hearth
煁 centers on a hearth: the fireplace or cooking area where a fire is kept. It is a specific architectural or domestic feature, not a general fire concept.
煁 combines 火 (fire) with 甚, which likely contributes the sound. The character is used for a hearth or cooking stove, but the detailed historical development of 甚 in this context is uncertain.
The left side is fire 火, and the right side 甚 looks like a sturdy container. Picture a strong, deep fire pit or stove: that is the hearth.
For シン, imagine a shiny new hearth being installed in a traditional kitchen: shiny -> シン, and the fire 火 warms the room.