In Finland, majority of energy costs comes from heating, and majority of that from water heaters. And majority of hot water is used by the shower. So when water from the mains comes in at between +1 and +8 degrees and used water leaves at +37 degrees, recovering some of that heat ought to mean massive savings in energy bills.
I found this video about how to make such a heat exchanger yourself, so I'd like to know /diy/'s opinions on this: https://youtu.be/I4wO3tjoQpw?si=hK0rpD0h3Cc53POY
How long would the exchanger need to be? Should I do it myself or are there better commercial versions already? Any other thoughts or recommendations?