Heat is a base aspect of the Ash Fysa which governs heat and flame. All chimia start with either this fysa, Flow, Light, or Land.
This is an active fysa. Users can sense heat more keenly. Its users can 'see' heat vision instead of light vision, though their vision is obscured by barriers which prevent heat waves from coming through.
A user is normally heat resistant, less likely to be burned by fire and other heat sources (they can disperse the heat right back into the air). They are good at regulating their body's temperature, able to hold onto heat for longer or disperse it faster.
One can manipulate fire, though it's hard to keep it from burning things and it will snuff out without fuel.