Codex Processing Tunnel Furnace

Tunnel Furnace

The Tunnel Furnace smelts in bulk. Items ride on trays through a long tunnel of Refractory Bricks and come out smelted at the other end. It needs no electricity at all: it runs only on process heat that comes in through its Heat Ports. The longer you build it, the more it smelts at once.

How it works

1. Items come in through the Item Input Port (or by hand) into the four input slots.
2. The furnace loads a tray with up to 4 items of one kind and sets it on the conveyor at the entry end.
3. Every tray takes 100 ticks (5 seconds) to ride through the tunnel. A new tray follows as soon as the last one has moved on far enough, so a full tunnel carries one tray per block of tunnel.
4. At the exit end the smelted items go into the eight output slots, from where pipes take them through the Item Output Port.

Without enough heat, or with full outputs, the whole line stands still. Nothing is lost: the trays simply wait.

Building

Shown size: 9 wide, 3 high, 3 deep (adjustable: Length 5–17).

One example of many: within the rules on this page, size and arrangement are up to you; the Build Projector shows the plan in every allowed size.

	Key	Block	Count
	A	Refractory Bricks	62
	D	Conveyor	7
	B	Industrial Sight Glass	6
	F	Heat Port	3
	C	Item Input Port	1
	E	Item Output Port	1
	0	Tunnel Furnace Controller	1

Step by step

Each picture adds one layer, from the bottom up; the newest layer is bright. The controller wall faces you on the left.

Step 1: Layer 1

Parts in this step: Refractory Bricks (A) ×27

Step 2: Layer 2

Parts in this step: Refractory Bricks (A) ×8, Conveyor (D) ×7, Industrial Sight Glass (B) ×6, Heat Port (F) ×3, Item Input Port (C) ×1, Item Output Port (E) ×1, Tunnel Furnace Controller (0) ×1

Step 3: Layer 3

Parts in this step: Refractory Bricks (A) ×27

Rules

• Size: 5 to 17 long, always 3 high and 3 deep. The length runs along the wall with the controller. The furnace finds its length by itself, starting from the block behind the controller.
• Edges and corners: only Refractory Bricks.
• Walls: Refractory Bricks, Industrial Sight Glass, ports and the controller.
• Inside: the tunnel is one block wide and one block high. Every block of it must be a Conveyor; air is not allowed.
• Heat Ports: up to 5.
• Controller: in the middle row of one of the two long walls, front facing out, with the tunnel directly behind it.
• Ports: Item Input Port (items in), Item Output Port (smelted items out) and Heat Port (process heat in). There is no Energy Input Hatch. Every port works only on its outer face.
• Direction: the line runs away from the end that is nearer to the first Item Input Port. Put the input port near one end and the output port near the other; the conveyors turn by themselves.

Tip

Data packs A data pack can add blocks to the tags qnet:multiblock/refractory and qnet:multiblock/industrial_sight_glass; they then count as bricks or glass.

What it smelts

Every smelting recipe the Induction Furnace knows with exactly one input item and one sure result: the furnace recipes of the game and of every other mod. Alloys (two inputs) and recipes with several results or with a chance are left out; the Induction Furnace or the Foundry make those.

Each tray carries one kind of item. The furnace fills the next tray from the first input slot that holds something smeltable.

Heat

• Each item needs 3,000 HU of process heat, spent evenly while it rides: 30 HU per item and tick. A tray that waits at the exit needs none.
• A full tunnel uses 30 HU × 4 items × the number of tunnel blocks per tick. The example (length 9, 7 tunnel blocks) needs 840 heat units per tick when it is full.
• The heat store holds one second of the full use. Heat sources only fill it as far as there is room.
• When the store holds less than the trays need this tick, every tray stops and the status line says so. The furnace never smelts with half the heat.

Heat sources must touch a Heat Port from outside:

• An Electric Heater gives up to 400 HU per tick out of its front (from electricity, 0.9 HU per E).
• A Geothermal Heat Exchanger gives up to 500 HU per tick deep down (600 in the Nether).
• Plants with heat outputs, such as the Molten-Salt Store, push heat into the Heat Port next to their own heat output.

Important

Enough heat for the length The example and the Build Projector place as many Heat Ports as Electric Heaters need for a full tunnel: 3 for length 9 (1,200 for 840 heat units per tick), four from length 13 and five from length 16 (2,000 for 1,800 heat units per tick at length 17).

Throughput

A pass takes 100 ticks at every length; the length only sets how many trays ride at once. The tunnel holds one tray per tunnel block (the length minus the two end walls), and a new tray starts after the pass time divided by the number of tunnel blocks (rounded up). The example starts a tray every 0.75 seconds, about 27 items every 5 seconds; the longest tunnel starts one every 7 ticks, about 57 items every 5 seconds.

There are no upgrades or modules for the Tunnel Furnace: length is the only way to make it faster.

The screen

• A bar for the process heat in the store (HU).
• Heat use: what the trays used in the last tick (HU/t).
• Trays and Places: how many trays ride now and how many fit.
• Items inside: how many items are on the trays.
• 4 input slots, eight output slots and the status line.

What the status line says

Status	Meaning
Running	The trays are moving.
Waiting for heat (HEAT ports)	The heat store holds less than the trays need; add heat sources.
Output full – take the products out	A finished tray does not fit into the outputs; the line waits.
Idle – waiting for inputs	The tunnel is empty and nothing smeltable is in the inputs.

Automation

• Pipes put items in through the Item Input Port; only items with a fitting smelting recipe are accepted.
• Items can only be taken out of the output slots, through either item port.
• A Grid Monitor can pause the furnace; while paused the trays stand still.

Error messages

Message	What to do
The controller must sit in a wall, front facing out	Put the controller into the middle row of a long wall, front outwards, with the tunnel behind it.
Structure too small (…)	Build at least 5 long, 3 high and 3 deep. A controller in an end wall also gives this message.
Structure too large or not closed near …	Close the wall at the position shown, or build at most 17 long.
Wrong block at … (expected: Refractory Bricks)	Only Refractory Bricks on edges and corners.
Wrong block at … (expected: Industrial Sight Glass, refractory)	Only bricks, sight glass, ports and the controller in the walls.
Wrong block at … (expected: Conveyor)	Fill the whole tunnel with Conveyors.
Too many ports of one kind	At most 5 Heat Ports.
Port not allowed here	Only item ports and Heat Ports work; an Energy Input Hatch does not.
Second controller at …	Remove the other controller.

Breaking the controller

The items in the slots drop, and so do the raw items still riding on the trays. The heat in the store is lost.

In the world

Behind the sight glass you see the trays riding on the running belts: raw items on the first half of the way, smelted ones on the second. While it works, the bricks glow from inside, brighter the more heat the trays use.

Building the controller

	Blast Furnace		→ Tunnel Furnace Controller

Tip

Smelt behind a crusher Crush ores first: the Crusher gives two dusts per ore, and the Tunnel Furnace smelts dust like any other item.