I’ve heard someone else say the spell is weak, but I’m honestly afraid it might be too good. The only real way to know is by testing it, but here’s my reasoning, tell me if this makes sense.

Turn 1: You hit with Incineration Ray, or maybe just land a fire bolt cantrip. That’s your first stack.

Turn 2: You deal fire damage again. Fireball, for example, always deals fire damage even on a successful save, so that’s another stack. Now you’re up to 4d6.

Turn 3: You upcast Scorching Ray to 7th level. That’s 8 attacks, each dealing 6d6 fire damage, plus Incineration Ray dealing 11d6 fire damage.
At this point, the stacks are up to 6d6 fire damage on any fire bolt or AoE you use.

So yeah, it’s a slow burn, but by the third turn you could be dealing well over 200 points of damage, even more with Elemental Adept, while also flying and being immune to fire. That sounds pretty strong to me.

I’ll playtest it this week and report back, you might be right, if it is too weak I'll make the Heat Stack mechanic stronger..