火 (fire) anchors the meaning: a 炉 is a furnace, stove or brazier - anything that contains and channels fire. The radical groups 炉 with 烧 (burn), 烤 (roast), 烟 (smoke). Visible flame, visible meaning.
户 (hù) supplies the sound, drifted to lú. This is the modern simplified phonetic - traditional 爐 used the much heavier 盧. The reform swapped in 户 as a clean four-stroke stand-in, sacrificing the precise sound match for shape simplicity.
