Q. The new gas water heaters come with a “Honeywell Vesta, self powered, self diagnostic gas valve. Alerts the user to the operating status of the heater.” This device is powered by the heat generated by the boiler and I believe it also has a backup battery that recharges from the heat. It includes a blinking light that blinks differently when the flame is on to boil the water. Can this be used on Yom Tov?
Thank you

A. From reading the available information on line on this product, it seems that the diagnostic and operating data the heater provides is activated by the thermostat reacting to the drop in temperature of the water only, as opposed to a drop in water pressure when you open the tap.
If that is correct, Horav Shlomo Miller’s Shlit’a opinion is that it is not different than any other water heater that people use on Yom Tov. Poskim (Horav S.Z. Auerbach zt”l – Meor Hashabos 1: p.509, Shemiras Shabbos Kehilchoso 2:7) are lenient for a number of reasons. The lowering of the water temperature by extracting the hot water, is not immediate (grama), nor a certainty, (psik – reisha). On a large tank, taking out a reduced amount of water may not turn on the gas flame at all, it happens by itself anyway, and one does not have intention for it (eino miskaven), besides some heaters have already an existing pilot flame on, therefore many are lenient and use the hot water on Yom Tov. Having extra light indicators or gauges react to the temperature of the water is then on no consequence.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a