Plumbing and Gas

Plumbing and gas piping are where good colonies are made and where careless ones flood. ONI simulates liquids and gases as physical elements with mass, temperature, and pressure, and moving them around is a system in its own right, separate from digging or building. Overlapping pipes and wrong-facing bridges are the top cause of base flooding, so learning the packet model early pays for itself many times over. This page covers how the two pipe networks work, the parts you route them with, and the atmosphere management that keeps your dupes breathing and your hazards where they belong.

Two networks, one packet model

Liquid pipes and gas pipes are separate systems, and both move fluid in discrete packets. A liquid packet holds up to 10 kg and a gas packet up to 1 kg, and each pipe advances one packet per section per second. That gives a hard throughput ceiling: a single liquid line tops out near 10 kg/s and a single gas line near 1000 g/s. If you need more, you run more lines. Understanding that a pipe carries packets, not a continuous flow, is the key to reading why a line stalls or why two sources fighting for the same pipe leave gaps.

The parts you route with each have one job. Pumps push fluid from the world into pipes. Vents release it back out. Valves meter flow to a set rate. Filters split one element out of a mixed pipe. And bridges, the part beginners misread most, act as a one-way segment that also lets you route a line behind another object.

Bridge direction is not decoration. A bridge has an input and an output, and it takes priority over a normal pipe joining the same line. Get the direction wrong and fluid goes where you did not intend, which is how bases flood. When a pipe network misbehaves, check your bridges first.

Liquid: the water cycle

Water is the central resource. It feeds electrolyzers, farms, and cooling, and it comes from geysers, ice melt, and reclaimed polluted water. The reclaiming is the important loop. A Water Sieve turns Polluted Water into clean Water plus Polluted Dirt, using a filtration medium like Sand or Regolith. Two facts about the sieve trip people up: it does not change the water’s temperature, and it does not kill germs, so it passes both straight through. That is why you cook food and use Chlorine rooms for sanitation rather than relying on the sieve to clean anything but the pollution label.

Closing the water loop is a core mid-game goal. Lavatory and shower output is Polluted Water, which a sieve turns back into clean Water, so with a Water Sieve and a bit of routing your colony recycles most of what it uses. A Carbon Skimmer fits the same loop: it removes CO2 using clean Water and power, outputting Polluted Water, and because the water in equals the liquid out, it runs a closed loop with a Water Sieve.

Gas: atmosphere management

Gas management is about keeping oxygen breathable and everything else contained. Gases stratify by density, and you use that constantly.

Gas	Behavior	Why it matters
Hydrogen	Lightest, rises to the top	Electrolyzer byproduct, burned for power or used as coolant
Oxygen	Light, sits high	The breathable gas, but only above a minimum pressure
Natural Gas	Dense, sinks	Generator fuel
Chlorine	Dense, sinks	Best germ killer and a strong insulator gas, used in sanitizing rooms
Carbon Dioxide	Heaviest, sinks to the floor	Unbreathable, pools where dupes work and sleep

Because CO2 is the heaviest common gas, it sinks into low spots, which you can use by digging a carbon pit for it to collect in, away from where dupes live. Hydrogen rising to the top is why a SPOM separates it there for the Hydrogen Generator. The general rule is that any gas that is not oxygen at breathable pressure is a suffocation risk, so you separate, pump, and contain rather than let them mix.

Overpressure, the missed failure

The failure new players miss most is overpressure. Gas producers throttle or stop under back pressure. The Oxygen Diffuser stops when its tile holds more than about 1.8 kg of gas, and the same principle applies across gas producers: if you do not vent or pump away what they make, they stall. High gas pressure also gives dupes Popped Eardrums stress. So atmosphere management is not only about which gas is where, it is about keeping pressure low enough at your producers that they keep running. Pumps and vents are how you do that, which brings the whole system back to the packet model you started with.

Next: the SPOM oxygen build is the biggest single plumbing and gas project, cooling loops run liquid coolant through pipes, and the crisis order cheatsheet shows where plumbing comes due.