Q. During these pandemic times when still many, despite the cold weather, daven on outside backyard minyanim, and seek ways to keep warm even during the longer tefilos of Shabbos, a number of questions have been raised. Firstly, can one wrap himself with a connected electrically heated blanket to keep warm when outside during Shabbos? The fact that when someone wraps himself with that blanket left in the cold outside and now contributes his body heat to it, will be sensed by the thermostat and it will diminish the electrical current used by the blanket a fact that should prohibit its use?
A. Poskim permit the use of an electrical blanket during Shabbos, if left on and connected before Shabbos, when the controls are properly covered and protected, to avoid involuntarily resetting or turning them off during the night. Taking care as well that the connection to the wall’s electrical outlet is set and secured, so moving in the bed will not disconnect it. (Igros Moshe, O.H. 3: 50, Minchas Shlomo 1; 9 – 1: 14, Yechaveh Da’as 5: 28; Chelkas Yaakov, O.H. 118, Minchas Osher I: 33 and others).
Horav Shlomo Miller’s Shlit’a opinion is that the fact that the users body heat may lower the electricity use, especially when being outside in the cold, is not a necessary immediate fact (P’sik Reisha) and is also unintentional (Eino Mrchaven) and therefore permitted. (This being similar to entering a heated house or opening a fridge etc.).
Rabbi A. Bartfeld as advised by Horav Shlomo Miller and Horav Aharon Miller Shlit’a