heat, temperature, fever, mania, passion
熱 centers on intense heat, whether physical (high temperature, fever) or emotional (passion, mania). The common thread is a state of high energy or excitement.
熱 combines 埶 (a phonetic component with a sense of planting or setting) and 灬 (fire), suggesting heat or warmth. The modern form is consistent with this phono-semantic structure.
The top part 埶 looks like a person kneeling over a mound of earth, and the bottom 灬 is fire. Imagine someone tending a fire, feeling the intense heat rise—enough to cause a fever or passionate excitement.
For ネツ, picture a net catching all the heat from the fire: net -> ネツ, and the net traps the intense warmth.
heat
hot (to the touch)
passion; enthusiasm; zeal
zealous; enthusiastic; ardent; fervent; keen; eager
being enthusiastic about; being wild about; being absorbed in; being engrossed in; being devoted to
heating; application of heat
wild enthusiasm; being crazy about
zeal; enthusiasm; ardor; ardour
generation of heat
tropics
white heat; incandescence; (emitting) white light; white glow
ardent; passionate; vehement
high fever
insulation
heat; hot air
boiling water
heat-resisting
to heat; to heat up
piping hot; scalding
overheating; excessive heating
tropical fish
slight fever
impassioned speech; fervent speech
fierce fight; close contest