From Old English 'hǣtu,' the noun form of 'hot' — from Proto-Germanic *haitaz.
The quality of being hot; high temperature.
From Old English 'hǣtu' (heat, warmth), from Proto-Germanic *haitį̄, from *haitaz (hot). The noun form of the adjective 'hot.' Key roots: *haitaz (Proto-Germanic: "hot").
The vowels in 'heat' and 'hot' look different but represent the same root at different stages — separated by the Great Vowel Shift.