Engströms also gives the suggestion of "firebox". I have looked at this term in the context of boilers, and it does seem to be a component of the boiler itself (see my link below).

I think the term has transferred from the context of steam locomotive boilers, i.e. the firebox was where the coal was inserted to heat the water in the boiler and turn it into steam for power. It now appears that "firebox" can be used more generically as the component of the boiler where any sort of fuel is combusted so as to heat water for heating/power generation.

I hope this might be of some help.