I like the idea of heating the water up and circulating it around to get the desired temp.
I personally wouldnt have all them drums to much faffing around.each to their own though.
How long would it take to heat a 500 up and how much gas etc would that use.at present i have a immersion heater and was trying to compare heating prices with you.
How much battery usage would it use trying to circulate the water heating in a 500.
Nice setup if it works for you

Merry xmas