炎
エン, ほのお, ぬく

inflammation, flame, blaze

JLPT1 G7 S8 F1242

火 fire (doubled) as an abbreviated form of 焱, 火 fire (tripled) → consuming blaze that encompasses objects in flames → *flame*. The phonetic evidence suggests that 焱 is itself a variant of a non-attested character indicating a great fire that consumes. Compare 焰.

[Full Kanji Etymology List]