Boilers

A boiler burns a coal pile beside a water vessel and pushes the steam into the pipe run on its top outlet. One litre of water becomes 16 L of steam. There are two: the Cornish Boiler, the iron-age entry tier, and the Lancashire Boiler, the steel tier that carries enough pressure to run a Cornish Engine.

Cornish against Lancashire

fact	Cornish	Lancashire
vessel capacity, water and steam together	800 L	1200 L
water needed before it will heat	150 L	200 L
most water it will hold	500 L	800 L
level the automatic intake fills to	400 L	600 L
steam output while boiling	32 L/s	48 L/s
water that costs, at full output	2 L/s	3 L/s
choke pressure	5 atm	12 atm
explosion radius	3 blocks	4 blocks
vessel material	iron or steel	steel only

Both share the rest: 16 L/s of exhaust, an exhaust back-pressure ceiling of 0.8 atm, 180 s of heating before the first steam, a 10 s grace after the fire dies, 10 L/s through the automatic water intake, 200 L/s out of an open lid, 16 L/s out of an unpiped steam outlet, and 50 L a day lost to evaporation.

At full output a Cornish drinks its 500 L of boil water in about 4 minutes and a Lancashire its 800 L in about 4 and a half. Nothing runs for long without a feed.

Water

The vessel takes water three ways. Opening the lid and pouring from a container fills it up to the boil-water ceiling. A water line on the passthrough bend under the boiler block fills it automatically at up to 10 L/s, but only to half the vessel's capacity, which leaves room for steam. Condensate from an engine and water from a Steam Condenser are ordinary water and count the same.

Two cautions. Pour one container at a time: pouring from a stack loses the contents of every container but one. And watch the pressure of a pumped feed. Water admitted above 1 atm flashes to extra steam as it enters a boiling vessel, one extra litre of steam per litre admitted per atm of feed pressure over 1 atm. A Manual Fluid Pump delivers at exactly 1 atm and flashes nothing, a Mechanical Fluid Pump at 1.5 atm flashes half a litre per litre, and an engine Fluid Pump delivers at three quarters of its engine's inlet pressure, so a Watt Engine on a 3 atm line pushes water in at 2.25 atm and adds 12.5 L/s of steam on top of the boiling while the intake runs.

A bucket bails water back out while the lid is open, but only down to the level the boiler needs to run; the rest sits below the reach of a bucket.

Fire and exhaust

The fire is a vanilla coal pile in the firebox slot, lit by hand. The boiler counts as burning only while that pile burns and its exhaust has somewhere to go. Exhaust leaves through the pipe outlet at the far end at 16 L/s, and the boiler will not push it into a network already at 0.8 atm.

If the exhaust backs up to that ceiling the boiler is choked, the look-at line says so, and after 10 s the fuel pile is snuffed out as if the flue were blocked. A chimney standing on the exhaust outlet draws exactly the 16 L/s the fire makes. A bare open pipe end only bleeds 8 L/s, which is half of what is needed, so a run that ends in open air chokes the fire instead of venting it.

When the fire goes out the boiler keeps running for 10 s, then shuts down. Leftover steam condenses back to water at 200 L/s while it stays under the boil-water ceiling; anything above that stays trapped in the vessel until the lid is opened.

Pressure and the explosion

Internal pressure is the steam divided by the space the water is not using. A Cornish holding 300 L of water and 400 L of steam has 500 L of free vessel and reads 0.8 atm.

The boiler pushes steam out to its steam line the way two connected vessels equalise: it moves steam until the line reads what the vessel reads, and it never empties itself into the run. It stops pushing altogether at the choke pressure, 5 atm on a Cornish and 12 atm on a Lancashire, and stops boiling there too.

A boiler explodes when three things hold together for 30 continuous seconds: it is boiling, the fire is lit, and its internal pressure is at or above the choke pressure with the lid shut. The blast shatters every block within its radius under 20 blast resistance, which is pipes, ports, coal piles and soft ground, and scatters 40 per cent of the construction materials. Taking a boiler down with a pick returns all of them.

Three things do not blow a boiler up: a choked exhaust (it snuffs the fire instead), a full vessel (there is no upper water cutoff), and an unpiped steam outlet, which bleeds 16 L/s and is nowhere near enough to save an over-pressured vessel.

How players lose a boiler

No route for the exhaust. The commonest one, and it kills the fire rather than the vessel. 16 L/s of exhaust against 8 L/s through an open pipe end chokes the boiler and the fuel pile is snuffed 10 s later, and players who then piped the exhaust into a run that was already carrying something else at pressure got the same result. Give the exhaust a chimney or a stack of its own.

A pump straight into a boiling vessel. Pressurised feedwater flashes to steam, the pressure climbs while the player watches the water level rise, and the 30 s timer starts. Put a Piping (Valve) in the feed line and shut it while the boiler is at pressure, or feed a boiling vessel from a low-head pump and keep the engine pump for filling a cold one.

Trusting the relief valve through a restart. Boilers have been found at critical pressure immediately after a chunk reload, a relog or a server restart, with a relief valve fitted and the hatch open. That case is open as of 2026-09-14 and no fix exists. The only mitigation players have used is to vent the boiler and stop the run before a restart.

Not knowing about the lid. Holding right-click on the lid opens it, which vents 200 L/s and resets the over-pressure timer while it stays open. It is the emergency release, and a boiler in its danger band shows warning vapour before it goes.