hearth, fireplace, furnace
鈩 centers on a contained space for fire: a hearth, fireplace, or furnace where heat is held and used.
鈩 combines 金 (metal) with 户 (door/household), suggesting a metal structure for containing fire, like a hearth or furnace. The exact historical development is uncertain.
Metal 金 forms the frame, and 户 suggests a household enclosure. Picture a sturdy metal hearth or fireplace built into a home.
For ロ, imagine a low, glowing fire in the hearth: low -> ロ, and the metal hearth contains the low flames.