Chapter 23
Housekeeping and Use of Domestic Facilities on Shabbath and Yom Tov.
CENTRAL HEATING
26.a. On Shabbath, one may not place on a radiator anything to which the prohibition against cooking applies (as described in Chapter 1, paragraphs 5 to 11), if indeed it could reach 45 degrees centigrade.
26.b. This is prohibited
1) even if one intends to leave it there only for a short time in order to remove the chill and
2) even if the heating system is not operating at the time, but will be activated later (by a non-Jew or a timer).
27.a. Humidifiers (containers filled with water and intended to prevent the air in the room from becoming too dry) may be hung on radiators, provided (if it is Shabbath) that the water inside will not reach a temperature of 45 degrees centigrade.
27.b. The fact that the water will evaporate is in itself irrelevant.
