Nice use of the theme. The dual heating/cooling system is an interesting idea that deserves more exploration. Controls are fairly smooth. The layouts of the enemy waves were clever, and surprised me more than once. I grinned to myself when I got to HARD MODE. :)
A few suggestions:
I would make the heating/cooling setting (hot vs cold) visible even when the bar is at zero. I expect that the current setup is intentional, but I still found it a bit confusing.
The flamethrower should display a stream of fire originating from the player. It took me a couple of plays to realize that it was me damaging the opponents. :)
The music gets grating after the first few seconds.