Question:
Are you allowed to wrap a perculator with a towel BEFORE Shabbat to keep the water very hot?
Answer:
A percolater can not be fully wrapped even from before shabbos because hatmana when there is a heat source, called “hatmana b’davar hemosif hevel”. What may be done is to leave one side uncovered, or to leave a thick line around the percolator uncovered. This is since hatma ionly applies when the item is covered from all sides.
Sources:
O:CH 253-1, Igros MOshe O:CH 4-74 Hatmana 6, The Shabbos KItchen pg. 47 in the name of the Debericiner Rov zt”l, Shmiras Shabbos Khilchoso- 1 ftnt 233, See Shefvet Halevi 5-30 who says that if the urn has a thermostat that it is permitted, however it seems that the other poskim disagree with this.
