This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
炆 combines 火 (fire) with 文, which likely contributes the sound. The exact historical development is uncertain.
The left side is fire 火 and the right side is writing or pattern 文. Picture a fire that writes patterns of warmth in the air, but remember its standalone meaning is unclear.
For ブン, imagine a bun warming by the fire: bun -> ブン, and the fire 火 gives it a gentle warmth.