If a person keeps on meditating, then a lot of heat will be generated from his/her body. Hiranyakashyapa (or some demon) was meditating then all God's couldn't bear the heat and rushed to Vishnu.
Is heat generating from the body related to kundalini? Or related to highest chakra?
Yes, continuous meditation generates an inner heat if the Manipura Chakra of the Yogi becomes active. The Manipura Chakra which is located in the spinal center of the region of the navel is the seat of the Tejas tattva (fire). An active Manipura Chakra would indeed generate an inner heat due to the Tejas tattva. The best reference for this type of esoteric knowledge is the book 'Serpent Power' by Arthur Avalon (Sir John Woodroffe).
There are three types of tapas in a human being , when we meditate with a goal to be liberated from wordly matters we are actually removing all three gunas (satva,rajas,tamas) and become nirguna, to attain the divine position in lotus feet of lord.
In bhagwat-mahapuran one shloka describes shri krishna as
Sacchidananda rupaya jagadutpayadi hetave tapatraya vinashaya shri krishnaya vayam namah.
tapa+traya=three tapas
The tapa means heat and that's exactly what we libreate when we meditate and do hari kirtan as described in shloka.When we see sacchidananda swaroop of lord krishna who is reason for creation and destruction of this world , we burn our karmas and release three type of tapas and then we see lord in a nirguna avastha.
Refrence for tapas and it's types.
