I like the idea of using a wrench for different things. It works pretty well and swinging the wrench looks and sounds really good.
The idea of moving to the back of the level and than having to go back to the start under time pressure is pretty interesting. What I did not like was that when the heater is activated that you change parts of the level. I get why you do this but I think it would feel much better if the player is causing the change in the level somehow so at least you know why the level is different. Or just shake the screen and simulate and earthquake that explains why the path is different on your way back :)
Overall a nice demo that could turn into an interesting puzzle platformer.