An alternative mechanic would be to treat heat like any other kind of power transmission system, possesing both a “quality” and a “quantity”.
In electricity, those values are the Voltage and the Current, respectively.
With pneumatics or hydraulics, even if they don’t exist in Gregtech, would be the Pressure and Flow (Amount of fluid/s).
With rotational power, those are the speed and torque. I do not know if in that order tho.
And now with heat you would have to manage its temperature and its HU/s. So now Burning Boxes have a maximum temperature they can deliver that could or could not depend on the type of fuel being fed and the material of the box itself, along with their HU/t that determines how fast can the box heat up a given mass.
For example, a low temperature burning box with high “heat current” could bring a crucible full to the brim with stuff to that maximum heat quite fast, albeit would be limited to stuff like melting meat or lead.
A High temperature burner with low heat current, on the other hand, could struggle with heating the empty crucible itself, but after a while it would eventually melt it down if not stopped.
Would be cool enough or way too complex to be worth it?