What is The Problem Using Hot Water on Shabbat?

Turning the hot water on in a faucet on the Sabbath is prohibited. This is because it causes hot water to be released from the water heater tank that supplies the hot water in a home. When hot water is released from the water heater tank, new fresh cold water immediately enters the tank. This new cold water instantly comes in contact with the hot water already in the tank, and becomes hot as well. The process of causing new cold water to become hot by causing it to enter the tank of hot water is a form of “cooking”, and is PROHIBITED.

One handle faucet carries the issue of “cooking”. This is because the cold water becomes “cooked” on contact with the hot water as they merge in the pipes and within the faucet.

The hot water in the faucet must not pour directly onto dirty or greasy dishes. This is because the water coming from the tap must be regarded as pouring directly from the water heater, which is prohibited on the Sabbath.


Some people switch off the hot water heater on Friday afternoon, thinking that this makes using the hot water permissible on The Sabbath. However this is a serious error because switching off the heater does not prevent fresh water from entering into the tank as the hot water is turned on in the faucet.
The ShabHOT™-Mode control system regulates the temperature accurately; this in turn allows you to keep the temperature at a permissible point on Shabbat. The ShabHOT™-Mode system while operating on S-mode has a patented system that uses a double random control process, thus removing the operation of the system from being a direct action of hot water usage.
אגרות משה (יורה דעה ב’ לג’) כותב:

“ורק מי שיש לו בית לעצמו יכול לכבות את האש ערב שבת כשתי שעות קודם השבת שיפסקו מלהיות רותחין שיותר קרוב שלא יתחממו המים קרים הנכנסים לשם במידה דהיד סולדת ויהיה מותר, או כשיוכל לתקן שלא יכנסו הקרים ביום השבת יהיה מותר.”

“And only who has a house of his own able to turn off the fire before Friday sunset about two hours prior to the Sabbath so it would stop from being hot being better it would not heat up the cold water coming in at the point that the hand springs and it will be permissible, or when he would fix that no cold water enters on the day of the Sabbath it will be permissible.”

פסקי תשובות (הלכות שבת שכ”ו א’) כותב:

“והנותן מים חמים שהוחמו מערב שבת לתוך מים צוננים בשבת על מנת לחממם (בדרך היתר דהיינו שלא יהיו הצוננים מתחממים לשיעור יד סולדת בו או ששופך עליהם מכלי שני), לא מיקרי חמין שהוחמו בשבת ומותר לרחוץ בהם פניו ידיו ורגליו.”

“And the one who adds hot water which was heated before sunset into cold water on the Sabbath in order to heat it (in a permissible way meaning that the cold water would not get hot to the point that the hand springs or pouring on it from a second vessel), we do not call it as being heated on the Sabbath and it is permissible to wash with it his face hands and legs.”