasing a flame.
In Yalkut Yosef Hilchot Yom tov page 243 he forbids increasing and decreasing as well.
R’ Moshe Feinstein zt”l in Igrot Moshe O:CH 1-115, 4-103 held that the flame may be lowered if it is going to help the food, even if the food will not burn, but will dry out, etc. it is permitted. Even if the person could make a different, smaller flame, he doesn’t have to and he may lower the existing flame.
Regarding getting benefit from this food:
According to the Mishneh Berurah 502:4 in the name of the Magid Mishneh and Rashba, that bedieved if a new fire was created for the food, one can eat from this food. Likewise, a kal vechomer can be made that if one lowered or increased the flame, it would be allowed in a bedieved way as well.
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