As many others, couldn't really get the first level completed due to there seemingly being nothing else to heat and the door not opening.
At first I tried holding the space button, which turned out to be a mistake :P
You know already about the space thing, so no point in reiterating that, but the controls also coul've been a bit better. I'd suggest adding an ability to slow down (holding S) that would make the turning a lot better and obviously remove the space thing. Having the player heating the FLOWs at all times would be a much better thing to do, making the player focus on the movement and predicting the enemy flow instead of trying to find the right balance between heating up and having cramps :P
Great idea though, with some polish and more levels (with some obstacles maybe?) it could be a lot of fun!