This Halacha is an excerpt from our Sefer
Buy me here or on Amazon.com
The custom of the Rebbe Rashab: The Rebbe Rashab avoided using sugar due to suspicion that a Chametz ingredient was used in the boiling process, or alternatively the workers would dip their breads into it, and there is thus a suspicion that a crumb of Chametz remained in the sugar. There is a story with the brother-in-law of the Rebbe Rashab who owned a sugar production plant and had made a specially supervised batch of sugar to be used by the Rebbe Rashab on Pesach, despite that even throughout the year there was no suspicion of Chametz in the sugar. When the Rebbe Rashab was brought the sugar cubical his face became stern with concern, and he followed to break open one of the cubicles, and unexplainably a wheat kernel fell out.
The Rebbe’s directive: Regarding if the above stringency of the Rebbe Rashab applies for all Chassidim, the Rebbe once said that if one knows for certain that there is no suspicion of Chametz in the sugar then he does not see a reason for one to be stringent.
The custom to boil the sugar: Some are accustomed to boil sugar before Pesach and thus make sugar water which they use rather than actual sugar. Some have the custom to then filter the sugar water afterwards.
 See Sefer Hasichos 5700 p. 37; Otzer Minhagei Chabad Nissan p. 50; Shulchan Menachem 2:267
 Hamelech Bemisibo 1:307; See Shulchan Menachem ibid
 Chayeh Adam 127:3 in name of Noda Beyehuda
 The reason: This is done in order to ascertain, that even if there were to be a speck of Chametz in the sugar, it would dissolve through the cooking and become nullified in 60x before Pesach in Lach Belach. [ibid]