Symbol 3397 from ISO 7000 - Title/Meaning: Temperature, high Function/description: To identify the high temperature position of a control. To indicate that the temperature is at a high level.
pay attention to orientation of symbolAdditional information: 
Colour may be red to signify warm or hot temperatures.
Own work using: iso.org/obp/graphics/grs/091d2315-053f-442c-a904-28757267f0a3 200.png
Vectorization:  Mrmw