noun
heat from a flame; fire heat
Refers specifically to heat produced by fire or flames, as opposed to general heat (熱). Often used in technical or descriptive contexts.
火熱で金属が溶けた。
The metal melted from the heat of the fire.
火熱を利用して料理をする。
Cook using the heat from the flame.
熱 is the general word for heat, while 火熱 specifically refers to heat from a flame or fire.
Compound of 火 (fire) and 熱 (heat). The exact historical derivation is uncertain, but the meaning is transparent from its components.