Got an idea: What about a HU/t boost mechanic for Burning Boxes where you can “overclock”, boost up their heat power output (And the fuel consumption) via injecting either air, oxygen or KU power, just like if making steel?
The more KU power you put into the burning box, for example, the more powerful it becomes. Its efficiency would be kept the same and the boost would be more of a sort of multiplier, incentivizing the usage of more powerful burning boxes anyways.
I do not know if it should also add the small quirk of not being able to stop with the front face of the block blocked, as it now has another source of air, kind of. You would need to both cover the front and to stop the flow of KU or air into the box.