1.must one use a blech on yom tov?
2. When can one start preparing food for second night Yom tov?
3.what are the laws regarding lowering /highering flames on yom tov?
1. We do not have to use a blech on yom tov. The reason is since the whole reason we need a blech for shabbos is so we won’t come to stir the coals, and make the fire bigger. On yom tov we are allowed to do this, therefore there is no need for a blech.
2. You may start preparing food for the second night of yom tov only after it is already considered nighttime. This would be from 72 minutes after shkiya, which is best. Other than that
3. Although we are allowed to make a fire from an existing one on yom tov, we may not extinguish a fire. Therefore essentially we cannot make the fire smaller on yom tov. However if the reason we are lowering the flame is so the food shouldn’t burn, then it is permitted, because it is considered for ochel nefesh.However there are different opinions how far this heter goes. R’ Moshe Feinstein zt”l was of the opinion 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.
However most poskim say that we may only lower the fire if the food is actually going to get ruined or won’t be good to eat, and there isn’t an option to make another fire. Therefore according to these poskim, one should not lower the fire if it too high, rather light another burner, (by bringing the fire over from an existing fire) instead of lowering the existing burner.
Everyone however agrees that we may not lower the fire just to save gas, etc.
There is also another issue here. In the last number of years sotve tops no longer are made with pilot lights, but rather with a spark igniter. This may not be used on yom tov. Besides this there is a safety feature to stove tops that the fire will not stayt on unless the knob is pressed for a few seconds. This is to insure that the knob wasn’t pressed by mistake. It is activated using, and electrical current, and according to some poskim it is not allowed on yom tov.
Therefore in the final analysis, if you have a stovetop that has the above feature, then it is better to lower the existing flame, but if your stovetop doesn’t have this feature, it is better to light a different burner.
O:CH 514-1, Igros Moshe O:CH 1-115, 4-103, Chelkas Yackov 1-60, Minchas Yitzchok 1-56, Be’er Moshe 8- 128, Mishna Halachos 7-75, Shmiras Shabos Khilchoso 13-10 in the name of R’ S. Z. Auerbach zt”l, .