So I managed to build the seared furnace, and it's working normally, until I remembered that I need to also place drains if I wanna use it, but no matter where I place the seared drain in the furnace it stops working and the fire goes out.

Versions:

• Minecraft: 1.12.2
• Forge: 1.12.2 -14.23.5.2847
• Mantle: 1.3.3.55
• Tinkers Construct: 2.13.0.183

Your issue description explains your problem exactly, it stops working without drains. Why are you placing a drain in the furnace? It does not handle fluids, just items. You are likely confusing it with the smeltery which uses drains.