In order to properly analyze and understand this question, we need to review a few important general concepts about what is allowed or not allowed to be done on Yom Tov, as well as to understand how a tankless water heater works.
Heating Water on Yom Tov via fire
Generally speaking, one is allowed to heat water on Yom Tov, provided that they do not create a new fire in order to do so. The fire used in the heating of the water, needs to be either lit from before Yom Tov, or ignited off a preexisting flame that was lit before Yom Tov. It’s important to note, that the majority of halachic authorities consider the prohibition of creating a new fire on Yom Tov not to be a Biblical prohibition of ma’avir (igniting), but rather a Rabbinic one, under the category of nolad (generating or creating). This important distinction will be relevant later on.
Grama vs Psik Reisha
Another important distinction one needs to be able to differentiate between, is that of a Grama versus a Psik Reisha. Both these concepts are incidental actions, i.e. you are not doing the prohibited act directly, but rather it is happening as a result of something else. However, a grama means that the incidental causation may or may not happen or it is not instantaneous. A Psik Reisha, on the other hand, is an inevitable and unavoidable outcome of your action, and happens almost instantaneously.
For example, if a fire is spreading and you put jugs of water in the path of the fire, that can later burst and put out the fire, that is a grama. However, if for example, you drag a very heavy bench on the ground, that is so heavy, it inevitably and unavoidably would leave a groove in the ground (an offshoot of plowing), that is a psik reisha.
Generally speaking, a grama is (Rabbinically) forbidden on Shabbos (except for a few narrowly tailored scenarios). However, it is permitted on Yom Tov (and most certainly when it’s for a needed activity). A psik reisha, on the other hand, is forbidden on both Shabbos and Yom Tov, whether it is a psik reisha of a Biblical or Rabbinic prohibition.
[Editor’s Note: there are other categories such as: melacha sheino tzricha l’gufo, dovor she’eino miskaven, psik reisha delo nicha lei and psik reisha d’lo ichpes lei, that are outside the scope of this article, but should be reviewed in detail as it pertains to grama and psik reisha in other matters.]
How does a Tankless Water Heater Work?
Tankless Water Heaters (“tankless water heater”) were created for the purpose of providing a steady supply of hot water throughout a house. When a hot water faucet is turned on, the flow sensor in the tankless water heater immediately senses the flow of water and sends a message to the motherboard that the incoming thermistor should take the temperature reading of the incoming water. If it detects that the incoming water is below the required set temperature of hot water (usually about 120°F) then it immediately fires up the gas burner, as well as the air fan, so that the water flowing through the coils will heat up very quickly. There is another outgoing thermistor that measures the outgoing water temperature to ensure that a consistent fixed temperature of water is exiting the tankless water heater and making its way to the faucet.
Is a tankless water heater a Grama or Psik Reisha?
Being that as a soon as there is water flow, there is an instantaneous reaction of the flow meter (which is unavoidable), namely, that it immediately starts sending a signal to the motherboard to start the temperature sensing and to fire up the fan and gas burner (which is also inevitable - since the incoming water from the water line will obviously be cold), the inevitable and unavoidable action of starting up the tankless water heater electronic mechanism, as well as starting a fire anew, is a definite and immediate psik reisha which is forbidden even on Yom Tov.
Recirculation Line / pump
There is one major flaw with the standard tankless water heater system. Namely, that it only heats water on an ‘as needed’ basis, once it detects water flow. However, what if your tankless water heater is in your basement and you are now trying to take a shower on the second story of your house? All the water that is in the water line between the tankless water heater and your faucet, will need to drain out before some hot water finally makes its way to your required outlet! Not only does that waste a lot of time, it also wastes a lot of water on an ongoing basis.
tankless water heater with no recirculation line
Thankfully, there is a rather simple solution for this issue: installing a recirculation line with a circulation pump. This recirculation line goes throughout the house and creates a loop where water is constantly flowing around and around through the tankless water heater (thereby keeping the flow sensor always working). Whenever it detects that the water temperature in the recirculation line drops below the desired hot water setting, only then does the fan and gas burner kick on and heat the water back up to the desired temperature. This ensures that there is always hot water at, or near, your water faucets, thereby enabling you to access hot water quickly and efficiently.
tankless water heater with recirculation line/ pump
Halachic Ramification of Recirculation Line
Installing a recirculation line has a dramatic impact on whether the tankless water heater can be used on Yom Tov or not.
First of all, since the water is constantly circulating, turning on the hot water faucet does not trigger the flow sensor, since it is already always on. Second of all, being that the system is itself constantly circulating the water and ensuring that it stays hot in the pipe, by turning on the hot water, all you are doing is syphoning off some hot water from the recirculation line, which then causes some cold water to be reintroduced into the recirculation line downstream, which will then be heated up by the system. All of this is not instantaneous and is therefore classified as a grama, which is permissible on Yom Tov.
Even if one wanted to argue that perhaps the gas burner was off when you turned on the hot water faucet and the whole process of the gas burner going on is very quick and perhaps an immediate psik reisha, that too is not an issue. As stated above, the prohibition of igniting a new fire on Yom Tov is only Rabbinic in nature. Since every time you open that faucet, the gas burner may already be on (via the recirculation line/ pump) or not, worst case scenario, we are talking about a sofek psik reisha (perhaps it’s on already, perhaps it’s not). The halacha is that a sofek psik reisha is permitted, especially in the case of a Rabbinic prohibition.
IMPORTANT CAVEAT
It is important to note that many recirculation line pumps come with a timer. This means that you can either turn it on, off, or have it work on a timer for a set amount of hours. Often people set the recirculation pump to go off late at night and go back on an hour or so before people in the house would start using the hot water. Obviously, if the circulation pump is off, the tankless water heater would not be allowed to be used for all the reasons mentioned above. Since one does not control when people in one’s household may get up and use hot water, one must absolutely make sure that the timer is set to ‘ON’ at all times on Yom Tov, to avoid any accidental violation of Yom Tov, chas v’shalom.
Conclusion
While a tankless water heater, as is, may not be used on Yom Tov because of the issue of a psik reisha, if one has a recirculation line and pump installed and ensures that it is on throughout Yom Tov, there is no halachic issue with using a tankless water heater throughout Yom Tov.
[Editor’s Note: this article was reviewed by several Rabbonim and Poskim, who concurred with the halachic premises and conclusions contained in this article.]