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I was looking into buying a more efficient water heater, and was dismayed to discover the significant halachic issues surrounding tankless water heaters (https://www.star-k.org/articles/kashrus-kurrents/681/insights-from-the-institute-winter-2008/). Upon further research, I discovered that there are hybrid systems that use a normal storage tank but a tankless heater for rapid replenishment (http://www.rinnai.us/hybrid-tank-tankless-water-heater/how-it-works). Does this mitigate the halachic problems of tankless heaters on yom tov?

Additionally, shabbos 146a declares hatmanah as assur, using the example of a pipe of cold water inside hot springs (which would nominally have been permitted) as the source of the decree. What about the inverse case - where hot water pipes are circulated through a cold water tank?

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  • If you get a solar heated water tank (the standard here in Israel), then you can heat water on Shabbat that even reaches yad soledet bo, and shower on Yom Tov (cf. Yalkut Yosef 326:3, 326:19). Highly recommended.
    – Aryeh
    Apr 26, 2015 at 18:44
  • @aryeh an interesting option, but that's outside the scope of the question. (In my case, roof mounting a tank isn't an option, and I have no attic for standard electric pump mounting... Most american systems have active electric pumps, making use on shabbos problematic.)
    – Isaac Kotlicky
    Apr 26, 2015 at 19:20
  • Has anyone found a case in halacha where reverse hatmanah occurs? This is classically "tapig tzinasa" territory, like the throwing of rods into the mikvah mentioned by yom Kippur...
    – Isaac Kotlicky
    Apr 27, 2015 at 11:52
  • sorry this isn't an answer- but was wondering if you ever got any input on the halachic status of the hybrid system you noted? We are installing a new water system in our home and are looking for options that could allow hot water use on yom tov (and possibly even shabbos!) Thanks
    – darren
    Jul 28, 2015 at 23:43
  • @darren thanks for reminding me! I'll look for the email they sent...
    – Isaac Kotlicky
    Jul 29, 2015 at 10:56

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I received the following answer from the Star-K but it doesn't include detailed sources:

The way this system works is that, when you open a faucet, hot water exits the tank. Cold water enters the tank in order to replace the hot water which has exited. The system recognizes the change in temperature in the tank, and activates the tankless heater. Presumably, it takes at least a few seconds for all this to happen. Since there is a time delay, Rabbi Heinemann would consider the activation of tankless heater to be a gramma, which is permissible on Yom Tov. Therefore this should be fine to use on Yom Tov.

His only concern was that maybe the system is so efficient and the thermostat so sensitive that there is not a time delay of a few seconds before the tankless heater activates. He feels that this is unlikely, especially if the system is not set at the hottest setting.

Perhaps you could check with the installer (or manufacturer) that there is a delay of a few seconds between the opening of the faucet and the activation of the tankless heater.

Basically, the ruling (according to the Star-K) is directly dependent upon the sensitivity of the system to fluctuations in temperature - if it turns on immediately (or perhaps even if it preheats the inflow of water?) when the tap is used, then it would be problematic.

This need for grama would seem to apply especially when the (gas-based) tankless water heater doesn't have a pilot light, since any time it activates you would be starting a new fire without an existing flame.

I have not received an answer to the "counterflow" question of hatmana of chamin bitzonen.

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  • Isaac - did you ever get an answer from the manufacturer regarding the activation delay (if any)?
    – user13475
    Oct 7, 2016 at 4:56

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