Try searching l5 I don’t recall a specific thread but defo talked about however at 18 lpm will give you in theory 360 ltrs but as it returns to the tank it will mix with cold and you would lose all the heat quickly
We use immersion heaters and to get 650 ltrs of cold to a decent temp takes around 4 hrs overnight
I’m not sure you could raise the temp enough to be of use
Darran