Holy crap that re-circulation feature is luxurious!
I'd love to have something like that!
Whenever I take a hot shower I fantasize about putting in a liquid-to-liquid heat exchanger so the hot water can heat up the cold water and you can run the hot water at a much lower temperature.
This application with the bath tub feels way more justified.
That does exist. Simple versions just have a large, deep drain with coils the cold water runs through. Fancier ones have a proper heat exchanger next to the shower (requires a pump) or in the basement. Water sanitation rules probably make it non trivial to get a design certified.
Many people have thought about the drain heat exchanger. However drains need to be way oversized for various reasons and so you can't get good contact with the water: it is mostly air in the drain pipes.
I'd love to have something like that!
Whenever I take a hot shower I fantasize about putting in a liquid-to-liquid heat exchanger so the hot water can heat up the cold water and you can run the hot water at a much lower temperature.
This application with the bath tub feels way more justified.