Chapter 23
Housekeeping and Use of Domestic Facilities on Shabbath and Yom Tov.
CENTRAL HEATING
18.a. 1) On Shabbath, it is forbidden to turn on the taps of central-heating radiators or of the pipes which conduct hot water from the boiler to the radiators.
2) This is prohibited because the cold water in the pipes and in the radiators would, as a result, flow into the boiler, where it would be heated. (Compare Chapter 1, paragraph 39a.)
3) The taps must not be turned on even if the heating system is not yet operating but will be turned on during Shabbath, for example by means of a timer (time-switch) or by a non-Jew. (Regarding the latter, see paragraph 24 below.)
18.b. On Yom Tov, these taps may be turned on even when the heating system is operating, and it is the custom to permit this even if the weather is not particularly cold.
19.a. On both Shabbath and Yom Tov, one is allowed to turn off the taps of central-heating radiators and of the pipes which conduct hot water from the boiler to the radiators.
19.b. 1) However, on Shabbath it is proper to refrain from doing so as long as the water inside the radiators or inside these pipes is in the process of being heated and has not yet reached a temperature of 45 degrees centigrade (113 degrees Fahrenheit).
2) This is because turning off the taps causes the water in the boiler to reach a temperature of 45 degrees centigrade more quickly than it otherwise would.
