[alexpotato]

This does exist btw.

It's a stainless steel coil that you can put on your A/C and then run water from your pool over it to heat the pool.

https://www.youtube.com/watch?v=J7fB8ul9dZw

[ryukoposting]

The sticker on the side of the doohickey in the video really gives the whole thing a feeling of "a dude makes these by hand in his garage." I looked up "AC pool heat exchanger" and lo and behold, the same company showed up as the first result:

https://www.hotspotenergy.com/titanium-pool-heat-exchangers/