fireplace, stove, oven, furnace
爐 centers on an enclosed space for fire: a fireplace, stove, oven, or furnace. The common thread is a contained heating structure used for warmth or cooking.
爐 combines 火 (fire) with 盧, which likely contributes the sound and may have originally referred to a type of vessel or container. Together they form a fire-containing structure.
The left side 火 is fire, and the right side 盧 looks like a complex container with a base (皿) and a top structure. Picture a fire burning inside an elaborate stove or furnace.
For ロ, imagine a low fire burning in the fireplace: low -> ロ, and the flames are contained inside the 爐.