This root was connected to “heat” for most of Tolkien’s life. It first appeared as unglossed ᴱ√URU in the Qenya Lexicon of the 1910s, with derivatives like ᴱQ. úrin “(blazing) hot”, ᴱQ. uru “fire”, G. urin “hot, very hot”, and G. urna- “blaze, burn” (QL/98; GL/75). In this early period it was blended with the root ᴱ√UŘU with variant ᴱ√USU which had derivatives like ᴱQ. urna “oven” and ᴱQ. usta- “burn” (QL/98). This second root had derivatives in the contemporaneous Gnomish Lexicon such as G. gund “boil, pus” and gusta- “burn, destroy” from primitive ✱gudh (GL/42), indicating the actual root form was ✱ᴱ√ƷUÐU.
✱ᴱ√ƷUÐU seems to have been abandoned in Tolkien’s later writings, but ᴹ√UR “be hot” appeared in The Etymologies of the 1930s with derivatives like ᴹQ. úr/N. ûr “fire” and ᴹQ. urya- “blaze” (Ety/UR). This entry was deleted and replaced by ᴹ√UR “wide, large, great”. This was likely a transient change, however, as ᴹQ. Úrin as a name of the Sun remained in contemporaneous Silmarillion drafts (LR/240). Indeed primitive √UR appeared several times in later writings as the basis for words for “sunlight”, “fire” and “heat” (PE17/120, 148; PE21/71; PE22/160).