Synergy skills

[Kristal]

Pressure Handle lowers the pressure. Lower pressure means better chance at better results and less damage on explosions. (Not 100% sure about it, but that's how I see it.)
Safety Lever removes impurities. Overloading can only occur at or above 35% impurity, I believe, so lowering it when it gets close prevents overloads.

Repair Furnace indeed increases the health of the furnace more with each level, but you rarely need it.

Recycle is pretty much useless, since you cap cinders out very quickly, and the only thing to spend cinders on is carving evolith slots in low level gear.