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One is obligated to disown/destroy all his Chametz by the start of the 6th hour of Erev Pesach. Prior to the 6th hour of the day, one can free himself from the prohibition of owning Chametz by giving or selling the Chametz to a non-Jew, or by disowning the Chametz and placing in a public area. After the start of the 6th hour, it no longer helps to disown the Chametz in a public area and one must rather destroy the Chametz from the world in a way that no person can get benefit from it. Even prior to the 6th hour, the disowning of the Chametz simply refrains one from transgressing ownership once the 6th hour arrives, however it does not fulfill the Mitzvah of destroying Chametz. It is for this reason that we burn the Chametz on Erev Pesach prior to the 6th hour, in order to fulfill the Mitzvah of destroying Chametz with at least some part of our Chametz that we own. There are various valid methods of destroying the Chametz, however, initially, the best method to be used is to burn the Chametz until it becomes charcoal, and so is the custom today. This burning process if coined “Biur Chametz.”
When to burn the Chametz?
Night of 14th versus day: The custom is that the Chametz which one finds by the search is not burned immediately that night but is rather hidden away to be burned the next day. Nevertheless, if one wants to destroy the Chametz immediately after the search [at night] in order to avoid the possibility of having children or mice and rodents take from the Chametz, then one may do so.
What does one burn?
One is to burn any Chametz that he found during the Bedika, including the ten pieces of bread [and the vessels used for the Bedika, such as the spoon and candle]. If one did not find any Chametz during his search, then it is proper for him to burn the vessels used for Bedika. Some have the custom to use the Aravos which were hit on Hoshanah Rabba as fuel to burn the Chametz.
Repeating the Bittul-Kol Chamira:
After performing Biur Chametz, prior to the 6th hour, it is proper that one repeat the bittul, and nullify once again all the Chametz which he has in his possession. The Bittul cannot be performed once the 6th hour has arrived. The Bittul should only be said after the Chametz is burned, or at the very least after burning one Kezayis. The following is the wording of the second nullification: “Kol Chamirah Vechamia Deikah Berishusi Dichaziseiy Udilo Chaziseiy Dechamisei Udelo Chamisei Diviartei Udilo Biarteiy Livatel Ulihavei Hefker Kiafra Diara/All the Chametz [and yeast] which I have in my property which I saw or did not see, which I destroyed and which I did not destroy etc….”
Receiving benefit from the Chametz fire:
When one is burning the Chametz prior to the 6th hour on Erev Pesach, one may benefit from the burning Chametz and use its heat to cook with. Nevertheless, the custom is to burn it separately [from ones cooking oven, and not benefit from it] even when burning it prior to the 6th hour. However, when one burns his Chametz after the 6th hour, it is forbidden to receive benefit from it.
Prior to the 6th hour on Erev Pesach, one is to burn all his leftover Chametz, ten pieces of bread and Bedika set. Immediately after the Chametz is properly burnt, one is to recite the Bittul, prior to the 6th hour.
Q&A on Biur Chametz
When before the 6th hour is one to burn the Chametz?
One may burn it anytime prior to the 6th hour, so long as the family has finished eating Chametz, in order so the Bittul can be said.
If one does not own any Chametz, is he to buy Chametz in order for him to be able to burn it before the 6th hour?
There is no obligation to do so, although one who desires to be stringent upon himself may do so, although he may not rule this way for others.
Should one destroy the Chametz himself rather than appoint someone else to do it for him?
Yes. It is best for the owner of the Chametz to burn it than to have someone else do so for him.
Where should the Chametz be burned; in a public area or in one’s property?
Some say that the Chametz should be burned in one’s property, or in a public area but in a vessel that belongs to oneself. In such a case, however, one must verify that all the Chametz is burned, as explained above.
What material should one use to fuel the flame to burn the Chametz?
May one pour flammable fluids onto the Chametz to help it burn better?
Some Poskim suggest that one should not pour lighter fluids onto the Chametz, as it renders the Chametz inedible for a dog before it is burned, and one thus loses the ability to fulfill with it the Mitzvah of Tashbisu, to burn the Chametz.
Which hand should one use to throw the Chametz in the fire?
Some write that one should throw the Chametz into the fire with his right arm, as is the case by all mitzvahs which should be done with ones right.
Must one make sure that all the Chametz he threw into the fire gets burned?
Before the 6th hour: If the Chametz is being burnt prior to the 6th hour in a public area, then it is not necessary to make sure that all the Chametz is burned, as in any event it is considered disowned, however the custom is to do so. At the very least, one is to verify that one Kezayis is burnt in order to fulfill the Mitzvah of Tashbisu. If, however, it is being burnt in a private area, such as the open garage of a private building, then one must verify that all the Chametz is burned.
After the 6th hour: When burning the Chametz past the 6th hour, one must make sure that all the Chametz has been burned, whether one is burning it in a public or private area. Therefore, one must remain by the fire there until all the Chametz is burnt, as many times if the flame is not stoked it will not penetrate and burn all of the Chametz.
Q&A on nullifying the Chametz
If one destroys his Chametz early, while his family is still eating, when is the second nullification to be said?
There is no use in saying it while his family is still eating Chametz. Rather, it is to be said after the time that Issur Chametz has arrived.
Q&A on discarding the Chametz
May one discard his Chametz trash into a garbage bin, if it will remain there after the 6th hour on Erev Pesach?
See Chapter 2 Halacha 8 in Q&A!
Q&A on benefiting from the burning Chametz
May one barbecue hotdogs or marshmallows over the flame used to burn the Chametz?
The custom is not to do so even when burning the Chametz prior to the 6th hour on Erev Pesach. When burning Chametz after the 6th hour on Erev Pesach, it is forbidden to do so from the letter of the law.
Yehi Ratzon-Burning the spiritual Chametz:
During the burning of the Chametz, and recital of Kol Chamira, it is customary to recite a prayer discussing the spiritual destruction of the side of evil, represented in the Chametz.
 See Chapter 2 Halacha 6 and 8 for the full details of this subject!
 Admur 445:7
 The reason: The reason for this custom is based on the same reason for why the custom is to destroy the Chametz through burning it, being that we compare it to Nosar which is also burned with fire. The Mitzvah in burning Nosar is to be done only during the day and not by night as the verse states “And the leftover from the sacrifice on the 3rd day is to be burned in fire”. [Admur ibid; Alternatively, the reason is in order so one have something to remind him to do the Bittul.]
 Admur 434:12; 445:1
Background: Although one does not transgress a negative command when owning Chametz prior to the night of the 15th, nevertheless, he is Biblically obligated due to a positive command, to destroy all Chametz from his home from midday of the 14th and onwards. This obligation is called Tashbisu. If he did not destroy his Chametz [at that time], then he transgresses the positive command of Tashbisu for every moment thereon. [Admur 431:1] The above obligation to destroy the Chametz by midday is only from a Biblical perspective, however Rabbinically once the beginning of the 6th hour of the day has arrived one is to destroy the Chametz. [Admur 443:1] Now, from the letter of the law, one is only commanded to destroy the Chametz starting from the beginning of the 6th hour and onwards and not beforehand. [Admur 445:1] Nevertheless, the custom is to destroy the Chametz prior to the beginning of the 6th hour, as explained here.
 The reason: The reason for this is because since one needs to nullify the Chametz at the time that he destroys it, as explained above, therefore he must destroy the Chametz prior to the 6th hour, as from the 6th hour and onwards one can no longer nullify it, as explained in chapter 433. [Admur 434:12]
 Likkutei Taamim Uminhagim of Rebbe on Haggadah
 Admur 445:7
 Admur 445:7
The reason: The reason for why this is done is in order so one will not forget the obligation to burn his Chametz the next year and also in order so he not forget to nullify his Chametz a second time [the next day.] [Admur ibid]
 Admur 445:12
The reason: The reason for this is because anything used to do one mitzvah it is proper to do with it another Mitzvah. [Admur ibid]
 Admur 445:11 “When destroying the Chametz prior to the 6th hour, [being] that the Chametz is still permitted in benefit, one does not need to make a separate flame [from that of his cooking oven] for the Chametz, and he rather may burn it in his cooking oven, and use its heat to cook with, as well as he may get all other types of benefit from it. Furthermore, even after the 6th hour one may continue to benefit from the heat derived from the Chametz if the Chametz has already been burned, before the 6th hour, to the state of being inedible even to a dog. Nevertheless, the custom is to burn it separately [from ones cooking oven, and not benefit from it,] even when burning it prior to the 6th hour.”
 Admur 445:8
The reason: When one burns his Chametz after the 6th hour, [being] that the Chametz is [now] prohibited in benefit, one needs to burn it individually, and may not place it in his fire used to bake or cook on. Furthermore, one may not use its light or warm oneself up from it or receive any other forms of benefit from it similar to these. Furthermore, even the charcoal/ash of the Chametz is prohibited in benefit. However, the charcoal and ash of the wood used to burn the Chametz are permitted in benefit. Thus, if one recognizes the wood ash and knows for certain that the ash is from the wood and not from the Chametz [then it may be benefited from]. [Admur ibid]
 See Admur 445:7 that one may burn it also at night if he chooses, so there is no need to wait to burn it until the 5th hour of the day; See however Piskeiy Teshuvos 445:3 for other opinions
 See 1st opinion in Admur 436:21 “The mitzvah of “Tashbisu” is not a bodily obligation that one is obligated to have a house in order to destroy the Chametz from within it. Rather, the mitzvah is that if one has a house, and in that house he has Chametz, then he has the Mitzvah of TaShbisu. However, if he does not have a house, then he does not have any obligation at all.”; Admur ibid concludes “Practically, one may be lenient like the second opinion, and one who wants to be stringent may do so; however, he should not rule this way for others”; See Chapter 2 Halacha 6B for other opinions; Chapter 3 Halacha 12A; See Piskeiy Teshuvos 445:6
 Piskeiy Teshuvos 445:1
 The reason: As a) Mitzvah Bo Yoser Mibishlucho, just like we say by Bedikas Chametz; b) So he has what to remind himself to nullify the Chametz, as explained in chapter above.
 See Piskeiy Teshuvos 445:1
 This is because once the Chametz is placed in a public property he is no longer obligated Biblically to destroy it, and thus he only fulfils the mitzvah of burning it Rabbinically.
 Rashash Shabbos 63
 Minchas Yitzchak 2:53; See Piskeiy Teshuvos 445:2
 Beir Moshe 5:122
 Piskeiy Teshuvos 445:5
 See Piskeiy Teshuvos 445:1
 See Admur Piskeiy Teshuvos ibid
 M”B 445:10 in name of Beis Meir, Chayeh Adam, Chemed Moshe; See Halacha 5E that the Biblical owning prohibition only applies to a Kezayis; See Kaf Hachaim 445:18
 Rav Avraham Chayim Naah in Yagdil Torah 6:2604
 Siddur Admur; Siddur Arizal