Friends:

What is the Heat in any Cluster of Form?

The Blessed Buddha once explained:
What is the cluster of Forms (rūpa-khandha)?

It is the 4 primary elements and all the forms derived from them...
Which are these 4 Primary Elements (mahā-bhūta)?

The 4 Primary Elements are:
1: The Element of Solidity... (pathavī-dhātu)
2: The Element of Fluidity... (āpo-dhātu)
3: The Element of Heat... (tejo-dhātu)
4: The Element of Motion... (vāyo-dhātu)
 

What, now, is the Element of Heat? The Element of Heat may be internal,
one's own, or it may be external. And what is one's own Element of Heat?
Whatever in one's own person or body, there is of kammically acquired
heat or hotness, such as that whereby one is heated, warmed up, metabolic
active, consumed, burning, scorched, that which digests, chemically breaks
and absorbs, what has been eaten, drunk, chewed, & tasted: this is called
one's own internal Element of Heat. Now, whether it be one's own Element
of Heat, or whether it be the external Element of Heat, as fires and suns,
they are both only the Element of Heat. One should therefore understand,
according to the utter reality & true Wisdom: This does not belong to me!
This I am not! This is not my self!

One may add:
This is not lasting, this is not a permanent property, this is not safe...
This is not pleasure, this is not happiness, this is also only suffering... 
In order to induce releasing disillusion with attractive warm objects...
Whether internal or external, whether alive or dead, present or not...
Clinging to any kind of Heat, whether internal or external, is Suffering...

Source (edited extract):
The Middle Length Sayings of the Buddha. Majjhima Nikāya. MN 28.
Mahā-hatthipadopama Sutta: The Great Elephant Footprint Simile:
Splendid Book: https://store.pariyatti.org/Middle-Length-Discourses-of-the-Buddha_p_1543.html
Text: https://www.accesstoinsight.org/tipitaka/mn/mn.028.than.html

Comment:
The 4 Primary Elements are not 'real substances out there', but more
realistically: Experienced qualities or properties appearing 'in here'...   
They are manifestations of form, and not lasting 'substantial entities'!
Their 'material' appearances depend on the level of observation:
Macroscopic Heat is a mere manifestation of microscopic vibration!
In all forms of materiality coexists all 4 primary elements in graduation.
In water, the element of fluidity is dominating at ambient temperature.
When frozen into ice, the solidity element becomes the dominant aspect.
In stone, the element of solidity is domination at ambient temperature.
When melted into lava, the fluidity element becomes the main feature...
Here one thus realizes, that it is the presence or absence of Heat defined
by temperature, that determines the relative degree of manifestation of

the two other elements: Solidity and Fluidity. All 4 are interdependent!

On these 4 Primary Elements (mahā-bhūta):
https://What-Buddha-Said.net/drops/IV/What_is_Solidity.htm
https://What-Buddha-Said.net/drops/IV/What_is_Fluidity.htm
https://What-Buddha-Said.net/drops/IV/What _is_Heat.htm

https://What-Buddha-Said.net/drops/IV/What_is_Motion.htm

Heat is an elemental property of materiality!