the brick-bed in northern China
炕 refers specifically to a heated brick bed used in northern China, a traditional platform warmed by fire underneath.
炕 combines 火 (fire) with 亢, which likely contributes the sound. The character reflects a bed heated by fire, matching the meaning of a heated brick bed.
The left side is fire 火, and the right side 亢 looks like a raised platform. Imagine a fire burning under a raised brick bed to keep it warm.
For コウ, picture a cozy brick bed where you can comfortably 'co' (コウ) up with a blanket: co -> コウ, and the fire underneath keeps you warm.