So my suggestion is to modify the furnace recipe to use the previous tier furnace.
I see two set of recipe changes without modifying the overall costs:
- without bricks being used:
- stone furnace: 5 stone
- steel furnace: 4 stone furnace, 6 steel
- electric furnace: 1 steel furnace, 4 steel, 5 red circuits
- with bricks, but slightly more complicated steel furnace:
- stone furnace: 5 stone
- steel furnace: 2 stone furnace, 5 bricks, 6 steel
- electric furnace: 1 steel furnace, 4 steel, 5 red circuits
- stone furnace: 3 bricks (2 bricks)
- steel furnace: 4 stone furnace, 6 steel
- electric furnace: 1 steel furnace, 4 steel, 5 red circuits