Heat is produced and leaves the stream of fire at the moment of its burning every moment, therefore we cannot consider heat as part of the essence of fire, just as we cannot consider sweat, tears, skin dust, exhalation, other waste products as our own body and our Self, our sanatana.
These are simply waste products of matter into the external world, which ceases to be a continuation of our form, our formation, ceases to be internal, captured by kamma and becomes external. In the same way, fire throws out waste products of “life” in the form of smoke, heat, light and ash. But when it begins to die out, it reduces the emission, when it dies out, it stops emitting. It is not that, having ceased to burn, it leaves behind a certain main emission that represents it, but precisely at the moment of burning it throws out, and at the moment of dying out, it stops throwing out. The heat after the fire is the remnants of the emission that it threw out at the moments of burning before, and they continue to sound like an echo when the source has already fallen silent.
By such reasoning we understand that in fact fire has no essence on its side, which would remain, exist after it. Fire is empty of essence, it is only an effect from the combination of conditions. As soon as the conditions cease - it stops arising, as if it had never existed at all. Emissions also stop escaping into the external environment. Fire is not some substance, but a specific form of a process.
If you destroy a beautiful painting, its form will disappear forever, as if it had never existed, it will disappear without a trace, but the substance of the painting will simply pass into the environment. A painting is not reduced to a substance, a painting is only a form of a substance, information recorded on it. If you dig deeper, then substance is also such forms and manifestations, created by a combination of causes and conditions, like fire from the example or a painting recorded on canvas. Atoms and molecules are only forms/formations, conditioned processes, etc. They will disappear as soon as the conditions change, as if they had never existed, leaving no trace of some essence.