indoor boilers require about 1/2to1/3 of the wood to heat a home that an outdoor boiler uses ,heated my home with one for 40yrs. list it on heating help .com classified
i my be wrong but it looks like that fitting would have a nut on each side locking it in place on the tank .you may have to go in through the sending unit to undo it. i would try to loosen the nut with out turning the pipe fitting.