Back to the boiler for the time being -thanks to Mr Pete Vill, i now know that what i took for an empty space between the boiler itself and the steel plates aboves was in fact covered with ropes and then with plaster, so that it could do some sort of isolation layer and therefore keeping the energy inside the boiler so there it goes: first unglue the steel plates (*sigh*, the lenghths you have to go to be accurate.. ;)