18. When it is very cold, it is permitted to ask a Gentile on Shabbos to
light a furnace, because, in the face of cold, [the Sages] gave everyone
the halachic status of a sick person. If, however, [having the heat] is
not so essential, one should not [ask a Gentile to perform the act]
(1). Also, one should not allow a Gentile to light a furnace on Shabbos
afternoon so that the room will be warm by Saturday night.
(1) It is difficult to define when an increase of heat is considered
essential. If someone might become ill due to the cold, it is permitted to
ask a Gentile to turn on the heat or to increase the temperature setting.
Similarly, if an air-conditioner is making the house too cold, and as a
result someone might become ill, one may ask a Gentile to turn it down.