10. Opening and closing the hot water of a sink:

A. May one open the hot water tab on Shabbos?[1]

So long as the water in the boiler is still Yad Soledes, then even if the boiler is no longer on, it is forbidden to open the tab, as turning on the hot water automatically enters cold water into the boiler.[2] [Furthermore those which have a type of boiler which automatically heats the water as it is turned on, are forbidden to do so, as in addition to cooking the newly entered cold water, they are also lighting a flame.]

B. May one open the hot water if one knows for certain that the water in the boiler is no longer Yad Soledes?[3]

This is allowed only if one knows for certain that the heating system will not turn on later on Shabbos and heat the new cold water added into the boiler. Regarding a solar heated boiler in which the sun will heat the new water on Shabbos, then from the letter of the law if the water is no longer Yad Soledes, one may open the faucet, although it is proper to avoid doing so.

C. May one ask a gentile to open the hot water tab?[4]

• Example: The Mikveh water is too cold for immersion; may one ask a gentile to open the hot water?

Yes, one may ask a gentile to do so [even if the heating system of the boiler is turned on in the process].[5]

D. May one open the hot water if he closed the pipes, thus not allowing any cold water to enter into the boiler?[6]

Yes, this is allowed even if the water is Yad Soledes as no water can enter it. [However, one must take care not to let the water hit the sink unless the sink is dry, as otherwise it will cook the water remaining in the sink which is forbidden.]

E. May one leave the hot faucet open from Erev Shabbos?[7]

Yes [although he may not use that water to wash with if it has been warmed up on Shabbos[8]]

F. May one further open the faucet on Shabbos when left open before Shabbos?[9]

No.

G. May one close a faucet of hot water that was left open before Shabbos or was accidentally opened on Shabbos?[10]

No, this may not be done unless a) The heating system is not on [and will not turn on at a later point], and b) there is no hot water left in the boiler. However, if there are only a few drops coming out from the faucet and one wants to close it to save those drops then seemingly it is allowed in all cases that doing so will not cause the heating system to turn off.

H. May one ask a gentile to close the hot water faucet that was left open?[11]

Yes, this is allowed even in a case that doing so will shut off the heating system.

I. If the hot water tab is open may one open the cold water in order to cool it down?[12]

This is only allowed if he opens enough cold water that when it mixes with the hot water it will not reach Yad Soledes.

^[1] Ketzos Hashulchan 124 footnote 27

[2] As by doing so cold water will be caused to enter into the boiler, and the boiler is defined as a Keli Rishon.

^[3] Shabbos Kehalacha Vol. 1 p. 129

^[4] Shabbos Kehalacha Vol. 1 p. 70

[5] As the adding of the hot water into cold water is allowed, as will be explained in “Iruiy Keli Rishon”, and the adding of the cold water to the boiler is a mere Pesik Reisha which the gentile has no intent to do which is permitted to be done by a gentile, as explained in chapter 253.

^[6] Shabbos Kehalacha Vol. 1 p. 129

^[7] Shabbos Kehalacha Vol. 1 p. 127

^[8] Admur 326:4

^[9] Shabbos Kehalacha Vol. 1 p. 127

^[10] Shabbos Kehalacha Vol. 1 p. 127

^[11] Shabbos Kehalacha Vol. 1 p. 128

^[12] Shabbos Kehalacha Vol. 1 p. 128