It'd be somewhat custom, but there is a commercial heat exchanger (https://www.hotspotenergy.com/pool-heater/ https://www.hotspotenergy.com/pool-heater/) that a

It'd be somewhat custom, but there is a commercial heat exchanger (https://www.hotspotenergy.com/pool-heater/ https://www.hotspotenergy.com/pool-heater/) that an HVAC tech should be able to install