it's not really financially worth it to try and store heat from one season to the next, you'll spend way more on the tank, getting it in position, plumbing it in and then super insulating it than you'll save on heating
you can super insulate your house for less cost and see more benefit
by my quick maths...
volume in litres x 4 x temperature rise in degrees centigrade / 3412 = kwh stored
20,000 litres, from 20'C to 90'C
20000x4x70/3412 = 1641kwh
at 4p/kwh (gas) = £65
at 16p/kwh (elec) = £262
even if prices double... it's a long pay back period... cash much better spent on insulation and solar power
Indeed. I did the maths too. I worked on 1800kwh, but near enough the same. Doesnt fly. Even at proper kwh prices. I think a lot of people would be happy at 16p. Even at triple that, it doesnt work. I dont have gas.
Where insulating becomes problematic is with an old solid wall house. Most of the insulation solutions will cause more problems then they solve. And can only be effictive up to a point. Which will be well short of a decent new build.
Shame. Free heating forever. If only the capital cost was sensible......................