In D&D cosmology, Avernus is a layer of the Nine Hells which are part of the Outer Planes. These places don't necessarily follow physical laws, as the prime material plane/mortal plane does. Instead, they are shaped by the beliefs of mortals, the powers of the gods that live there and the other inhabitants, like fiends, celestials and so on. You have chasms of infinite depths, places where it's raining blood and deserts that drain your colours. I also wouldn't be surprised, if the overheating heart wasn't a flaw, but deliberate design by Zariel to secure her soldiers' loyalty. That said, I still think it would have been great to find Stygian or Canian (cold layers of the Nine Hells) ice somewhere and have Dammon and the Gondians finish the job.
