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?