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I have been trying to understand the issues with tankless water heaters on Shabbos and Yom Tov. (The simple description of how this works can be found here and here).

I found this answer on Mi Yodeya and the related teshuva from Star-K (which has moved to a new address recently).

I would like to confirm that I understand things correctly. What I understand right now is as follows:

  • Cold Water is not problematic on Shabbos or Yom Tov
  • Hot Water is a problem of heating directly on both Yom Tov and Shabbos (as opposed to a water heater with a tank, where the heating is a gramah and therefore permitted on Yom Tov but prohibited on Shabbos)

So, if someone has such a water heater installed in their house, there is no problem going there, for example, and using the cold water from a faucet?

In addition, the hybrid system including both tankless and with-tank units, which could be thought of as problematic for hot water on Yom Tov, would be an issue (from the linked Star-K teshuva) only if the sensor is extremely responsive and there is no delay between turning on the faucet and pouring the cold water through the tankless component, thereby heating it without a gramah. But even in this case, turning on cold water is not going to be a problem.

Please confirm or correct this, if possible. I would appreciate any sources dealing directly with the issue, if you know where to find them. Thank you very much.

  • I see two parts to your question: about cold water usage, and about hybrid tanks. The latter seems to be a duplicate of the question you linked to already. – Double AA Oct 26 '18 at 14:28
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    @DoubleAA neither one an independent question, I am just trying to verify if I have the correct understanding of the halacha. – gt6989b Oct 26 '18 at 14:31
  • I have heard of countries like Russia for example when hot water is a commodity and there is always hot water being pumped directly into the faucet which would mean one isn't heating any water and therefore may not be an issue even on shabbos. – Dude Oct 26 '18 at 15:16
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    Except with a common water tank more water is added as you use the hot water from the faucet causing more hot water to heat up. – Dude Oct 26 '18 at 18:13
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    There's the issue of "secondary" heating, when the cold water refilling the water tank is boiled by the hot water already present in the tank. To my best knowledge the only way to circumvent this is to have hot water below the halakhic definition of boiling water. – Kazi bácsi Oct 28 '18 at 7:56

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